8. VIBRA TION Prepared by E. M. Roth, M. D., Lovelace Foundation A. N. Chambers, Bellcomm, Inc. https://ntrs.nasa.gov/search.jsp?R=19690003259 2020-03-12T05:53:11+00:00Z ,TABLE OF CONTENTS VIBRATION ....................... 8- 1 Nomenclature and Principles ............... 8-1 Biomechanical Responses of Body to Vibration ....... 8-6 Longitudinal Vibrations (±Gz) ............. 8-9 a) Supine Vibrations (±Gz) ........... 8-11 b) Semi-Supine Vibrations ............ 8-13 Transverse Vibrations (±Gx) ............. 8-19 Vibrations Transmitted to the Hand .......... 8-21 Skull Vibrations ................... 8-2Z Transfer Functions and Power Spectra of Random Vibration ................... 8-22 Tissue Impedance ................. 8-3Z Ultrasonic Vibrations ................ 8-33 Patho-physiological Response to Vibration ......... 8-34 Permanent Pathological Changes ........... 8-34 Physiological Response ............... 8-35 Subjective Response .............. 8-36 Longitudinal or Vertical Vibration (±Gz) ..... 8-36 Semi-Supine and Other Vibrational Axes ..... 8-41 Threshold of Vibration Sensation ........ 8-46 The Cardiovascular System and Blood ...... 8-48 Respiratory Physiology ............. 8-48 Metabolic Factors ................ 8-51 Human Tolerance to Vibration ............ 8-51 Visual Effects ................. 8-67 Visual Acuity ............... 8-67 Effects on Visual Tasks .......... 8-75 Vigilance, Concentration and Reaction Time During Vibration .............. 8- 78 8-iii Simple Motor Performance ........... 8-78 Performance of CornpIex Tasks ......... 8-79 Performance Under Combined Vibration and Linear G .................. 8-89 Performance on Launch Towers During Horizontal Oscillation ........... 8-91 Protection Against Vibration ............. 8-91 References ........................ 8-95 8-iv 8. VIBRATION The vibration environment in space operations covers a wide range of amplitudes and frequencies. One must consider vibration due to spacecraft booster and control rockets, aerodynamic loading, cabin machinery, and equipment, as well as vibration in land and water surface vehicles supporting operations. Much can be preplanned regarding the vibration environment but unexpected contingency resonances, especially in emergency modes of operation, may suddenly appear on the scene. A study of the accuracy of vibration predictions in spacecraft design is available (14). Vibration seldom occurs in the operational situation as a single isolated variable. Other environmental variables such as weightlessness, linear acceleration, etc., can be expected to interact with vibration to either reduce or increase the debilitating effects. Equipment variables include such things as size of graduations or illumination of instruments, inflated pressure suits, etc.; procedural variables include such things as task load, variations in time of task performance, etc.; and finally, personal variables, such as fatigue and deconditioning from weightlessness or inactivity, etc., all may have effects. The effects of some of these can be predicted at this time; others must await further research. After a review of nomenclature and principles, data will be presented on the biomechanical characteristics of the human; the physiological and bio- chemical response, and finally, degradation of performance in response to vibration. NOMENCLATURE AND PRINCIPEES Vibration is defined as the periodic motion of the particles of an elastic body or medium in alternately opposite directions from the position of equilibrium when that equilibrium has been disturbed. The vibration, there- fore, may occur in air, liquid, or solid media; however, the primary concern here is with the oscillation of solid or semisolid bodies (e.g., structures of the spacecraft) in contact with the human body. Some nonauditory effects on the body are produced by transmission of vibration through the air; however, most of this occurs in the audio range and therefore is covered in Sound and Noise, (No. 9). The parameters of vibration which are significant for an understanding of the effects of vibration on the human body are as follows: Frequency Frequency is usually specified in cycles per second (cps) or Hertz (Hz) and may extend from greater than zero to the megacycle range. The band of concern with respect to the effects on the human body is usually confined to that less than 1000 Hz. Most of the significant effects of vibration occur 8-i within the 0.5 to 50 Hz range. Within this range the human body responds as a complex system of masses, elasticities, and dampers having lumped parameters. Above 50 Hz, it is more appropriate to regard the body as a continuous viscous-elastic medium for vibration applied to the surface of the body. Below 0. 5 Hz vibration is experienced as single jolts and as such, may be experienced as motion sickness. See Acceleration, (No. 7). Displacement Displacement, which is usually expressed in inches or centimeters, is defined as the maximum half-wave (single amplitude) or full-wave (double amplitude) displacement. The term amplitude is not synonymous with displacement. One refers to displacement amplitude or acceleration ampli- tude. Velocity Velocity, in inches/second or cm/second, is the first derivative of displacement and ordinarily is not used in describing the effects of vibration on the human body. Acceleration 2 2Acceleration inches/second or cm/second , is the second time deriva- tive of displacement and ordinarily is expressed as maximum or peak g. For sinusoidal vibration: G = A(2_ f)2386 (1) Where A = single amplitude of displacement in inches f = frequency in Hz For random vibration the acceleration is given as root mean square (r.m.s.) acceleration, or r.m.s, per frequency band. Jolt Jolt, in inches/second 3, is the third derivative of amplitude. The rate of onset of linear impact is a significant factor in evaluation of human tolerance to vibration of low frequency such as the I/2 cycle vibration of impact. Duration Duration of vibration, in seconds or minutes, is important in establishing the effects of vibration. This includes short-term exposures, long-term exposures (over I hour) for which there is very little research data, and repeated exposures for which there is even less. 8-2 Figure 8-1 represents the relationship of some of these parameters. Vibrations may be considered sinusoidal when composed of a single sine wave; complex, when composed of any other wave form; and non-periodic, which may approach randomness, when composed of waves which do not repeat systematically. Random, or semirandom waves, are usually charac- terized in terms of a power density curve (PSD) which is derived from the frequency bands that characterize the random vibrations. 0 -A v. _ltfA CO_ _t a* -4WZf2A sin wt (_ ""- 4_'_A f 1_ 81r_Af I ), -e _3f 3A COS _tt \ f\ PRODUCT OF ANGULAR FREQUENCY (=) ANO TIME (t) (rodlont) Figure 8-1 Relationship of the Parameters of Sinusoidal Vibration (After Morgan et al (eds.)(114)) Figures 8-2a and b represent a nomogram allowing interconversion of frequency, amplitude, and acceleration factors in sinusoidal vibration. The transmission of vibration to and within the human body is dependent on the following: • Direction of application of vibration to the human body which includes one, or any combination of the axes of the body. (See Figure 7-i in Acceleration, (No. 7) for nomenclature of axes). Most vibration re- search has been confined to the longitudinal axis (Z) of the body, since this has been the most common condition in most vehicles. However, in space vehicles, vibration can be expected to also occur in the direction of the fore-aft (X) axis of the body, or laterally along the Y axis. • Point of application of vibration to the human body which includes whole body vibration transmitted through seats (when subject is seated), through the feet (when standing), through the length of the body (when reclining), or combinations of these. Also, vibration may be transmitted via a body part such as the hand (when holding a vibrating control). Vibration of instruments may also be included in the latter category where the effects are primarily confined to the visual system. 8-3 8-4 Figure 8-2 Interconversion of Physical Factors in Vibration I00- 80- 60. 40 2O ! 8-t_ 6 4 2 I J / j/ 10 4- . io3_ I02-: I0- _2 J i0-1. iO-Z iO-3- 10-4. .z/ I Z g I-- r_ _J ..J W (.J 10- ° 4: / / 4: i0-1. . 6: 4: IO-Z. 6 4: 10-3 c b.I CI .J in = HALF PEAK-TO-PEAK AMPLITUDE Example of use: To determine the acceleration-ampl}tude of a v'tbration of 6 Hz at 1 in. (half-wave) amplitude (i.e. 2 in, peak-to-peak), lay a straight-edge across the chart joining 6 Hz with 1 in. amplitude. Answer: The acceleration-amplitude is 4 g (approximately) a. Nomogram of Frequency, Displacement-Amplitude and Acceleration-Amplitude for Sinusoidal Vibration (After Guignard (80)) Figure 8-2 (continued) b. Conversions Between the Parameters of Vibration (After Morgan et al (eds.) (114)) i I ' ' ' _01 ' =- '° I I I0 IO0 I=recluency (Hz) • Position of the body also determines the effects of vibration on the body and is related to the point of application. Effects are different depending on whether the person is seated, standing, reclining, prone, supine, or crouched. • Materials interposed between the body and source of vibration affects transmission of vibration to the body. This includes the effects of padd- ing on seats, spring mountings, etc. • Restraint of the body may attenuate or increase the effects of vibration depending on the frequency and amplitude. This includes the restraint of the person by seat belts and harnesses against a vibrating seat, sup- port for the arm in operating a control, wearing of girdles (which restrain the abdominal region), etc. • Interaction with other environmental variables may also enhance or reduce vibration and vibration effects. The presence of acceleration forces (e.g., linear +Gz) , reduced gravitation, and null gravity can be expected to affect the transmission characteristics. The transmission of vibration within the human body is dependent on the following: • Body size, build, and weight with greater attenuation involving larger masses and densities Posture, whether erect or slumped; and Muscular tension and fatigue ( 91 ). The response of the human body, which establishes the criteria for this Compendium, can be considered under the headings of physical or bio- mechanical, pathological, physiological, and performance. Of these, the biomechanical responses, which cause discomfort and pain, establish most of the physiological tolerance limits to vibration. Each of these responses will be described in some detail below. 8-5 BIOMECHANICAL RESPONSES OF BODY TO VIBRATION Basic concepts in the biomechanical structure of the body have been covered under impact in Acceleration, (No. 7}. The physical responses of the body are primarily the result of the body acting as a complex system of masses, elasticities, dampings, and couplings in the low frequency range (i.e., up to 50 Hz). The impedance of the body and its parts and organs damp vibration over certain frequency and amplitude ranges. For certain other frequency/amplitude ranges, there are resonances which amplify the vibration within various portions or all of the body. In general, the impedance of the body and its parts and inherent resonance factors influence the transmission characteristics of the vibration through the body (137, 139}. Figure 8-3 is a model of the dynamic mechanical properties of the human body. The body system for the low-frequency range below 100 Hz can be approximated partially or in toto by a lumped-parameter system, i.e., a system consisting of rigid bodies and restraining elements of negligible mass. PRESSURE -HEAD EXCHANGE WiTH LUNG _. VOLUME fo~ 30 Hz TRACHEA _,!;i _J z-sPI'JAL coLu,,,,,o-... UNDER +G z LOAD, fo ~60 S_-_-__r _ T r _ _ (STIFF ABDOMINAL MAS fo ~ 4-8 Hz _/////////_/ PERIODIC FORCE"_.APPLIED TO IMPACT JSITTtNG SUBJECTS INPUT IMPEDANCE fo ~ 4-6 Hi: I PERIODIC APPLIED TOFORCE] _MP_CT J STANDING SUBJECTS Figure 8-3 Mechanical Model for the Human Body for Gz Vibration and Impact Loads and External Pressure Loads (Acoustic, Blast, Decompression) (After von Gierke (67)) Calculation of the response of such a system to static, transient or dynamic forces presents a "network" rather than a field problem. The characteristics of such a system can be determined experimentally by studying its resonance behavior when exposed to steady state vibrations of varying frequency (33). The smaller the masses involved, the higher is the resonant frequency of the subsystem. The main torso resonances of the sitting or standing man are between 4 and 6 Hz,whereas the resonance of the head relative to the shoulder is in the order of ?.0-30 Hz ( 30, 33 ). 8-6 Mechanical energy, per se, transferred from the environment to the man is primarily deterministic of biologic effects ( 69, 179, 180, 182). The impedance model, then, is essentially an energy transfer model. As such, its purpose is to delineate the energy exchange characteristics of the human body. There are several pertinent aspects to the energy characteristics: the distri- bution of the real or dissipated energy over the frequency range, the distribu- tion of the reactive or stored energy over the frequency range and the total amount of each. The model defines the energy transfer patterns and, while it gives no a priori estimate of absolute levels, it does indicate that certain types of environments will be more likely to present tolerance problems than others. Also, the energy transfer to the human from the environment can be given in terms of the impedance and the power density spectrum of the velocity ( 105, 140, 141, 14Z, 143, 144, 171). This means that one can reduce a par- ticular acceleration-time history to its power spectrum in order to judge its effect. This has the advantage that it eliminates the need for approximating very erratic acceleration-time histories. Therefore, knowledge of impedance and phase angles permits calculation of the coupling of mechanical energy transmitted to the body under different modes of application and protection. (See Figure 8-I0.) (137, 139) This approach is especially useful in non-sinusoidal forms of vibration as are often met in operational situations. In the case of the steady state non-sinusoidal excitation it is necessary to resolve the force and velocity wave forms into a finite or infinite (Fourier) series of sinusoidal components. In the case of the transient excitation it is necessary to break the aperiodic waveform into an infinite series of sinusoidal components by means of the Fourier integral. In the case of random excitation the problem is more difficult because the waveform cannot be broken into sinusoidal components. However, it is still possible to calculate the impedance factors by dealing with the auto- and cross-correlation functions of force and velocity. These functions are actually definitions of the average characteristic of the signals and they do have Fourier integrals. The electric analogues of these problems in power circuits have been amenable to computer treatment ( 85, 142, 179 ). Above roughly i00 Hz, lumped-parameter models become more and more unsatisfactory; a distributed-constant, continuous medium must be introduced to describe the wave phenomena observed in these frequency ranges ( 68 ). The characteristics of their propagation and the internal damping of vibrations at higher frequencies tend to localize effects of such stimuli. For higher frequencies, through the audio-frequency range and up to about 100 kHz, the wave propagation of vibratory energy becomes more and more important but the type of wave propagation (shear waves, surface waves, or compressional waves) is strongly influenced by boundaries and geometrical configurations. Above 100 kHz and up into the mHz range, compression waves predominate and are propagated in a beam-like manner. This viewpoint permits not only a phenomenological description of the body's mechanical properties but, in an increasing number of cases, forms the basis for attempts to explain the behavior of tissue in terms of microscopic tissue- and cell-structure. 8-7 Vibration measurements of the body's response are usually made when- ever possible by non-contact methods. New cineroentgenographic methods have been used successfully to measure the displacement of internal organs, while optical, cinematographic, and stroboscopic observation can give the displacement amplitudes of larger parts of the body (106, 181 ). Small vibrations can sometimes be measured without contact by capacitive probes located at small distance from the (grounded) body surface. The probe forms a condenser with the body; capacity changes due to vibrations can be measurec in a high frequency carrier system. If vibration pickups in contact with the body are used, they must be small and light enough so as not to introduce a distorting mechanical load. This usually places ,_weight limitation on the pickup of a few grams or less, depending on the frequency range of interest and the effective mass to which the pickup is attached. For example, Figure 8-16b illustrates the effect of weight and size on the response of accelero- meters attached to the skin overlying soft tissue. New laser techniques are being developed to detect and measure vibration of mechanical structures {I19). Measurer_l_nt of transmission of external force through fluid systems of the body under vibration is currently under study (15). Living organisms have linear characteristics only in the low frequency range and for very small deformation. (See discussions of Figures 8-17 and 8-18.) For higher frequencies and large forces, a non-linear response must be expected. The damping coefficients, for instance, may depend on the instantaneous velocity of the organ displacem_::nts. This deviation from the assumed viscous damping and the participating mass may change with higher accelerations. There will be a difference, therefore, between the response of an organism to steady-state vibration with relatively low acceleration and to impact pulses with short, high accelerations ( 27 ). This consideration hold for the responses of the whole body as well as for a single organ or organ complex. Since the natural frequency of the studit:d organ system in a given animal differs much fror_] the related organ of man, their responses to a certain form of excitation cannot be compared. Also the posture of the living subject will have an influence on his dynamic response, necessitating a careful control of the subject's position during the test. Establishing tolerance criteria for man, using animals as substitutes, presumes that the tissue strength in animals is the same as in man. This is not _ttoo unreasonable assumption (133). Scaling factors are now under study (66). In evaluating impedance curves, it should be kept in mind that the impedances, reflecting resonances and tissue damping are not independent of the shaking intensity whether expressed in terms of amplitudes or peak velocity or peak accelerations. It has been shown that the force between subject a_d support does not increase linearly with these parameters ( Z4, 48, 103, 185 ). {See Figure 8-9a.) The nature of the padding material between shaker and subject as well as the level and direction of linear acceleration bias can ir_f]_lence the determination of the resonance spectrum of the body ( 26, 48 ). (See Figure 8-11a and b.) 8-8 Excellent reviews of impedance and transmission parameters of man are •vailable from which the following discussion is taken ( 27, 28, 29, 33, 179, 180). Longitudinal Vibrations (±Gz) The average response of sitting human subjects to steady-stage vibration in the frequency range 0 to 20 Hz is illustrated in Figure 8-4. Up to two Hz, the impedance of the whole body is the same as the impedance of a pure mass (m_), indicating that the body moves as a whole unit. Around 5 Hz, a reson- ance peak produces the greatest deformation of the whole body, and a second peak around 10. 5 Hz demonstrates a resonance of a subsystem in the body. % t . Median _ oo 2 / -.:._._................... "_ zl neomaJ)-Ipr_ng )y_tern I Frequency - Hz 0 2 4 6 .8 IO 1.2 _4 1,6 18 20 2,2 24 26 28 30 32 34 36 38 40 n Median and the twentieth and eightieth percentile of the impedance of eight different subjects, compared with the impedance of a pure mass (m_) and a one-mass-spring system with damping. Figure 8-4 Impedance of a Sitting Man (After Coermann et al(28, 29)) The calculation of the parameters for these two resonances leads to a model consisting of two mass-spring systems with dampers. The first system (dotted line) (Figure 8-4) represents about 90 percent of the body mass, has an elasticity of about 685 Ibs per inch and a damping factor of about 0.32 of the critical damping; the second represents about i0 percent of the body mass, has an elasticity of about 186 Ibs per inch and a damping factor of about 0. 13 of the critical. These parameters change considerably if the subject assumes a relaxed position or if the abdomen of the subject is restrained by a semi-rigid envelope (Figure 8-5a). While the first resonance frequency changes only little, the damping factor for this resonance varies over a wide range, indicating that the body reacts almost as a pure mass for frequencies up to 8-9 Figure 8-5 The Effect of Posture, Clothing or Envelopes, and Relaxation on Impedance and Transmission of Vibration in Man (After Coermann et al (27' 33)) n_ass 90, 000 d_e x =ecZ/ ¢m X Z = hi.5 X 4 "_.\ _" _tt,ng relaxe_ • " "_. s_tting erect o o , z 3 4 5 e 7 8 _ ,o _, _2 ,a ,4 ,_ ,6 ,7 ,6 _ 20 Frequency - Hz E I-- b° The Effect of a Semi-rigid Envelope Around the Abdomen on the Impedance of One Sitting Subject at Erect and Relaxed Postures The Transmission of Vibrations from the Seat to the Head of One Subject at Varied Body Postures Compared with the Transmission Factor of a One-Mass-Spring System with Damping 8 -I0 6 Hz. The second resonance practically disappears. The same effect can be seen if the transmission of vibration from the seat to the head of a sitting subject is measured (Figure 8-5b). The peak of the fundamental resonance at around 5 Hz is reduced in a relaxed posture, and the higher resonances become insignificant. (See also Figure 8-10 comparing impedance in the semisupine seated and standing man.) The greatest loads occur in the region of the eleventh thoracic to the second lumbar vertebra which can therefore be assumed as the hinge area for flexion of the upper torso. Since the center of gravity of the upper torso is considerably forward of the spine, flexion movement will occur even with the force applied parallel to the axis of the spine. Changing the direction of the force so that it includes an angle with the spine (for example by tilting the torso forward) influences this effect considerably. Similarly, the center of gravity of the head can be considerably in front of the neck joint which permits forward-backward motion. This situation results in forward-backward rota- tion of the head instead of pure vertical motion. Examples of relative amplitudes for different parts of the sitting and standing body are shown in Figure 8-6. The curves show an amplification of motion in the impedance resonance range and a decrease at higher frequencies. The impedances and the transmission factors are changed considerably by individual differences in the body and its posture, as well as support by a seat or back rest of a sitting subject;or by the state of the knee or ankle joints of a standing subject. The resonance frequencies remain relatively constant whereas the transmission ratio varies (for the condition of Figure 8-6b). Transmission factors as high as 4 have been observed at 4 Hz (145). Above approximately i0 Hz vibration amplitudes of the body are smaller than the amplitudes of the exciting table and decrease continuously with increasing frequency. The attenuation of the vibrations transmitted from the table to the head is illustrated in Figure 8-7. At 100 Hz this attenuation is around 40 dB. The attenuation along the body at 50 Hz is shown, although not for pure longitudinal excitation. (See Figure 8-8). In the seated position, between 20 and 30 Hz, the head exhibits a reson- ance as can be seen clearly in Figure 8-6b. In this range the head amplitude can exceed the shoulder amplitude by a factor of 3. This resonance is of importance in connection with the deterioration of visual acuity under the influence of vibration. Another frequency range of disturbances between 60 and 90 Hz suggests an eyeball resonance (30). (See also Figs. 8-17 and 8-18.) Maxima in horizontal seat-to-head transmissibility have been reported in the range 1.5 to 2.5 Hz (40, 91 ). Resonance near 2 Hz is the dominant mode in horizontal vibration of the seated man. If the seated subject is vibrated in more than one plane simultaneously, his physical response and subjective appreciation of the motion depend upon the amplitude and phase relationships between the component vibrations. a) Supine ±G Z The impedance of the human body lying on its back on a rigid surface and vibrating in the direction of its longitudinal axis has been determined by ballistocardiograph studies ( 73, 187). The total mass of the body forms a 8 -ii Figure 8-6 Transmission of Vertical Longitudinal Vibration to Body Parts in Standing and Sitting Subjects o _-1.5 tr z o W _J IJJ (.) .5 /O_ A "f HI P/TABLE / --_ SHOULDER/TABL E I 1 I "r"--- 1 I I m 2 3 4 6 8 I0 20 30 40 FREQUENCY IN Hz a, Transmission of Vertical Vibration from Table to Various Parts of the Body of a Standing Human Subject (After Dieckmann (41), data for transmission to belt after Radke (145)) 3 sl 2.5 O 4_ rc _- 2 O 0 < / ! It HEADI----"_/ SHOULDER I / I I /- -- SHOULDER / i L 1 l _ ____L___ J_ Frequency in Hz b. Transmission of Longitudinal Vertical Vibration from Table to Various Parts of Body of Seated Human Subject (After Dieckmann (41)) Figure 8-7 Attenuation of Vertical and Horizontal Vibration for Standing and Sitting Human Subjects (Continuous lines after von Bekesy (5). Shaded area is range of values for 10 subjects after Coermann (30)) (After yon Gierke (70)) TABLE - HEAD HORIZONTAL (SUBJECT SITTING ) FOOT --HEAD VERTICAL [SUBJECT TABLE --HEAD VERTICAL ( $L_IJECT SITTING) 4 5 ,o zo 3o 4o_o6o 8o,oo ,_o Frequency in Hz 8 -12 Figure 8-8 Attenuation of Vibration at 50 Hz Along Human Body Excitation of a) hand and b) platform on which subject stands. (After von Gierke (70), adapted from yon Bekesy (5)) 45 40 30 20 Vibration Attenuation in dB t ,I / FORCE ",,, 0 20 3O Vibration Attenuation in dB simple mass-spring system with the elasticity and resistance of the skin for tangential vibration. For the average subject the resonant frequency is between 3 and 3.5 Hz and the Q of the system is about 3. Q is defined as the rate of increase caused by resonance of the amplitude of an oscillating system and is measured as the ratio of the mass or spring reactance at resonance frequency to damping resistance. Restricting the subject's mobility by clamping the body at the feet and shoulders between plates connected with the table changes the resonant frequency to approximately 9 Hz and the Q to about 2.5. More recent studies on shake tables in the 0.2 to 0.5 g range indicate that at 4-8 Hz, as in the sitting position, subjects can tolerate less intensity than at other frequencies. Tension-relaxation factors also hold here. These results are indicated in Figures 8-9a and b. In Figure 8-9a the nonlinearities of the intensity response are noted as g-load is increased. b) Semi-Supine In semi-supine position used in spacecraft liftoff, the response of the human body to steady-state sinusoidal vibration is definitely different, as shown in Figure 8-i0 (middle curve). The general course of the impedance curve follows very closely the m o_-line up to about 15 Hz, except at around 8 Hz where a slight peak due to a resonance in the pelvic area becomes evident. The second enhancement of the curve around 14 Hz indicates a very high damping in the body, and the phase angle signifies the domination of the damp- ing forces at higher frequencies. The other curves of Figure 8-10 can be used to compare the impedance factors for steady-state sinusoidal, longitudinal vibration in the seated, semi-supine and standing human. Figures 8-Ii a and b indicate the effect of a constant, linear-G bias on semi-supine impedance and resonance peaks (176). The subjects were placed in a seat position similar to that described in Figure 8-24a and vibrated in the Z axis, exposed to vibration (_=0.4 G at 2-I/2 to 20 Hz in i/2 Hz increments) combined with linear acceleration of l, 2-I/2, and 4 G. The absolute quanti- ties (impedance magnitude and phase angle) shown in Figure 8-11a and 8-11b for one of the subjects are the magnitudes of the undistorted fundamental 8 -13 Figure B-9 Mechanical Impedance in the Supine Position (After Edwards and Lange (48)) 32 I ,J I ! i 20 ! i ' ,/_ L 8 I . O,2g - 4 0 0 2 4 6 8 I0 12 14 16 18 20 FREQUENCY (Hz] 9O klJ '" 60 t_ W 30 L_ _J z 0 bJ •_ -30 .... [ . L __ "7- n J ! i [ i , 0 2 4 6 8 I0 12 14 16 18 20 FREQUENCY (Hz) 32-! ] 28 _ 24 W ,_, 20 -J W u 12 Z a 8 0. lIE I , TENSED _ ___ RELAXED _ --:---- 4 0 0 2 4 6 8 I0 12 14 16 18 20 FREQUENCY (Hz) 90 __/ T ] r ] [ TENSED_] '_ ( ! I f ( ,E_.,,(ED......-( "' 6o ." ! Q_ 3 0 - - i " +" -T: I ' _ " Z 0 ..... : .,- , -<_ -60 0 2 4 6 8 I0 12 14 16 18 20 FREQUENCY (Hz) a. Whole-Body Mechanical Impedance Versus Frequency, with Table Acceleration Level as a Parameter, Subject RGE, Supine Relaxed b. Whole-Body Mechanical Impedance Versus Frequency, Subject RGE, Supine Tensed and Supine Relaxed 8 -14 O10$ 105. (11 I*J tO IQO 20.0 QI FREQUENCY- Hz | IIWJW_B¢I" MAllt __ _,o,_ .....................i o ,o _o2o0 -_ _i F"_°°_._L. i_t FREQUENCY- Hz gT?ING ERt_T___ ........ _'.o ....... _o _o FREQUENCY - Hz 1.0 I0,0 200 FREQUENCY- Hz ........ Lo ....... _;.o "_o FREQUENCY-Hz Figure 8-10 Human Mechanical Driving Impedance in the Steady-State Sinusoidal (Vibration) Environment (After Weis et al (179), from data of Coermann (27)) 8-15 Figure 8-11 Effect of Linear G Bias on Vibrational Impedance (After Vykukal (176)) I00 92 84 ,- 76 _ 68 _ 60 N 52 _' 44 Z g 36 _ 2a 2O 12 100SUBJECT; RG I g _v"" ......... 2.5g f ..... 4g J t,] I i / ,,",,i\ I I I I I L I I I I 4 8 12 16 20 FREQUENCY, Hz BO -o 70 Z 50 z 40 123 _ _o 90 _:,,.r ,v,+vxr,,, \ ,x 201_ _ 10| I l I I I ] I l t 0 4 B 12 16 FREQUENCY, Hz i o 2o a. Whole-Body Mechanical Impedance Measured b. at Constant Vibration and Bias G Levels of 1, 2-1/2, and 4 G + IMAG 96 p 90° +f80 ,_ 72 il\x _64 .- t_ "/ \+" --"_ / i | I/ t L___\\ 56+ t2 " J ...... ! y--, 4 X goo +.,+ N ....32 a. 16, /> SUBJECT:RG Ig ......... 2.5g ..... 4g .:'I ! / B[_ 0° V I J t I l I I I I I 0 8 16 24 32 40 48 56 64 72 80 88 9 +REAL IMPEDANCE Z, Ib sec/in Measured Whole-Body Mechanical Impedance Plotted in the Complex Plane waveforms of the frequencies tested. As shown by these results, the influence of an increased bias G on the dynamics of the human body is quite significant. The differences in the nature of the body position and seat damping structure are probably responsible for the difference in impedance between the 1 G condition in 8-1]a and 8-1lb. Resonances at 7, i], 13, 15, and 18 Hz were detected as expected (48, 179 ). The complex plane or polar coordinate repr_;sentation of impedance data in Figure 8-11b allows for a more detailed analysis of the system dynamics. Resonances and curve trends appear as deviations from ideal element re- sponses (mass, spring, dampers). As noted, the resonances indicated appear as small loops (actions of subsystems resonating by themselves); the major loop trend represents whole body response in the frequency range tested. The most significant effects of increased bias G on dynamics of the body are increased stiffness, reducing damping, and higher energy transmission to internal organs. The observance of pain by the subjects at certain measured resonant frequencies would indicate a lower tolerance to vibration at that frequency when combined with higher linear accelerations. No significant subjective impressions were noted at the I-G bias level. However, at 2. 5 and 4g, the subject became dramatically aware of local resonances, such as in the abdomen, chest, and extremities. Visual decrements were also noted. The correlation of resonance measurements and subjective impressions is quite significant. For example, in the frequency range of 9. 5 to IZ.5 Hz at Z.5-G bias, the subject noted the following sensations: at 9.5 Hz, "awareness of stomach vibrating," 11.5 Hz, "stomach vibration continued," IZ.5 Hz, 8 -16 "stomach senaations decreasing." The polar plot (Figure 8-11b) indicates a deviation from the major loop in this frequency range. Subjective observations indicate a direct relationship with other deviations also shown in Figure 8-10c. Additional studies are needed to determine the effects of increasing the vibra- tion G magnitude on impedance for higher bias-g levels simulating booster liftoff to orbital insertion. One of the most important subsystems of the body, which is excited in the standing and sitting position as well as in the lying position is the thoraco- abdomen system ( 151, 187). The abdominal viscera have a high mobility due to the very low stiffness of the diaphragm and the air volume of the lungs and the chest wall behind it. Under the influence of both longitudinal and trans- verse vibration of the torso, the abdominal mass vibrates in and out of the thoracic cage. These vibrations not only take place in the (longitudinal) direction of excitation but, during that phase of the cycle when the abdominal contents swing towards the hips, the abdominal wall is stretched outward and the abdomen appears larger in volume; at the same time the downward deflection of the diaphragm causes a decrease of the chest circumference. At the other end of the cycle the abdominal wall is pressed inward, the diaphragm upward, and the chest wall is expanded. This periodic displacement of the abdominal viscera has a sharp resonance between 3 and 3.5 Hz. (See Figure 8-12.) Oscillations of the abdominal mass are coupled with the air oscillations of the mouth-chest system ( 8, 15 ). Measurements of the impedance of the latter system at the mouth (by applying oscillating air pressure to the mouth) shows that the abdominal wall and the anterior chest wall respond to this pressure. The impedance has a minimum and the phase angle is zero between 7 and 8 Hz. The abdominal wall shows maximum response between 5 and 8 Hz, the anterior chest wall between 7 and II Hz. Vibration of the abdominal system resulting from exposure of a sitting or standing subject is clearly detected as modulation of the flow velocity through the mouth. (At large amplitudes, speech can be modulated at the exposure frequency.) Elec- trical equivalent circuits and mechanical models for the abdomen-chest-mouth system are available ( 8, 67, 73 ). Recent studies focused on the transmission of sinusoidal abdominal pulses to the cerebrospinal fluid of animals suggest that the amplitude ratio of intro- cranial pressure to the airflow velocity is relatively constant for 2-30 Hz driving frequencies ( 15 ). The bear is often used as a substitute for man in vibration and impact research because it is assumed that his body structure is most similar to that of man. In the steady-state impedance curve of a ig6-1b Himalayan bear, the first resonance frequency is a little lower than that of the average man but still in the range of the standard deviation. Also at higher frequen- cies the characteristic of the impedance curve is similar if the difference in weight is considered. An anatomical review of organ morphology related to vibration is available (73). The movements of the visceral organs can be a limiting factor for human tolerance to steady-state vibration in the frequency range 4 to 8 Hz. To measure the resonant frequencies of the thoraco-abdominal system as a whole, an air-filled balloon on a catheter was inserted in the 8-17 5_= ;4 5 3 ,=. z: b. Figure 8-12 Thorax Expansion and Oscillating Air Volume of a Human Subject with and without Envelope Around Abdomen (supine position, shaken horizontally) (After Coermann et al (33)) • ABDOMEN WALL DISPLACEMENT ._D 6O0 I00 a. Without Envelope about Abdomen With Envelope about Abdomen 1400 PO0 _200 ;00 iO00 _00 800 IO0 z • 600 500 400 .PO0 ZOO IO0 0 0 , 2 3 • _ 6 7HzS , ,0 ii 12 is 14 THORAX EXPANSION ANO OSCILLATING AIR VOLUME OF A HUMAN SUOJECT WITH ENVELOPE AROUND AIBOOMEN { SUPINE POSITION; SHAKEN HORIZONTALLY ) 8-18 colon of human and animal subjects sitting on a vertically vibratory shake table (185). The changes of the pressure within the balloon and the phase of this pressure relative to the movement of the shaker were recorded at frequencies from 1 to 20 Hz. In Figure 8-13 the pressure changes per G of the shaker are plotted versus frequency for a man sitting erect, sitting relaxed, and semi-supine, as well as for a sitting rhesus monkey and mouse. 20 LB 16 r21 I O C- I0 El N 4 ,2 ...... HEAD/TA_E i _ _LDERI TABLE HIP/TABLE '//_. J9 SITTING -'_ l I 1 I ........ I 2 3 4 5 FREQUENCY IN Hz Figure 8-13 Transmission of Transverse, Horizontal Vibration from Table to Various Parts of Sitting and Standing Human Subject (After Goldman and von Gierke (73), adapted from Dieckmann (40)) Man and monkey show in the sitting position main resonances between 4 and 5 Hz, while the man in semi-supine position has maximum movements of the thoraco-abdominal viscera between 7 and 8 Hz. The abdominal viscera of other animals have different resonance systems ( 131, 132, 150 ). Thus, before animals are used to study the influence of transient forces on the abdominal organs, it must be assured that their thoraco-abdominal systems have dynamic characteristics similar to those of man. There appears to be little evidence that anesthesia, 100% oxygen, or feeding have significant effects on resonant frequency of organs in animals (130). Transverse Vibrations (+Gx) The physical response to transverse vibration -- i.e., horizontal in the normal upright position -- is quite different from that described for vertical vibration (73). Instead of thrust forces acting primarily along the line of action of the force of gravity on the human body, they act at right angles to this line. The direction of the body masses along this line is therefore of the utmost importance and greater differences must be expected between sitting and standing subjects than for vertical vibration where the supporting structure of the skeleton and especially the spine have been designed for vertical loading. 8-19 Impedance measurements for transverse vibration are available. In fore and-aft vibration, the resonant frequency occurs due to flexion of the lumbro- dorsal spine and articulation at the hip joints. There is also a lateral bending mode at 1.6 Hz (80). The transmission of vibration along the body is illustrated in Figure 8-14 ( 40)o For a standing subject the shoulder and head amplitudes are of the order of 20 to 30 percent of thetable amplitude at about l Hz and decrease rnm Hlllg 70 li, ANG_ OF INTESTINAL RESONANCE IN DOG ('5 60 55 1_4AN SITTIN_ ,, RE_XEO iA I .o //i IA-. 5, TINoERECT 25 ":"" "-. .......... -. "'-''° .'5 RHESUS f z ] '_ ; J ; ; 9 lo II lz 13 _._ ,s 1_> 17 _8 ,9 20 HZ FREQUENCY Figure 8-14 Comparison of Intestinal Pressure Change During Sinusoidal Vibration in Man at Different Postures and in Rhesus Monkeys (Anesthetized) (After Coermann (27)) with increasing frequency. Relative maxima of shoulder and head amplitudes occur at 2 and 3 Hz respectively. The sitting subject exhibits amplification of the hip (1.5 Hz) and head (2 Hz) amplitudes. All critical resonant frequen- cies appear to be between i and 3 Hz. Investigation of experimental results of the type of Figure 8-I0 in connection with phase measurements shows that the transverse vibration patterns of the body can be described as standing waves, i.e., as a rough approximation one can compare the body with a rod in which transverse flexural waves have been excited. One has, therefore, in agreement with the experimental results, nodal points on the body which become closer to the feet as the frequency increases since the phase shift between all body parts and the table increases continuously with increasing frequency. At the first characteristic frequency at 1.5 Hz, the head of the standing subject is observed to have a 180 o phasc_ shift compared with the table; between 2 and 3 Hz this phase shift becomes 360 ° ( 40 ). There are longitudinal head motions excited by the transverse vibration in addition to the transverse head motions shown in Figure 8-14. The head performs nodding motion due to the anatomy of th_ upper vertebrae and the location of the head's center of gravity. Above 5 Hz the head motion for the sitting and standing subjects is predominantly w, rtical of the order of i0 to 30 percent of the horizontal table motion. 8 -20 Many kinematic processes, physical loadings, and gross destructive anatomical effects can be studied on dummies which approximate a human being in size, form, mobility, total weight, and weight distribution in body segments ( Z7 ). Several such dummies are commercially available. They have been used extensively in aviation and automotive crash research and in other studies to precede work with human subjects and to study protective seats and harnesses. Such dummies attempt to match the "resiliency" of human flesh by some kind of padding, but they are crude simulations at best and the dynamic mechanical properties are, if at all, only reasonably matched in a very narrow low frequency range (27). This and the passiveness of such dummies must be kept in mind as important mechanical differences between them and living subjects. Vibrations Transmitted to the Hand In connection with studies on the use of vibrating hand tools, several measurements have been made of vibration transmission from hand to arm and body ( 2, 6, 39 ). The impedance measured on a hand grip for a specific condition representative of hand tool use is presented in Figure 8-15. +90 °l -90 106 MI • SPRING IRES'STANCC JMASS 1 I I IO 20 40 70 FREQUENCY IN Hz Figure 8-15 Impedance and Phase Angle of Arm Measured at a Vibrating Hand Grip Elbow flexion 20 ° to 25° , static pressure on grip 22 Ibs. Measurements on one subject. (After Goldman and von Gierke (73), adapted from Dieckmann (39)) The impedance has one maximum in the range below 5 Hz probably determined by the natural frequencies for transverse excitation of the human body between l and 3 Hz. A second strong maximum appears between 30 and 40 Hz; the effective mass of the hand (approximately 2.2 Ibs. I kg. ) is here in reson- ance with the elasticity of the soft parts on the inside of the hand. This elasticity between force and hand has been estimated at 2. I0-8 cm/dyne. With a practical hand tool, which operates between 40 and 50 Hz, it has been found that the vibration amplitude decreases from the palm to the back of the 8-21 hand by 35 to 65 percent. Further losses occur between the hand and the elbo, and the elbow and the shoulder. Figure 8-8 which shows this decrease of vibration amplitude from the hand to the head, indicates that the strongest attenuation occurs in the shoulder joint. Thresholds of sensitivity to vibrating tools vary with age, experience, and simultaneous exposure to noise (172). Skull Vibrations The vibration pattern of the skull agrees approximately with the pattern of a spherical elastic shell ( 7, 57 ). The nodal lines observed suggest that the fundamental frequency lies between 300 and 400 Hz with resonances for the higher modes around 600 and 900 Hz. The frequency ratio between the modes is approximately 1.7 while the theoretical ratio for a sphere is i. 5. From the observed resonances, the elasticity of skull bone can be calculated. The value obtained for Young's modulus (1.4 x I0 I0 dynes/cm Z) agrees fairly well with static test results on dry skull preparations. (See also Sound and Noise, (No. 9). Vibrations of the lower jaw with respect to the skull can be explained by a simple mass-spring system, which has a resonance, relative to the skull, between 100 and 200 Hz (35). Transfer Functions and Power Spectra of Random Vibrations Recent studies have attempted to quantitate responses to random vibration from power-frequency encountered in ground vehicle simulators ( 88, 174). Random vibrations best describe the environment in most transportation systems. Power-spectral histories and other techniques such as time histor- ies, peak distribution, and probability records describe the vibratory input to the subject but often fail to quantitate the response especially where angular mode such as pitch and roll are also present. Transfer functions relating "average absorbed power" to onset of fatigue haw_ received preliminary study (140, 142). Since absorbed power is a time-sensitive scalar quantity it is hoped that this parameter may be summed in complex multidegree of freedom environments and symptoms related to this value. The essential premise controlling the use of the transfer function is that the system be linear. Therefore, application of the transfer function to human dynamics requires the establishment that man behave as a linear system within the bounds of interest. The body cannot always be treated as a linear, passive mechanical item. Linearity is an idealization which holds only for relatively small amplitudes. Nonlinearity must be kept in mind if mechanical injury to tissue is considered (73). There is actually considerable non- linearity of response well below amplitudes required for the production of damage. An example of this is given in Figure 8-16a. Studies at 0.2, 0.35, and 0.5 g indicate not only that non-linearities exist between intensity and impedance, but that the non-linear response is most prominent at resonance frequencies (48). Animal data up to 3 g's have corroborated these findings ( 24, 103, 185). Another variable controlling impedance and resonance 8-22 patterns is the nature of the absorbing medium between shaker and subject (48). A technique should really be developed by which the impedance of the padding may be subtracted from the total impedance to yield that of the subject alone, principally in the same mannerby which the impedance of the mass of the support is now subtracted (179). The adjusted impedance of the subject may then be compared to that of the impedance of the subject without padding, and consequently a true measure of the force-transmitting characteristics of the padded support may be obtained. Distortions also arise from the effects of the mass of the accelerometer {Figure 8-16b). Studies have recently been performed in an attempt to define the range of linearity of the human response to :_G z vibration for subsequent determina- tion of transfer functions ( 14aJ. The following discussion is taken directly from these studies. To gather response data concerning random vibration, experiments were conducted using a uniform spectrum, white-noise vibration filtered through a 2-Hz band-width filter at center frequencies of 3, 5, 7, I0, 13, 15, 20, 25, Figure 8-16 Measurement Distortion and Non-Linearity of Tissue Response to Vibration I0 X IO 5 ® a. Effect of Loading Distortion of Body Surface on Surface Impedance of Soft Tissue for Two Experimental Human Subjects A and B (After Goldman and yon Gierke (73), adapted from Franke (56)) 16 14 12 At. A F 8 6 .4 2 4 .... m • 3 grams .. m • 12groml % ....... _-j .-._ _ r • j. 5 cm - I I I A l I I 10 20 40 60 80 tOO 200 400 FREQUENCY IN Hz b. Amplitude Distortion Due to Size and Weight of Accelerometer Attached to Body Surface Over Soft Tissue of Human Subject Exposed to Vibration A L This graph gives the ratio,----, of the response of a A F loaded accelerometer of mass m and radius r of con- tact surface area for several values of m and r. (After Goldman and von Gierke (73)) 8-23 and 30 Hz. The tolerance condition was a combination of vibration severity where pain, loss of physical stability, or advanced stages of blurred vision were considered unacceptable. When tolerance was reached, the subject actuated a buzzer which began the rms data collection procedure. Each sub- ject was held at the tolerance level for 20-60 sec. The time interval was dependent upon the stability of rms data. In order to examine more carefully human response to random vibration of greater frequency content, a third experiment was conducted using 10 subjects exposed to random motions filtered with a 10 Hz band width filter. These data established the upper boundary of human response in accordance with the experimental control described for tolerance testing. To validate the hypothesis that whole body response is linear below this level, tests were performed at seven different acceleration levels to very low intensity. Regression analysis (Figure 8-17a) indicated strong linearity throughout the frequency spectrum. To evaluate the variability of human response with changes in wave form, the same test was conducted using white noise passing through 2 and 10 Hz band-width filters. A comparison of the response_ at a center frequency of 5 Hz is shown in Figure 8-17b. Although response varied with the three types of input, strong linear characteristics were evid_ent for each input. The study of linearity was also approached in a manner analogous to that of a constant-rate spring: k : I(F/D )Z - (_:_)Z (2) Where D = Deformation -{deflection} F = Force k = Spring constant a = Damping oa = 2rrf For head acceleration responding to body displacement or deflection, D, (see Figure 8-18a): D {Ain - AH) 386 4 2f 2 (3) Where A H = Head acceleration A I = Input acceleration f = Frequency, Hz. When the applied or restoring force is proportional to the spring deformation, the system is considered to be linear. However, if the force magnitude is not proportional to the displacement, or if the damping is not proportional to the velocity, the system is nonlinear. Figure 8-17c describes the use of 8-Z4 Figure 8-17 Determination of Acceleration Transfer Functions from Human Tolerance Data to Random Vibration in the _+ G z Axes (Seated) (After Pradko et al (144)) =, _ ! g, _ //, 3., r- P'i , LIJ m .j.- + 3 ' -i / 0 / _ i 120Hz 4.4__ .+ .... _ /_ _,/ / _ _,/ 0 . I .7,2 .3 .4 .5 .6 SINE INPUT AGCL, g(RMS) IJO .8 _".7 w o o a0 Frequency Response Comparison .40 • j¢/ / _ W/tO Hz FILTER o i " 11 _ l')" ./ , CENTER FREQ- 5Hz o ._ .z .3 .4 .5 .6 .7 INPUT ACCELERATION g(RMS) b, Response of Human Head to Different Types of Vibration in the _+ Gz Axis (Seated) .32 o_ .24 z z o I- • .16 w .OS ; J --i t , i r i _/ IO _'O 30 INPUT FORCE ,, POUNDS 3Hz /- / / , / _OHz I 20Hz 40Hz ! 60 Hz 40 50 6'_ ?b (RMS) C. Human Deflection Characteristics in the + G z Axis (Seated) 8-25 Equation (3) again verifying the previous findings that the mechanical response characteristic of man in a seated erect position within the limits of this evalu- ation may be treated as a linear system. With the establishment of linearity, the transfer function can be ex- perimentally determined. Transfer functions are expressed as continuous The schematic representation isfunctions, using the Laplace transform. shown a s IElem ent transfer] [Input, I(s)]x L function, G (s) J = Output, O(s) Output (s) O(s) G(s) = Input (s) = (4a) where I(s) = O(s) = G(s) = Laplace transform of the input Laplace transform of the output Transfer function The general form of the transfer function G (s) is the ratio of output to input transform, G(s) - Response function O(s) (4b) Input function = A graph of the ratio of the output to the input is a graph of the transfer function. The curve is then mathematically fitted by a ratio of polynomials. The early manual techniques of curve matching were laborious and now have been replaced and simplified by computer methods (50). For the specific case the data used were the means of rooted values. The transmissibility of head acceleration to input acceleration gave a curve similar to the dashed line of Figure 8-5b. From the ratio described in Equation 5a, the complex variable (S)is obtained: Ahead GA(S) = -_. input (S) (5a) where GA(S } = Acceleration transfer function (S) = Complex variable This transfer function, under both sinusoidal and random vibration, should be in good agreement with experimental data. The force transfer function, GF(S), can be defined as GF(S) = _ (S) "'head (5b) 8-26 The GF(S) can be determined from characteristic curves of F/Ainpu t and F/Aoutput shown in Figure 8-18a to c. By combining Equations 5a and 5b, the analytical transfer function can be obtained. There is close agreement between the statistical impedance data collected during these experiments (similar to dash dot line of Figure 8-5a) and the response calculated from the analytical transfer function. It is thus concluded that human response to sinusoidal and random vibration displays linear characteristics in the range where equilibrium or physical self-control is maintainable; transmissibility ratios of force and motion are sufficiently stable to be used for determining human transfer functions in the 1-60 Hz frequency spectrum; and transfer function statements of motion and force accurately describe human response to sinusoidal and random vibrations. Transfer functions allow analytical solutions for the dynamic response of the human body, but the transfer functions do not give any apparent way to determine the severity of the vibration. A parameter that will relate the vibration to the subjective response is required. Extensive testing has shown Figure 8-18 Force to Input and Output Acceleration Ratios Used in Calculation of Force Transfer Functions (After Pradko et al (144)) a. (right) Diagram of Force Accelerations and Deformation, D b. (bottom left) Force Input Acceleration Characteristics c. (bottom right) Force Ouptut Character- istics for Head 240 200 16C _ , l 12C _ 8o 4C 0 0 ,.... 10 2O 30 40 _0 FREQUENCY (Hz) q 200 160 12C ¢j 4G 0 0 AHEAD A,N \ \ J / IO 20 30 40 FREQUENCY (Hz) _o 6o 8-27 that the rate of flow of energy becomes the parameter that characterizes the interaction of the vibrating human and the environment. The energy flow takes place as a result of the complex damped elastic properties of the anatom _ This energy flow has been designated as average "absorbed power" {105, 143). These are two distinct concepts employed to obtain the analytical solution for a vibration environment: the transfer function and the absorbed power. The utility of the transfer function is manifested in the assessment of human response for skewed or oblique orientations (141). On the other hand, the absorbed power concept is a self-contained procedure that correlates with the nonlinear subjective response to vibration intensity { 143). The transfer function describes the mechanical response characteristics analytically as effective mass and inertial. These functions provide the means of conducting vibration analysis mathematically without using human test subjects in laboratory experimentation. The transfer functions were developec for a normal seated position without armrests or backrests. Consequently, when attempting to assess these or other specific features of seating arrange- ments, one must resort to direct measurement of absorbed power. It was noted that under linear vibration conditions when seat belts were used, ab- sorbed power was not appreciably affected. When man is seated, the vibration input to the feet becomes significant when the frequency is above 10 Hz. This is especially true when the man is seated on a cushioning device. In this case the input to the feet should be included in the analysis. Absorbed power has a physical significance and interpretation. Conse- quently, it is possible to measure the variation of this parameter for different people and different seating arrangements. A muscular person generally has, for the same body weight, a lower absorbed power for the same vibration than a more obese person. On the other hand, if a rigid mass were used in place of a human test subject, the rate of flow of energy would be zero because of its rigid, inelastic properties. A contoured seat generally has a larger contact area than one that is not contoured. This larger area will reduce body movement, owing to its elastic properties, and thus generally produce a lower absorbed power. Under very severe conditions where the man moves relative to the seat, the assumption of linear equations is no longer valid; consequently, transfer functions and absorbed power cannot be used under these conditions. Under normal vibration conditions, using absorbed power as a criterion, one can modify the input or a seating arrangement and measure directly the effect upon comfort. This versatiliby of absorbed power is not realized in other known methods of determining human discomfort due to vibration. If one is using sinusoidal boundary curves to assess the effects of various seating arrangements and positions, for the same vibration input, the curves are not helpful. Sinusoidal boundary curves are a function of the vibration input and are not sensitive to position and seating arrange_ent. Because of the importance of this approach, the following discussion is taken directly from Reference ( 10_. In the time domain, absorbed power can be written for an infinite time or a finite averaging time. For an infinite averaging time it is 8-28 I/0TAverage absorbed power = lira T F(t)V(t) dtT_ (6) where F(t)= Input force V (t) = Input velocity For a finite averaging time, it can be written in the form of a differential equation as follows: dZPav(t) dt2W 2 n where Pay(t) = 5 = W = n F (t) = V (t) = dPav(t)/26 \ + dt _)+ Pav(t) = KF(t)V(t) Finite average absorbed power Damping factor Lowest irequency to be averaged, rad/sec Input force Input velocity K = Conversion constant (7) Absorbed power can also be described in the frequency domain. It is computed as the product of the mean squared acceleration A 2 rms and the z parameter K.z, at each frequency "i". K.I is a function of frequency, but does not vary at any one frequency { 143). N P = E K-A-2 rms {8) av i ii=0 Since absorbed power is a scalar quantity, it possesses the desirable feature of being directly additive. For multidegree of freedom systems, the individual absorbed power values are readily summed for a single quantitative and qualitative measure of human vibration. From the previous equations it becomes apparent that absorbed power can be determined in both the time domain and the frequency domain. When absorbed power is determined in the time domain, it can be determined by direct measurement of force and acceleration or by obtaining force from the transfer functions. Absorbed power can be determined from the output of a triaxial accelerometer, utilizing the transfer functions to generate the force. (See Reference 105). Filters can be included only if one is interested in determining the contribution of specific frequencies to the absorbed power. For many purposes it is convenient to describe vibration in terms of its frequency domain characteristics. For deterministic vibrations this is usually done by stating the time-domain equations; for example, Fourier 8-29 series, single sinusoids, or line-spectra graphs. Random vibration charac- teristics, however, are commonly described in the time domain by correlation functions and in the frequency domain by power spectral density. Absorbed power can be calculated for each of these conditions when the iKi in Equation 8 are known at each frequency, or are known as a function of frequency. Transfer functions that relate force to acceleration can be manipu- lated to give phase angle, force, and power by the following three equations:(143). sin_ = (F2F 3 - glF4) w F4 + F3 F 1 + w F _F.__2 w2F2 4 1 2 Force = K 0 - + w F4 _3 A rrns (I0) t N (F 1 f 4 f 2 f 3) Power = i =0_" Klk 0 (F: + w F42 2)A2rmsi (11) The F quantities that appear in Equations 9-iI are the F that are given with the constant power graphs, Figures 8-38a to d. In the equations, ¢ is the angle between acceleration and force, force is in pounds when Airms is in feet per square second, and power is in watts when K I is equal to 1.356. The F and numerical constants are derived from experimental data. For vertical vibration they are (+G z) F 1 = -0.1024,5296 × 10 -9w 6 + 0.17583343 x i0"5w 4 -0.44600722 x i0 "2 w 2 + 1 F = 0.12881887 x lO-qw 4 - 0.93394367 × I0"4w 2 2 + 0.10543059 F = -0.45416156 x 10 -9w 6 + 0,37667129 x 10-5w 4 3 -0.56104406 x 10 -2w 2 + 1 F = -0.21179193 x 10"11w6 + 0.5172811 x 10 "7w4 4 -0.17946748 x 10 -3 w 2 + 0.10543059 8 -30 and W = 2_f, K0= 4.35373, K 1 = 1.356 The F and numerical constants for the feet are and F 2 = -0.18706955 x 10-4w 2 + 0.074n_6789 F = 0.33913154 × 10"6w 4 - 0.23697592 x 10"2w 2 3 F = 0.17013499 x 10-8w 4 - 0.39439090 x 10"4w 2 4 + 0.074036789 W = 2_lf, K 0 = 1.182, K 1 = 1.356 +1 The above analysis assumes that the vibration input is the same for both feet. Where this is not the case, K 0 must be divided by 2 and the transfer function used for each individual foot. The F and numerical constants for the fore and aft motion (_:Gx) are: _i = i F = 0.2J9106 2 2 F 3 = -0.0185309w ÷ 1 F 4 = -0.000618934w 2 + 0.219106 are: and W = 2_f, K0=4.3532 K 1 = 1,356 The F and numerical constants derived for the side to side motion (+Gy) and F1 0.24052124 x 10"3w 4 0.066974483w 2= - +1 F2 = 0.57384538 x 10"Sw 4 - 0.50170413 x 10-2w 2 + 0.33092592 F =-0.14979958 × 10"5w 6 + 0.0010088882w 4 3 - 0.10108617w 2 + i f 4 =-0.17137490 × lO'2w 6 ÷ 0.53137351 × i0"4w 4 - 0.011096507w 2 + 0.33092592 W = 2 _ f, K 0= 4.353, K1 = 1.356 8-31 Inserting these F into Equations 9-ii, one can solve for phase angle, force, power, or acceleration for any frequency, sum of frequencies, or power spectral density (PSD). This calculation is best performed on a digital computer. However, tables of constants are available for hand calculation of several different problems relating to comforl threshold. These are seen in Figure 8-38. Examples of these calculations are available (105, 141). Tissue Impedance (73) The mechanical properties of body tissue are summarized in Table 7-74 of Acceleration, (No. 7). Whereas bone behawes more or less like a normal solid, soft elastic tissues, such as muscle, tendon, and connective tissue, resemble elastomers having similar Young's moduli, S-shaped stress-strain relations and high stretchabilities. Their properties have been studied in connection with the quasistatic pressure-volume relations of hollow organs such as arteries, the heart, the urinary bladder, etc. (160), but linear proper ties have always been assumed when dynamic responses were studied. Soft tissue can then be described phenomenologically as a visco-elastic medium and plastic deformation has to be considered only if injury occurs. Impedance measurements of small areas (I to 17 cm Z) over soft human tissue have been made with vibrating pistons between i0 Hz and 20 kHz. This impedance starts out at low frequencies as a large elastic reactance. With increasing frequency the reactance decreases, becomes zero at a resonance frequency, and becomes a mass reactance with still further increas_ in frequency. (See Figure 8-19a.) ( 64, 71 ). These data cannot be explained by simple lumped-parameter models, but require a theory of wave propagation in a visco-elastic medium, such as the tissue constitutes for this frequency range (71). The high viscosity of the medium makes possible the use of simplified theoretical assumptions such as a homogeneous isotropic infinite medium and a vibrating sphere instead of a circular piston. The results of such a theory agree well with the measured characteristics. As a consequence it has been possible to assign absolute values to the shear viscosity and the shear elasticity of soft tissue. (See Table 7-74 i_ Acceleration, No. 7.) The theory, together with the measurements, shows that over the audio- frequency range most of the vibratory energy is propagated through the tissue in the form of transverse shear waves and not in the form of longitudinal compression waves. Such shear waves have a much smaller propagation velocity (and therefore wave length) than sound and a strong dispersion. The velocity is about Z0 m/sec, at Z00 Hz and increases approximately with the square root of the frequency. This may be compared with the constant sound velocity of about 1500 m/sec, for compressional waves. Some energy is propagated along the body surface in the form of surface waves which have been observed optically. Their velocity is of tee same order as the velocity of shear waves. From the mechanical impedance of the body surface one can calculate the acoustic absorption coefficients. This indicates what percentage of an incident air-borne sound wave is absorbed at the body surface and propagated through the tissue and what percentage is reflected (64). At i00 Hz a small area of the forehead or of soft tissue absorbs only about 2 percent of the incident 8-32 Figure 8-19 Response of Body Tissues to High Frequency Vibrations / _D Z_ ,' . Z ,o° !" /rrl i S,o.! | I • I • R*l,slo.c_, _ai_td FREOUENCY IN Hz a. Resistance and Reactance of Circular Area 2 cm. Diameter, of Soft Tissue Body Surface Smooth curves calculated for 2 cm. sphere vibrating in a) visco-elastic medium with prop- erties similar to soft tissue, b) frictionless com- pressible fluid, c) incompressible fluid. (After Goldman and yon Gierke (73), adapted from von Gierke et al (71)) tOO _" PREDOMINANT FAC TO_tS __ AFFECTING ATTENUATION, COLLAGEN FIBERS AND IO INTERCELLULAR SUBSTANC! / ¢ FLUID AND rTS PROTEIN 0 001 t 1 I I 11 I _ I I I I 2 3 4 6 .8 I 2 *4 4 6 8 I0 FREQUENCY f IN MHz b. Approximate Values of High Frequency Sound Attenuation in Various Tissues (After Goldman and yon Gierke (73), adapted from Goldman and Hueter (74), and Dussik et al (47)) sound energy. At higher frequencies a still smaller percentage is absorbed. Only the specialized structure of the ear allows a small area of the body surface, the tympanic membrane, to absorb much more energy; for example, at 1000 Hz, 50 to 80 percent. This is achieved by the middle ear transformer action which matches the tissue structures of the inner ear to the characteris- tic impedance of air. (See Sound and Noise, No. 9, for aural, local, and diffuse body effects. ) Ultrasonic Vibrations (73) Above several hundred kHz in the ultrasound range, most of the vibratory energy is propagated through tissue in the form of compressional waves and geometrical acoustics offers a good approximation for the description of their path (20, 156 ). Since the tissue dimensions under consideration are almost always large compared with the wavelength (about 1.5 mm at l mHz) the mechanical impedance of the tissue is equal to the characteristic impedance, i.e., sound velocity times density. This value for soft tissue differs only slightly from the characteristic impedance of water (74). The most impor- tant factor in this frequency range is the tissue viscosity, which brings about an increasing energy absorption with increasing frequency. At very high frequencies this viscosity also generates shear waves at the boundaries of the 8-33 medium, at the boundary of the acoustic beams, and in the areas of wave transition to media with somewhat different constants, (e.g., boundary muscle to fat tissue, or soft tissue to bone). These shear waves are attenua- ted so rapidly that they are of no importance for energy transport but are noticeable as increased local absorption, i.e., heating. From 500 kHz to 10 mHz the attenuation coefficient describing the decrease of the sound intensity in a plane ultrasound wave, is only in fair agreement with the value one would calculate from the tissue viscosity measured in the audio-frequency range. The tissue deviates in this frequency range from the behavior of a medium with constant viscosity. In Figure 8-19b: attenuation coefficients measured in different types of tissue are summarized c_ ( 47, 74 ). On this graph a curve, _-= constant, would be indicative of classical viscous absorption with constant shear viscosity. A smaller slope, or a change in slope, indicates a change in viscosity with frequency (relaxation phenomenon). The graph gives only a few examples and typical functions from a large body of attenuation data available. The absorption of most soft tissues is in the range from 0.5 to ZdB/cm/m Hz. The order of increasing absorption is: brain tissue, liver tissue, striated muscle, smooth muscle, kidney, skin and tendon. Bone has the highest value with approximately 10 dB/cm. PATHO-PHYSIOLOGICAL RESPONSE TO VIBRATION The effects of vibration on the physiological functioning of the body may be considered in two categories: a) those changes which are directly attribut- able to the differential vibratory movement or deformation of particular body structures giving permanent or transient damage, and b) generalized response_ to vibration as a non-specific stress. These are more dependent on magnitude and duration and less on frequency than the primary effects. Permanent Pathological Changes Permanent pathological changes may follow very severe vibration. Animals can be killed by vibration (73). There is a poorly defined dependence on frequency of the lethal accelerations above [0G which coincides with the resonant displacement of the visceral organs (I16). Post-mortem examination of these animals usually shows lung damage, often heart damage, and occasionally brain injury. The injuries to heart and lungs probably result from the beating of these organs against each other and against the rib cage. The brain injury, which is a superficial hemorrhage, is not yet interpretable in definite terms; it may be due to relative motion of the brain within the skull, to mechanical action involving the blood vessels or sinuses directly or to secondary mechanical effects transmitted through the cerebrospinal fluid (15). Tearing of intra-abdominal membranes is rarely seen. Exposure for several minutes to peak acceleration of about 5 g often produces heart damage as indicated by delayed changes in the electrocardiogram. An increase in body temperature is found on exposure to vibration. Since this occurs also in dead animals it is probably mechanical in origin. Calculations 8-34 of heat absorption based on body impedance data suggest that appreciable heat can be generated at large amplitudes. Exposure of monkeys to 5g at 10 and 20 Hz for several hours seems to produce some damage to the vestibular system but these findings require confirmation {148). Rarely has permanent damage been produced in humans by total body vibration. Observations on man have been made in a few instances and indicate that above about 3g, sharp pain in the chest may occur {184, 193 ). Traces of blood have occasionally been found in the feces after exposure of 6g at 20 to 25 Hz for about 15 minutes suggesting mechanical damage to the intestine or rectum. {See Figure 8-31 - uppermost curve. ) At very severe levels of vibration, chromosomal changes have been pro- duced in microbes and insects along with mutations { 3, 100 _. The signifi- cance of these changes to man is not clear. Chronic injuries may be produced by vibration exposure of long duration at levels which produce no apparent acute effects (73 _. In practice, such effects are usually found after exposure to repeated blows or to random jolts rather than to sinusoidal motion. When such shocks or blows are applied to the human body at relatively short intervals, the relation of the interval to tissue response times becomes very important. Exposure to such forces frequently occurs in connection with the riding of vehicles. Buffeting in aircraft or in high-speed small craft on the water, and shaking in heavy vehicles on rough surfaces, give rise to irregular jolting motion. Acute injuries from exposure to these situations are rare but complaints of dis- comfort and chronic minor injury are common. Truck and tractor drivers often have sacroiliac strain. Minor kidney injuries are occasionally suspected and, rarely, traces of blood may appear in the urine. The length of exposure and the details of the ways in which the body is supported play an important role. Chronic injuries are also produced by localized vibration, such as the pain and numbing of the fingers on exposure to cold which affects many people after several months of using such equipment as pneumatic hammers and drills or hand-held grinders or polishers ( 58, 71 ). The heavier, slow- moving devices appear to produce more severe jolting. Little is known of the mechanism of the injury or of the actual forces responsible, although many high frequency components may be present ( 2 ). The repeated insults to the tissues directly or through vasoconstrictor responses seem to gradually affect the arterioles, capillaries, and their nerve supply giving a Renaud's syndrome (58). Repeated vasoconstriction in response to high intensity sound may also increase the incidence of this syndrome {97 ). Injuries resembling these have been produced in the feet of rats exposed to 60 Hz at 8 to 9 g for 10 to 12 hours per day up to about 1000 hours (84). This vascular condition has been treated with ultrasound { 1 56, 191 ). Physiological 'Response The primary physiological responses to vibration include: 8-35 • Subjective sensation of pain and discomfort. • Neurological functionr-including the perception of vibration and regulation of posture. • Cardiovascular changes in blood pressure, electro- cardiographic outputs, and blood cell counts. • Respiratory changes in tidal volumes, rates, and valsalva response. • Metabolic imbalances. • Endocrine malfunction. Subjective Response The nature of the subjective response varies with the axis of vibration. Longitudinal or Vertical Vibration (+Gz) The subjective and symptomatic responses to vertical sinusoidal vibration are reviewed in Figures 8-20 and 8-31 (70, 94, 96, I07, 123, 168, 178, 193). The subjective responses of the subjects range from the perception of feeling through discomfort, apprehension to pain and are the primary sensa- tions correlated with the physical response of the body. Most tolerance criteri_ are based on these subjective responses. Testing to the point of serious physical or physiological damage to the body is, of course, unacceptable for human subjects. It may be assumed that, in most cases, these levels are well below those that would cause damage to bodily tissue. Table 8-20 indicates the gross variability in previous attempts to set thresholds for tolerance to vibration. Table 8-21 is a descriptive summary of the symptoms during vertical vibration plotted in Figure 8-22. Generally there are three apparently simple criteria: the threshold of perception, of unpleasantness, and of tolerance. The latter two are difficult to identify and reproduce, although agreement to within a factor of about 3 has been obtained. In studies lasting up to 20 minutes, no single subjective end point is found for a whole population, although some reactions are more common than others (I15, 126, 193). There is a general tendency for those body organs with high mobility to give symptoms at low frequency and those with little mobility to be affected at higher frequencies. Very long exposure to vibration much above the level of perception seems to be irritating and fatiguing. Subjective responses to vertical sinusoidal vibration while standing have been studied ( 19, 41 ). In general, standing subjects tolerate this form of vibration more readily than when sitting, probably because of the damping factor of leg flexion (90). Except for body sway and balance difficulty, physiological effects are similar. Figure 8-23a compares the subjective symptom threshold for standing and sitting subjects with acceleration recorded on the shake table. Figure 8-23b gives the body areas affected. At the higher 8-36 SIGNS * - Increue - - Decreue None - No Change FREQUENCY (Hz) A4. C- AY- This chart summarizes the reported reactions to a wide range of vibrations, plus some objective signs of physiological change, as reported in the literature up to 1960. Figure 8-20 Subjective Response to Vibration (After Webb (178), adapted from Linder (107)) 8-37 Regional Table 8-21 Symptomatology During Vertical Vibration (After Magid et al (117)) HEAD-NECK Head Sensations Pharynx Jaw Speech THORAX Respiration Dyspnea Valsalva Pain ABDOMEN Voluntary Abdominal Musculature Con- traction Pain Vibration or "tight" sensation of facial skin Pharyngeal tug or "lump in throat" Sensation of vibration Lower frequencies secondarily affected due to reactions of thorax and abdomen, high frequencies due to superimposed transmitted vibrations to laryngeal tissues and possibly main stem bronchi Decreased ability to perform physiological respiratory movements of the thoracic cage due to superimposed forces from oscillating platform Actual air hunger Partial or complete closure of glottis resulting in increased intrathoracic and intra-abdominal pressure Dull-to-severe pain of the precordium occas-ionally radiating to the sternum, no other radiations, pain subsiding immedi_ttely after cessation of vibration Degree of contraction or "bearing down" Usually periumbilical with tendency te radiate to right lower quadrant Skeletal Musculature Voluntary Muscle Contraction of Extremities Lumbosacral Pain SKELETAL MUSCULATURE Sensation of "muscle tightness" or possibly increased muscle tone primarily of the lower extremities, dorsum, and neck Muscular contraction in an effort to counteract movements of oscillating platform Dull-to-severe pain at midline with bilateral radiation, subsiding immedi- ately after cessation of vibration PELVIC- PERINEAL COMPLEX Micturate (Urge) Mechanical stimulation of bladder neck and proximal portion of urethra Defecate (Urge) Mechanical stimulation of distal portion of sigmoid colon and rectum GENERAL DISCOMFORT Overall estimation of each frequency ridden 8-38 100 8O 60 4C 20 0 1 2 3 4 5 6 8 9 10 FREQUENCY- Hz z o uJ n_ 180 _ uJ z ._l .,,( t- o i- This chart summarizes the subjective responses of ten experienced subjects exposed to vertical sinusoidal vibrations ranging from 35 inches double amplitude at 1 Hz to about 1/4" double amplitude at 6-10 Hz, which produced accelerations of 1.2 + 0.6 G. They were seated on a hard surfaced aircraft seat and were restrained by a lap belt and shoulder harness. The total number of subjective responses for each frequency was summed; they reached a peak at 6 and 7 Hz, as shown by the upper dashed curve and the right hand vertical scale. The vertical bars show the approximate magnitude of the following groups of symptoms: Chest (cross-hatched bars) - Respiratory difficulty, pain, and breath holding (Valsalva maneuver). _;keletomuscular (stippled bars) - Muscle pain, back pain, general discomfort. Abdomen (open bars) - Contraction, pain. The words ahove each set of bars state what symptoms set the limit for voluntary tolerance for that particular frequency. Figure 8-22 Subjective Responses to Vertical Vibration While Seated (After Webb(178), adapted from Magid et al (115, 117)) 8-39 Figure 8-23 Subjective Reactions and Body Areas Affected by Vibration in Seated and Standing Subjects (After Chaney (19)) EXTRE ME LY 2. ANNOYING a. ____ PEHCEI)TIt_LE T ' I ' I 4 6 8 10 12 14 16 18 20 27 FREQUENCY (Hz) Subjective Reaction Curves - Seated and Standing Subjects 30- O rt" "_ 20- .Q E z ,---- ! __ , LIMBS I _ _ 5 10 15 20 25 Frequency (Hz) b. Relationship Between Body Areas Affected by Vibration When Standing ! 3O 8 -40 frequencies, a higher acceleration is generally accepted on the second encoun- ter with a given frequency. Semi-Supine and Other Vibrational Axes There are few data available on symptoms during semi-supine vibration In transverse vibration(in the fore-aft direction)of a seated subject, there is a shearing force applied to the buttocks which must be considered (80 ). Data are available on symptoms during sinusoidal vibration in the X, Y and Z axes in contoured and adjustable couches proposed for spacecraft use. A detailed review will be presented of this study with the acceleration of gravity vectored through the X-axis. The contoured couch was similar to the project Mercury couch. Restraint was provided by two-inch straps; no helmet was used; the head was unrestrained. The adjustable couch had leg and foot supports, torso and lower extremity support straps, and provided closer coupling than the contoured couch. During exposures in the contoured couch each subject was told to maintain his position in the couch with his head in the headrest as long as possible by whatever bracing or straining maneuvers that were useful. Howeyer, if head buffeting against the headrest contour did become intolerable before other symptoms became tolerance limiting, he was to lift his head from the headrest and continue until forced to stop by other symptoms. In the contoured couch the head was restrained. Pertinent angles and g-load nomenclature are noted in Figure 8-24a. At a preset frequency, the double amplitude of sinusoidal vibration was increased at a rate of 0.75 mm per second until the subject signaled that he had reached his limit of voluntary tolerance. The acceleration level thus reached was considered a subjective tolerance value for that frequency. Curves describing the averages of these acceleration and velocity-time tolerance levels at each of the frequencies studied in the contoured couch are presented in Figure 8-24b and d. This figure reveals that the Y and Z axis curves are similar throughout their extent, rising from the 2 G level at the low frequencies to the 12 to 14G level at Z0 Hz. The Xaxis curve is slightly higher than the other two below 8 Hz but rises more slowly as frequency increases above this point, reaching a maximum of only about 50% of the Y and Z axis values at 20 Hz. The X axis curve is slightly higher than the other two below 8 Hz but rises more slowly as frequency increases above this point, reaching a maximum of only about 50% of the Y and Z axis values at 20 Hz. Curves tracing the average values of accelerations and velocity seconds tolerated in the adjustable couch are shown in Figure 8-24c and e. Although representing fewer data points, they permit axis tolerance above 8 Hz with a slight improvement below this frequency. The X axis curve is considerably higher below 12 Hz but flattens out at the higher frequencies. Except for some drop-off above 15 Hz, Z axis tolerance is quite similar to that attained in the contoured couch. Table 8-2`5a summarizes the symptoms occurring during X axis exposures. With both couches, the principal focus of complaints was the thorax. Using the contoured couch, these complaints were present throughout the frequency 8 -41 Figure 8-24 Couch Parameters and Tolerance Levels During Vibration in the X, Y, and Z Axes (After Temple et al (168) and Weis et al (179)) / "--,--# GRAVtTY VIBRATION :i: ng 1 +IGI t a. General Orientation of the Body (Semisupine) and the Principal Body Angles in Both Couches 114 #"I 3 //I l 11 Y,AXIS 1"/'///// "¢,,iO-zi /,-"/_// Z AXIS i! /'J / ./ j ..,"_ /./i" X AXIS // _- t" 2- I ; ; ,; "; ; ; ,'o ,', ,'= ,% 1'4 ,'s ,% ;7 i'l *'lz'bz', FREQUENCY (Hz) b. Average Levels of Acceleration Tolerance in the Contoured Couch 15" 14- I$" 12- II- _l. lO- v 7- u 6, Z AXIS "'-. ,-'-._."-_'.-"'-- xAxls 2- = _ ; ; ; _ -' _ ,'o L', t'= ,% ;4 ,'s ,'* ,'7 ,'l ,'ti'od, FREQUENCY (Hz) c. Average Levels of Acceleration Tolerance in the Adjustable Couch .6 A _.s (3.4 > .I :._ -- . _...-._.:_. __--.._--___t___-.-=__-:----_ .--- °o ....... ' ' 'o ' ' ' '4 ' ' ' ' ' z'o2 4 6 8 l 12 I 16 18 Frequency - Hz TEMPLE (CONTOURED COUCH) X AXIS Y AXIS ....... Z AXIS d. Velocity-Time Plots of Tolerance in Contoured Couch +6 _.51 =o C?.4 L9 O > TEMPLE (ADJUSTABLE COUCH) ......... X AXIS Y AXIS ....... Z AXIS i t l I f I I I I i i i ! i i i f i I i 4 s e _o ,z ,4 ,s _e _o Frequency- Hz e. Velocity- Time Plots of Tolerance in Adjustable Couch 8-42 Figure 8-25 Symptoms During Vibration in the X, Y, and Z Axes (After Temple et al (168)) • PR_RV **¢IUN O S[CO_ARY C0_ILAJNT L_ __ ADJUSTABLE COUCH SYMPTOMS _-_c_ __ _o_( _:Y ! tO = 5 ZO 6(NEnJ'. mscoewoRt SUOST[RNAL ._ SuOCOSTAL ---- PRECOAOtAL 1 T'OP_C 8O _?N _(SP_ M c R suBx_o PE_U_e_L ICAL s_ue,c .... S_A_N6 TO0 _M - - rAhGU_ .... _kouc, __'-_.F__SACRAL "BANGINC" Note: Frequency in Hz a, Occurrence of Symptoms in the X Axis oe _'R_MJR_ KAClC_SECO_RY CO_La_N? SYMPTOMS RESPIRATORY _NSPIRATION [_PtRAT _ON GENERAL DISCOMf O(q T THORACIC SUBSTE RNAL SUBCOSTAL pR'E CO_plJ L ?MORACOABD _T_ RESPIR _c=_- ABDOMINAL suow_,Horo E_CASTRIC PE _U_L _CAL SU_RAPUB_C 7EST_CUL JR MISCEL L ANE O_US IE AD SOI@E NEC_ CONFuSiON 1 OISOI_[NIl? _ S_A_INC too _uc_ "B'N_;,',C" _WSC_LLANEOUS SACRAL "BANGINC" MISCELLANEOUS CONTOURED COUCH FREQUENCY Io I :i !! °t ;°°!° "itf! i !i Note: 'Frequency in Hz ADJUSTABLE COUCH b. Occurrence of Symptoms in the Y Axis SYMPTOMS CONTOURE0 COUCH FREQUENC_ " t / • 1 ! Note: Frequency in Hz c. Occurrence of Symptoms in the Z Axis 8 -_3 range studied but with the adjustable couch they occurred at, and below, Ig Hz. In both systems the majority of the complaints were of pain or a poorly defined "pressure" referred to the substernal area. Considerable difficulty with respiration, particularly inspiration, was encountered in both couches. In the contoured couch this difficulty was almost invariably associated with abdominal or other thoracic symptoms. Complaints referable to the abdomen were relatively more frequent in the adjustable couch, occurring at and below IZ Hz, but with no predominant localization. These complaints were in the nature of a poorly defined "dull discomfort." There were complaints of headache in the contoured couch, occurring across the frequency range, but head complaints were relatively more common in the adjustable couch, almost entirely at the frequencies above 14 Hz. These included headaches and poorly characterized, transitory feelings of "confusion" or "disorientation" which cleared almost immediately upon cessation of the exposure. Sharp, repetitive impact of various body segments, occurring alternately against couch and restraint with each vibratory cycle, caused considerable difficulty in the adjustable couch at frequencies up to 15 Hz. This "banging, ''as it was described by the subjects, was a particular problem in the sacral area where, apparently, point-loading was occurring. "Banging" was not encountered in the contoured couch in this axis. Symptoms occurring during Y axis exposures are summarized in Table 8-25b. Again, with the contoured couch, the thorax was a major source of difficulty. In this instance, however, the complaints were primarily of a "pressure" referred subcostally. As in the X axis respiratory difficulties were present, albeit not quite as frequently, and were associated again with abdominal or other thoracic symptoms. Thoracic and respiratory symptoms were relatively less frequent with exposures in this axis in the adjustable couch and, with both couches, the symptoms were largely confined to the frequency range below I0 Hz. Abdominal complaints were more frequent in both couches here than in the X axis. They were present across the frequency range in the contoured couch but were largely limited to the region below 12 Hz in the adjustable couch. The symptoms were, again, poorly described "pressures" or discomforts. In the contoured couch, below 9 Hz, they were largely referred to the epigastrium and, above this level, to the suprapublic area while in the adjustable couch they were principally referred to the epigastrium and periumbilical area. In the frequency range 3 to 6 Hz subjects in the contoured couch frequently complained of significant general fatigue and neck muscle soreness and "spasm." There were some complaints referred to the head, principally headache, in the adjustable couch at and above I0 Hz. Another problem of significance in the contour_Ld couch in this axis was that of frictional rubbing against the couch or restraint straps, giving rise to erythema and abrasions (occasionally blisters) of the hips, thighs, and calves and other areas underlying the restraint straps. This was occasionally associated with reports of "banging" such as were previously encountered in connection with the adjustable couch in the X axis. Both problems became more severe as the frequency increased. Symptoms occurring during Z axis exposures are seen in Table 8-25c. There were more complaints referable to the head in this axis in both couches than had been previously noted. In the contoured couch these consisted primarily of headache and neck-muscle soreness in the 5 to 10 Hz range; and 8 -44 _nstances of "confusion" or "disorientation," such as occurred in the X axis, _cattered across the frequency range. In the adjustable couch the head symptoms occurred almost completely below 12 Hz and included headache, 'confusion" or "disorientation, " some head "banging" and general unpleasant- ness. There was a major difficulty with respiration, particularly inspiration, in the adjustable couch in this axis. This was quite often unassociated with • ny other complaints, in contrast with the other two axes, and occurred _cross the frequency range. Complaints of this nature were much less fre- quent in the contoured couch. Thoracic complaints were similar with both couches but were noted up to a slightly higher frequency in the adjustable couch. Abdominal symptoms in both couches followed patterns very similar to those present for the respective couches in the Y axis. In the contoured couch, below 9 Hz, the complaints were again referred to the epigastrium while above 6 Hz testicular symptoms, described as "fullness," "tapping," or "squeezing," replaced the suprapublic symptoms noted in the Y axis. These testicular symptoms occurred very infrequently in the adjustable couch. A number of subjects in the contoured couch stopped simply because they had just "had enough." This complaint was not frequency-related. Almost all symptoms, such as thoracic "pressure," abdominal discomfort, or headache, of which subjects complained following exposures in any of the axes were gone within 5 to 10 minutes. The abrasions and occasional blisters were medically treated and healed without complication. Infrequently, a headache or sore neck persisted beyond the immediate post-exposure period for 12 to 24 hours or, more rarely, for longer periods but no serious problems arose. Further analysis of the contoured couch data directs attention to two factors that influenced the results. The first relates to the complaints of intolerable frictional rubbing (and, less frequently, "banging") of body segments against the couch or restraints which occurred, especially at the higher frequencies, in the Y and, to a lesser extent, the Z axis. Two conditions appear to have been responsible for this problem. One was the variable, occasionally loose, body-to-couch coupling which often resulted in dead space between subject and couch, or in unavoidable slackness in the restraint. The other, arising from the first, was the development during vibration of phase shifts between body segments and the couch due to the mechanical response characteristics of the system. The second factor pertains to the lack of head restraint. One objective of the contoured couch exposures had been to determine the desirability of raising the head, when its buffeting against the couch became unbearable, in an attempt to prolong the duration of the exposure and increase the level of acceleration reached. It appears, however, that the benefits accruing to this procedure were negated by the resulting fatigue and the precipitation of, or accentuation of, head, neck, and thoracoabdominal symptoms. In composite vibration, the direction and magnitude of the resultant acceleration are important in determining its subjective acceptability (76). If a subject is exposed to simultaneous vertical and horizontal (fore-and-aft) vibrations of equal frequency and amplitude, a circular motion of the seat may be produced by suitable phasing of the two vibrations. It has been 8 -45 demonstrated that, if the direction of circular motion of the seat is backward- running (i.e., anti-clockwise as viewed from the right), the subject's physical and subjective responses are essentially those characteristic of horizontal excitation; but, if the circular motion is forward-running, the response resembles that to vertical vibration (80). Threshold of Vibration Sensation Whole-body accelerations of sufficient intensity can be appreciated as oscillatory at frequencies down to 0.1 Hz, a frequency corresponding with the slowest, noticeable, heaving-movements of a large ship, and possibly lower. The threshold of sensation for linear oscillation in the long axis of the body, as determined by parallel swings, is about 0.005 g at frequencies of l Hz and below (177). At such frequencies, the sensation is mediated by the vestibular organs and by somatic receptors in the areas of application of the vibration to the body. The sensation of motion may, of course, be augmented by seeing surrounding objects in relative oscillation. At higher forcing fre- quencies, where body resonances and phase shifts in the transmission of vibration occur, the sensation of vibration is presumed to be mediated by mechano-receptors throughout the body. The threshold of perception of whole-body vertical vibration in the range I to 20 Hz lies between 0.01 and 0.002 g, with the lowest threshold (maximum sensitivity to vibrational acceleration) at 3 to 6 Hz (72). When standing, the sensitivity at low frequen- cies less than in the seated position is shown in Figure 8-23a. The threshold of sensitivity of the finger tips to sinusoidal vibration is recorded in Figure 8-26. The difference limens (DE) for rate of intermittent stimuli at the finger tip in comparison with those of the ear are shown in Figure 8-27. The mechanism which allows these skin receptors to distinguish between rates in the vicinity of 300 Hz with less than a 10% error are under study (124). No gross change in sensitivity to vibration appears to be brought on by previous vibration exposure ( 80 ). Even though the pattern of the vibration be quite complex, habituation to vibration occurs, as it does to noise, if the quality of the stimulus is constant. For example, one can quickly cease to notice considerable intensities of noise and vibration on board ship. Low frequency (l to g Hz) steady-state oscillation of moderate intensity can be soporific in man, as exemplified by the practice of baby-rocking and the relaxation afforded by the rocking chair. Random vibration, on the other hand, tends to increase arousal, as does the sudden onset, alteration, or cessation of a particular condition of vibration. Irregu- larity and intermittency of stimulation oppose habituation. Habituation to sustained vibration is probably a central phenomenon, although some adapta- tion may occur at the receptor level. Sinusoidal oscillations at frequencies between 0.1 and l Hz, or random vibration with a spectral representation in that region, can induce motion sickness. (See Acceleration, No. 7). The nature of the synchronous electrical activity which can be recorded from the brain during vibration remains an open question ( l, 80 ). There is no published evidence that the human E.E.G. can be driven by low frequency vibration in a manner analogous to photic driving. The likelihood of motion 8-46 n. I FREQUENCY- Hz Figure 8-26 observed Relation Between Threshold Amplitude of Vibration and the Frequency of Sine waves for the Finger Tip (After Mowbray and Gebhard (124)) Figure 8-27 Different Limens for Finger Tip Vibration Sensitivity and Acoustic Sensitivity Upper curve: Relation Between Af and log f for the tactile data. Lower curve: The same relation for the auditory flutter data. (After Mowbray and Gebhard (124)) I <3 a n_ Q Z o (1: Z 0 I-- u.l L_ > .ot t 4 6 8 tO tO 40 $0 tO0 tOO 400 000 STANDARD FREQUENCY (F)- Hz tooo 8 -47 artifacts and the possibility of E.E.G. changes produced by indirect physio- logical mechanisms, such as hyperventilation, should be borne in mind if abnormal E.E.G. recordings are seen during vibration. The Cardiovascular System and Blood In man, measurements of cardiovascular activity during whole-body vibration have generally been restricted to the pulse rate, the blood pressure and the electrocardiogram. The changes in pulse rate, blood pressure, and cardiac output reported in the literature have been varied and non-specific, resembling the responses associated with alarm or moderate exercise (23 , I12, 149, 152). The rise in systolic radial arterial pressure during vibration at frequencies above 3 Hz was unaccompanied by a fall in the diastolic pressure (153). Figure 8-28a and b indicate the oxygen consumption and cardiovascular response to +G x vibration. At low frequencies of vibration (2 to 3 Hz), beating has been observed between the pulse and vibration frequencies during arterial pressure record- ings in the dog ( 4 ). A fall in the skin temperature has been noted in the lower limbs during whole body vibration of human subjects. Other changes in peripheral vasomotor activity, mediated by autonomic efferent impulses, have been demonstrated by workers using the galvanic skin reflex as a measure of vibration stress ( 41 ). Changes in the formed elements of the blood appear to be related to general stress response ( 80, 152). (See also Reynaud's phenomenon in section on permanent pathology. ) Respiratory Physiology The increases in oxygen consumption and respiratory quotient have been attributed to a raised metabolic activity due to the muscular effort of main- taining the posture during vibration. This explanation is supported by the fact that the increases in these respiratory indices were found to be greater when the subjects were standing up than when they were seated during low frequency vibration of the same intensity. The differences produced by varying g load and direction of vibration are seen in comparison of graphs in Figures 8-g8a and 8-28c. Exposure to altitude above 10,000 feet lowers vibration resistance in animals (121). The relation of respiratory response to resonance is not clear in that a significant increase only at forcing accelerations exceeding 0.5g at 9.5 Hz was recorded in one experiment ( 50, 51 ), while a fairly linear increase in oxygen consumption with increasing frequency from 6 to 15 Hz, at a fixed displacement-amplitude of 0. 13Z in. at 0.46 to Z.88 g was recorded in another (60). Increase in oxygen consumption has also been found to be greatest at the lowest frequencies of vibration, at constant acceleration- amplitude, in the range 2 to 7 Hz (46). Some of these responses are seen in Figures 8-Z8d and 8-29a and b. The changes are more pronounced early in the exposure and appear to be compounded of a passive mechanical effect added to a physiological response to vibration stress. Hyperventilation with hypocapnia, on occasion, may be caused by stres; or discomfort; also in- creased muscular demand for oxygen may be augmented by superimposition 8 -48 Figure 8-28 Cardiovascular and Respiratory Response to +G Z Vibration c E E x j )oo a z g k- _oo z C= o c3 o _ U _ 0 z < 35 U I I ] I i / / / I I i 1 r T 1 FREQUENCY iN HZ ---- -- CONTROL -----0.6G --|.ZG *gZ_ \ ".... ! L - I k Io IZ b= I ! E J o 5, z 1 zk. z Y .#.._ /_--'--._. __-- g 6 n FREQUENCY IN HZ --- -- CONTROL O._G I.ZG 1 ---m J Average oxygen consumption (a) and cardiac index (b) of four human subjects vibrated in the (front to back) direction. The subjects were in the semi-supine position and vibrated for 6 minutes at two vibration intensities (0.6G and 1.2G). (After yon Gierke (65), from data of Clarke et al (23)) (The bars indicate the range of the data), 100; 8O OX_G} N CONSUMPTION ....: _:;:iiJJ.i -20 I o 4 h i!:¸ : I I I, 8 12 16 FREQUENCY- Hz d. 100 b_ 60_ % o 6i _ 2o, ':/ - $ Z I -20 0 RESPIRATORY VENTI[ ATION l ! / 4 I t _E_N ] I [ | , 12 16 FREQUENCY- Hz The change in oxygen consumption for four male subjects during vibration is shown in (c) as a percentage change from the control value, with the mean values being connected by a line and the range shown with shading. Similar data are shown in (d), where respiratory ventila- tion is also shown to increase as a function of frequency, when amplitude is held constant. The subjects were exposed to 20 minutes of Gz vibration at an amplitude of 0.132 inch while seated on a chair and entirely unrestrained. (After Webb (178), adapted from Gaueman et al (60)) 8 -49 Figure 8.29 Respiratory Ventilation During Longitudinal Vibration While Seated a. _ 24 G.. I-- ._= E\ 20 _ 16 .J I.,- Z UJ >. >- _: 12 0 I'-- ,< ¢v LIJ _ 8 , ill I __<.,0. 4 6 8 10 FREQUENCY - Hz LEVEL 12 *GAS AT BODY TEMPERATURE,AMBIENT PRESSURE, AND SATURATED Average respiratory ventilation rates are shown for 12 men who were vibrated while seated on a hard platform at the amplitudes, rates, and acceleration shown. (After Webb (178), adapted from Ernsting (50)) b. =°-" 13 _ _11 c, I..- // ..A.E E_ - 8 H z g 10 , 2Hz >- 8 Q¢ 6 I 5 10 15 0 5 10 15 20 5 I0 CONTROL I EXPERIMENT I RECOVERY TIME - minutes *GAS AT STANDARD TEMPERATURE (°C), PRESSURE (I =Tin) AND DRY Mean values of respiratory ventilation are shown for 5 seated men exposed to a 20-minute period of vibration at 0.125 inch amplitude, at frequencies from 2 to 15 Hz as marked on each curve. (After Webb (178), adapted from Hoover and Ashe (89)) 8-50 of oscillations at forcing frequency ( 89, 192). Labyrinthine factors may be at play during very low frequencies of 1.76 Hz (52). The hypocapnia, occasionally seen, may affect performance (23, 42 ). Metabolic Factors The increase in oxygen consumption and endocrine changes in animals have been attributed to several stress factors in chronic exposure to vibra- tion (80). Loss of body weight can also be found (89). In humans, 9 minutes of vibration at the predicted 3 minute tolerances {I18) (see curves in Figure 8-30) for selected frequencies ranging from 1-20 Hz produced statistically significant increase in 17-OHsteroids and 17-ketosteroids which were in the upper reaches of normal. No significant changes in serum PBI were noted { 108). Combinations of thermal and vibratory stress in mice tend to show synergistic effects (122). Hematocrit, hemoglobin and SGOT responses reflected this synergism. HUMAN TOLERANCE TO VIBRATION Tolerance criteria for vibration exposure established by various investi- gators agree only within certain limits since evaluation criteria, positioning and support of the subjects have a marked effect on such limits. Threshold data on longitudinal sinusoidal vibration while seated have been summarized in Figure 8-30. Figures 8-31a and b elaborate on these thresholds with subjective and performance end points for several other vibration modes and conditions. Some past tolerance thresholds may be seen in Figure 8-20. Other reviews of tolerance criteria are available {I0 ). Tolerance thresholds to vibration in the X, Y, and Z axes while sitting in contoured couches or adjustable vibration-absorbing devices in spacecraft liftoff position are found in Figures 8-24b and c. (See below for restraint variables.) Sources of vibration in land and aircraft are seen in Figure 8-32. The range of vibration levels found in aircraft, trucks, and tractors is indicated in Figure 8-33. Individual points represent flight vibration data obtained in various types of military aircraft. The solid circles, squares, and triangles indicate vibrations at seat levels which were reported as excessive and un- desirable in actual service; the open marks indicate conditions accepted as tolerable. The linearized dividing line between tolerable and excessive vibrations is the WADC tolerance criterion shown in Figure 8-30, used as a long-time vibration tolerance criterion in military aviation aircraft. This curve is based on subjective pilot comments with respect to tolerable and intolerable levels in military aircraft {62 ). Levels below this line are tolerated, in general, for long times (up to 8 hours) without observed detri- mental effects on performance or health. For short flight durations {for example in helicopters) vibration levels considerably above this line but not above the voluntary tolerance area, are routinely tolerated. 8 -51 "_ VOLUNTARY TOLERANCE j ..... -+--- UNINJURED : l _ J&Ap F" rDEPENDING ON TIME Ii [ "i1_ II I --Sh .... i .... I...... ' " • ,V ..... t 4 i t ...."i.... "--"_ __l_'"'_-T;'°i _......,...... , / ._ ..<;.i.]!T_IL.?_%.:...[/.F,I:-t : _,o,,.,, ..... >. J 't:u_.,E,s,N+._s_I""-T['_,.,_0'_'o,..:,ptio° P - ,..,,-.-,; ,..,,-,-,--- t _ _-_ of vib,a_ion in handlrl l A till I ..... -# // i i#/ ' :: : * F ! _'"# : ! ;P E R C E P T I ONig"_- " _---- - y Threshold for l_lrceptlcln --- at fingertip (tongentlol vibration) il 11 I J ! 2 3 4 5 10 20 30 40 50 FREQUENCY-Hz i[ ! i i i I| III 1(30 200 3oo 5o0 1000 i _-+(sec "l) This illustrates schematically a number of tolerance cr:teria for vibration. The four shaded zones represent: threshold for perception; the unpleasant area of vibration; the limits of volun- tary exposure, unprotected, for 5-20 minutes; and the voluntary tolerance limits for subjects with lap belt and shoulder harness for three minutes, one minute, and less than one minute. Above this, minor injuries occur, depending on time. At the top of the chart is plotted the voluntary tolerance curve for impact, which may be considered to be half-cycle vibration of large magnitude. The conventional way of measuring durations (t) and amplitudes (A) is shown in the small diagrams at the left. Figure 8-30 Criteria for Vibration Tolerance in Longitudinal Axis While Seated (After Webb (178), adapted from von Gierke and Hiatt (70)) 8-52 a. Figure 8-31 Reactions to Mechanical Vibrations (After Mohr and yon Gierke (123)) Summary of Tolerance Limits to Sinusoidal, Who.le-Body Vibration (Numbers refer to Table 8-:31b. See original source for references.) SNORT-TERN TOLUAN¢| PNY$1OLOGIC RESPONSE P£RFORNANCE DECRENENT COMPI..& I NT O.Ot ANNOYING PERCEPTIBLE 0.001 0,1 1.0 FREQUENCY (Hz) 100 8-53 Figure 8-31 (continued) b. Table of Experimental Conditions for Figure 8-31a i. Seated subjects exposed for 20 rain or less to vertical sinusoidal motion. 2. Standing and recumbent subjects exposed for short durations (5 rain or less) to vertical and horizontal sinusoidal naotion. 3. Standing, seated, and recumbent subjects exposed for short durations (<20 rain) to vertical sinusoidal vibration. 4. Recumbent subjects exposed for short durations to vertical sinusoidal motion. 5. Seated subjects exposed for 10-rain periods to vertical sinusoidal motion. 6. Subjects seated in military aircraft exposed for periods of approx 1 hr to flight-induced vibration, primarily in the vertical direction. 7. Standing, seated, and recumbent subjects exposed for periods of 5-20 rain to vertical or horizontal vibration. 8. Seated subjects exposed for short durations to vertical vibration. 9. Standing and seated subjects exposed for periods of 5 rain or less to vertical sinusoidal vibration. I0. Seated subjects exposed for periods of approx I rain to vertical sinu soidal vibration. II. Standing and seated subjects exposed for short durations to vertical or horizontal sinusoidal motion (summary combination of limits). IZ. Seated subjects exposed for periods of 1 min to vertical sinusoidal vibration. 13. Seated subjects exposed for short durations to vertical vibration. 14. Seated subjects exposed for short durations to vertical vibration. 15. Recumbent subjects exposed for short durations to vertical and horizontal vibration. 16. Standing subjects exposed for short durations (<5 rain) to vertical sinusoidal motion. 17. Airline passengers exposed for continuous periods of i00 nlin principally to vertical vibration. 18. Recumbent subjects exposed for periods of I-2 rain to vertical and horizontal vibration. 19. Recumbent subjects exposed to vertical and horizontal vibration at increasing amplitude up to the maximum voluntary tolerance. Vibration limits (general summary).20. 8-54 O3 LIJ --J O; _O I F- LARGE BODY -- VIBRATIONS----_ I I I I 5 SECONDARY I ...... v._#._.!,o..,s_iOR,;,',_%,%AT SUSPENSION THROUGH SYSTEM _ SUSPENSION I RESONANTWHEEL _"iBOUNCE'REOuE_¢,ES L I I I _ L 1 1 1 I0 50 J I I I IOO I"..... PROPELLERS I'-----I_ CRANKSHAFTS L,I--_ ENGINE MOUNTING J-- FI--_--AND ACCESSORIES " " .%1 ACCELERATIONS AND 1 STRUCTURAL STRESSES CON'ROLSURI'ACES------ WING 1 I I l t llll i l I I l|Jii t.O IO I00 FREQUENCY (Hz) J I I I ] Jill I000 Figure 8-32 Vibration Frequencies Encountered in Aircraft and Automobiles (After McFarland and Teichner (114)) -G v Z O u I- nr ILl J W (.J O I00 / I / Vibrations wilh/Gunfire,bomber 2"///Chipping_ I fl ight-deck vibrations///Z/_/hommer_//////_ I,r=torvbro,ionsJ.H Ro,orF#Y/ I /.: " bond IO_)Is//"/_/_ ,V/I j,;,, _ ,//_J¢._< t_Jr'_stramed Maximum accelera- tions were, 3.{J'3 G. b. _L c_ uJ uJ Z o ,; N _ o_ r_ tY _u z , .-< .-q , .,q , -.'q _,, .'q-q xx I 4 3'16 1'8 1 '16 SEATED, • G z 14 3q6 1 8 I 16 AMPLITUDE-inches SUPINE, _ O x %', 14 3 16 I 8 1'16 SUPINE _ G z BODY POSITION AND DIRECTION OF VIBRATION The effevts of tll_* direction of vibration and (,f am_Htud_' _r_, shown for the same experimental situation as in a. 8 -82 Recent comparisons have been made of the effect of whole-body vertical vibration on tracking performance with compensatory wheel, column, and foot systems I Z0 ) in 4 subjects at frequencies ranging from I through 27 Hz. A good correlation was found between general subjective experience and tracking performance. Maximum effects were noted in the I0-20 Hz range. Within the limits of this experiment and over the entire range of vibration frequency and intensity, increased force requirements in foot tracking pro- duced the lowest degree of error but are increasingly affected by vibration. Thus, if precision is of utmost importance and the environment is to contain little vibration, higher force values (150 ibs.)are recommended; whereas, if vibration is to be present and consistency in performance is the objective, lower values 150 ibs.) appear most appropriate. Foot pressure constancy is most severely affected at 1.5 and 2.5 Hz at 43 lb./inches during transverse, and at 3.5 Hz during longitudinal vibration (91). Error increases with intensity. There is no trend of error over time, and recovery is virtually complete following exposure. The frequency dependence may vary with pressure. The effects of random vibrations or jostle on control tasks are seen in Figure 8-50. The difference between sinusoidal, random-amplitude and random amplitude-and-frequency vibration has received study (20, 25, 138 ). A comparison has been made of tracking ability during sinusoidal, narrow- band and wide-band random vertical vibration, produced by a dynamic flight simulator (20). The three types of vibration were equated in terms of their rms displacement-amplitude. The random vibration represented flight through turbulence having characteristic modes of vibration at 0.75 and 2.5 Hz and had a power spectral density representation with peaks at about 0.01 gg/Hz at I and Z.5 Hz. Sinusoidal vibration was applied at 0.75 and 2.5 Hz, which were also the nominal frequencies of the narrow-band random vibrations used. The report revealed no clear-cut difference in tracking performance during sinusoidal and random vibrations of the same rms intensity. Under the condi- tions of the trial, significant differences were found only when the design of the display was important in relation to the direction of vibration. A marked decrement in performance was measured during steady-state vibration at 2.5 Hz and 0.54 in. amplitude when the task involved vertical tracking with a control-display feedback delay. The time-constant of the feedback delay was Z sec, which is representative of an aircraft control system. In another set of experiments an attempt was made to study human subjects under whole-body vertical vibration to compare effects on performance of 5 Hz sinusoidal, 5 Hz random amplitude, and 4-12 Hz random vibration equated on the basis of power, and determine acceleration levels at which significant performance decrements are found for each type of vibration (183). The complex experimental task required two-dimensional compensatory tracking, visual monitoring, and auditory monitoring during 20-minute vibration exposures at levels equated to 5, 15, 25, and 30 percent of the l-minute human tolerance values for 5 Hz sinusoidal vibration. Performance decre- ments under vibration were restricted to tracking, the most demanding com- ponent of the task complex. Tracking performance deteriorated with increas- ing acceleration levels of each type of vibration. Overall performance differences associated with the different types of vibration equated on the basis of power were not significant. A number of task and procedural variables, 8 -83 Figure 8-50 Tracking Performance During Vertical Jostle (After Webb (178), adapted from Clark (22)) LM U W J 0 rw 0 O_ UJ Z i Z -v I.-- W U- 0 Z 0 e_ 0 0 ae ,= 10 0 -% bo u.l U Z e4 U_ .J 0 k- LU J o - l:3 UJ BEST SCORE I 7....., I I i i I 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 03 JOSTLE LEVEL - gzrms JOSTLE LEVEL - gzrms These two figures illustrate the increasing difficulty of performing a demanding control task as ran- dom vertical vibrations ("jostle") increased in magnitude. Pilots were held by a Navy torso re- straint device in the North American Aviation "G_seat, '' which was then programmed to move in the +-Gz direction with an electrical white noise input, and jostle levels of: 0.15 G z rms (root mean square) described by the pilots as similar to flying through mild turbulence; 0.35 Gz rms, described as severe turbulence; and 0.50 Gz rms, in which the largest jolts were classed as one-of-a-kind in a real flight. Power density spectra peaked near 1 Hz. A panel-mounted accelerometer showed maxi- mum and minimum accelerations of 0 to 2.5 Gz for 0.15 Gz rms; -1.2 to 4.0 Gz for 0.35 Gz rms; and -2.5 to 5.0 Gz for 0.5 G z rms. The demanding control task was to track in pitch, shown on a 5-inch oscilloscope screen, and hold roll to 0. Fourteen highly motivated and skilled Navy pilots made 70 runs; they tolerated the random vibrations well, although they had to brace themselves against the larger seat movements to avoid slapping of the head or body against the structure. Longer runs - 15 to 30 minutes at 0.35 Gz rms - produced muscular fatigue. Chart a shows what tracking error occurred as the jostle levels increased; the minimum tolerance for error was set to be 1/12 of the task amplitude. Chart b shows that as jostle increased, more and more tracking deviations appeared beyond the loose tolerance level of 5/6 of the task amplitude. 8 -84 including task difficulty, work-rest cycle, and prior experience appear to be Lmportant determinants of performance capabilities and fatigue effects found in vibration studies, indicating a need for further investigation of these variables. For various pairs of periodic vibration exposure having the same rms but different peak accelerations, subjective severity ratings seem to depend on differences in peak acceleration but motor functions such as holding hand and arm in fixed position depend on rms values. The evidence presented above indicates that differences in bandwidth and spectral density character- istics of vibration conditions equated on the basis of power (mean square acceleration) are of far less importance in determining performance capabili- ties than other experimental variables. Inasmuch as the vibration conditions tested cover a significant portion of the 1-20 Hz range in which the more significant effects of vibration are found, further study should be undertaken to test the generality of this finding. If performance equivalence of power values is found to hold within a number of frequency bands spanning the 1-20 Hz range, the way will be open to utilize data derived from tests with simple sinusoidal vibration in predicting performance under more complex spectra. A major difficulty of research in this area still centers on selection of performance tests representative of operational tasks for which difficulty levels and operator loading can be specified. If unitary activities such as tracking or visual monitoring are tested in isolation, measurement problems are simplified, but task difficulty must be increased in order to achieve adequate sensitivity to stress effects. An alternative is to employ complex tasks which require time-sharing of attention among several activities. The use of tasks of this type, which simulate more closely operational situations, will be facilitated if some effort is devoted to establishing the difficulty level of the total task complex and each of its component activities. One approach which merits consideration is the use of secondary task scores to measure operator workload. Techniques are available for deriving quantitative estimates of operator loading in a complex task by measuring ability to per- form an auxiliary task concurrently (49, 101, I02). An interesting comparison has been made between the long-time tolerance limits proposed and the result of visual acuity tests and tracking performance as shown in Figures 8-51a and b. The curves offer an excellent opportunity for display of the effect which experimental variables may have on the setting of performance thresholds. The following discussion is taken from this review (87). In Figure 8-51a, the geometric symbols refer to the studies of particular authors and their locations on the figure indicate the levels of vibration at which visual acuity was tested. The numbers inside the symbols show the relative level of decrement obtained within any one study. Number l denotes the greatest decrement with successive numbers indicating decreasing decre- ment. The absence of a number means that no decrement was obtained. There is nothing to indicate that visual acuity will be adversely affected at the levels of the two long-term exposure curves at the bottom of the figure. Of course, no data are presented at these levels. However, it seems extremely unlikely that visual acuity would be significantly affected at these low g values. (See also Figure 8-23a. ) 8-85 Figure 851 Performance During Vibration in Relation to LongTime Recommended Limits (Adapted from Harris and Schoenberger (87)) 2.0 1.0 .5 o_ .2 {.} < .! .05 .O2 A E3 O Dennis (36) Loeb (111) Mozell and White (125) @ [] <> Schmi,z,l_3, ® ,,, ® ® -\ N v_-._.___''. ("_-"L One Hour or Less t,Linder} | _ "'..'''_--O_e.ourorLo_o_"_r"_' Shoenberger) n_im mm im mm im mm i _ mm mm me im Be _ mm mm im im i _ _ i _ _ _ % _ Long Time Tolerance Curve For Military Aircraft (WADC) %% / Prolonged Exposure (Janeway) I I I I I I I I I I 0 2 4 6 8 10 12 14 16 18 20 Frequency (Hz) a. Performance on Visual Acuity Tasks (See text) 8-86 Figure 851 (continued) L| 2.0 1.0 .5 0 A [3 O © Buckhout (21) Catterson et al (17) Harris et al Mozel] and White (125) Schmit z (153) m _% I " t - I ' O @ O A O O O O % ,(_Of "°"e"°ur°r L°"s{Lind°'} ^ z_ ....::::"" _, l_ 0 ....'" • ,

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