The related literature was reviewed and a preliminary analytical model was developed for simplified acoustic and structural geometries for pressurized and unpressurized space station modules. In addition to the analytical work, an experimental program on structureborne noise generation and transmission was started. A brief review of those accomplishments is given

Vaicaitis, R.

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t - y
	
a
PROGRESS REPORT
FOR
NASA GRANT NAG-1-541
Vibrations and Structureborne Noise
in Space Station
For the period
January 1, 1985 - June 30, 1985
(NASA-CH-176291)	 YIBRATICNS AND	 .486-11220
i780C1UREBOaNE NOISE IN SPACE 31A11ON
?rogress Report, 1 Jan. - 30 Jun. 196
(Columbia Univ.)	 26 p HC A(,3/AE AJ 1	 Uncias
LSCL 22r) 0-3/18	 15861
Prepared by:	 Rimas Vaicaitis
Principal Investigator
Department of Civil Engineering
and Engineering Mechanics
Columbia University
New York, N.Y. 10027
N
REq yE n
^^, m
1
1
1. Summary of Major Activities
During the reporting period, the following major activities relating
to the proposed work have been accomplished.
1.1 Conference Papers (Presented)
1. Vaicaitis R. and Mixson J.S., "Review of Research on Structureborne Noise,"
26th AIAA/ASME/ASCE/AiS SDM Conference, Paper No. 85 -0786-CP, Orlando, F1.,
April, 1985.
2. Vaicaitis, R. and Bofilios, D.A., "Response of Double Wall Composite
Shells," 26th AIAA/ASME/ASCE/AHS SDM Conference Paper No. 85-0604-CP,
Orlando, F1., April, 1985.
1.2 Conference Papers (Prepared)
1.	 Vaicaitis, R. and Bofilios, D.A., "Noise Transmission of Double Wall
	
Composite Shells," ASME 10th Biennial Conference on Mechanical Vibration and
	
f
Noise, Paper No. H-334, Cincinnati, Ohio, September 1985.
(A copy of this paper is included with the present progress report)
1.3 Oral Presentations
1. Vaicaitis, R. and Bofilios, D.A. "First Progress Report on
Structureborne Noise for NASA NAG-1-541 Grant," Langley Research
Center, NASA, AM, March 25-27, 1985.
2. Vaicaitis, R., "Second Progress Report for NASA NAG-1-541 Grant,"
Langley Research Center, NASA, ANRD, Aug. 5-7, 1965.
z2.0 Technical Highlights
The technical background on structureborne noise generation and trans-
mission for aircraft, rotorcraft, automobile, spacecraft, ship and building
technology has been described in Ref Review of technical liter-
ature related to this subject suggests that theories on structureborne noise
generation, propagation and transmission are incomplete and significant amount
of theoretical and experimental work is needed before noise control measures
can be implemented. The analytical techniques which are emerging as potential
candidates for analysis; of structureborne noise are modal methods, finite
element procedures and wave propagation techniques. However, the success and
validity of using those methods is strongly influenced by the ability to
describe in detail input loads, vibrational energy propagation thrrwgh complex
structural systems and coupling of acoustic field to structural vibrations.
The first phase of the proposed work during the 1984-85 period was mainly
devoted in reviewing the related literature and then developing preliminary
analytical model for simplified acoustic and structural geometries for
pressurized and unpressurized Space Station modules. In addition to the
analytical work, an experimental program on structureborne noise generation
and transmission was started. In what follows, a brief review of those
accomplishments is given.
2.1 Theoretical Study of Structurebornhe Noise Transmission
Analytical models based on modal analysis have been developed for a cy-
lindrical acoustic enclosure shown in Figs. 1 and 2. Single wall (Fig. 1) and
double wall (Fig. 2) constructions have been considered. The shells are
S
1	 ,
1	 45
closed at both ends by circular plates of either single or double wall
constn:ctions. The forces acting on the structure are normal point loads
Aich can be located either on the shell or on the end plates (Fig. 3). 'fie
point loads are random and can be acting at any arbitrary location on *he
structure (inside and/or outside). Numerical procedures have been developed
to calculate modal frequencies, deflection response and noise transmission.
Pressurized and unpressurized modules were considered. The geometric parame-
ters selected in the first phase of this study are similar to those of the
proposed Space Station modules. Structural models for aluminum and fiber
reinforced ocaposite materials were developed for both single and double wall
shells. 'fie details of the theoretical development are given in Refs.
The highlights of the numerical results are presented in this proposal.
2.1.1 Single Wall Aluminum Shell
The aluminum shell shown in Fig. 1 has the following dimensions: L = 300
in, R = 58 in and hs = 0.1 in. Both ends are closed and the interior walls
are lined with a layer of porous acoustic material [ 3, 4) . The inputs to the
shell located at xl=x2 = 150 in., el = -900 , e2 = 900 are random point loads
which are characterized by a truncated Gaussian white noise spectral density
[-3, 43. The shell response calculated at x = L/2, A = 45 0 is shown in Fig. 4
for pressurized and unpressurized conditions. The deflection response levels
RL are calculated from
FL (x,e,w) = 10 log [Sw(x,e,w)ew/w2 1
	
(1)
where SW is deflection response spectral density, Aw is frequency bandwidth
and wref = hs . As can be observed from these results, pressurization has a
3
1
(
f
w -_
different for these two cases.
marked effect on shell response at low frequencies for the first few shell
modes. Similar results are presented in Fig. 5 where sound pressure levels
inside the shell generated by two point loads are given. These results sug-
gest that pressurization plays cray a minor role on noise transmission. It
should be noted that under orbital conditions noise can be generated only for
the pressurized shell while vibrations are important for both cases. The
results shown in Figs. 4 and 5 indicate that shell response is dominated by
low frequency structural resonances while transmitted noise is governed by
structural and acoustic resonances in the mid-frequenc , range (300 Hz-700 Hz).
This can be attributed to the fact that low frequency shell modes do not
couple well to the low frequency acoustic modes.
2.1.2 Single Wall Composite Shell
The composite shell is constructed from 10 laminae layers each reinforced
by fiberglass and/or graphite fibers. Each layer can be oriented in any arbi-
trary direction. The details of the theoretical development and parameters
chosen to characterize composite materials are given in Refs. 3-E.
Fiberglass and graphite fibers are used to reinforce the plexiglass materi-
al. The ratio of fibers volume to the plexiglass volume is 0.2. The weight
of the composite shell is about half of the weight of the aluminum shell. The
transmitted noise for an aluminum and composite shells is given in Fig. 6.
These results tends to indicate that more noise is transmitted at most fre-
quencies by a shell made from composite materials. It should be noted that
structural damping mechanisms uses to characterize vibration attenuation are
4
2.1.3 Double wall Aluminum Shell
Deflection response and noise transmission of a double wall shell shown
in Fig. 2 was calculated for point load inputs. For Spaoe Station applica-
tions, the outer shell could serve as a radiation and thermal protection
shield. The space between the two shells is filled with soft thermal insula-
ting material. The vibration coupling between the two shells is provided by
the soft thermal material. Numerical results are presented for a core with
he = 2 in (thickness), ks = 4.16 lb/in3 (uniaxial stiffness), and ps =
s
3.4 x 10-6 lb - sec2/in4 (density).
The natural frequencies of a double wall aluminum shell are plotted in
Fig. 7 for 10 longitudinal and 20 circumferential modes. The thicknesses of
the outer and the inner shells are hE - 0.032 in and hl = 0.1 in. For the
present double wall construction, the flexural (in phase) and the dilatational
(out of phase) moles are included. The highest modal frequency is that of
"breathing" mode for which n = 0 and m = 1 (simply supported shell at both
ends). Results plotted in Fig. 7 indicate that for the large shell dimensions
and the ratio radius/length - 0.1933 chosen in this study to characterize the
dimensions of a space station module the modal frequencies at n = 0 seam to
converge to a single point for all values of m = 1,2,...,10. This suggests
that in the vicinity of the "breathing" mode frequency large number of struc-
tural modes could couple to acoustic modes resulting in high levels of noise
transmission. In Fig. 8, sound pressure levels in the shell generated by two
mechanically induced point loads are given for reverberant (hard walls) and
absorbent (interior walls lined with porous acoustic materials) conditions.
For reverberant conditions, the noise levels inside the cylinder became rela-
tively large and are dominated by peaks at acoustic resonant frequencies.
5
2.1.4 Double Wall Composite Shell
The modal frequencies of a double wall composite shell constructed from
ten laminae layers for inner shell and three laminae layers for the outer
shell are shown in Fig. 9. From the results given in Figs. 7 and 9, it can be
seen that modal frequencies of a composite shell are significantly higher than
those of an equivalent aluminum shell. However, the mass of the oomposite
shell is about 50% less than that of the aluminum shell. Figure 10 depicts
sound pressure levels for an aluminum and fiber reinforced composite shell for
identical loading and damping conditions. As can be observed from these re-
sults, the noise levels generated by a oomposite shell are higher than the
noise levels for an aluminum shell at most frequencies. However, the oompos-
ite shell is nuch stiffer than the aluminum shell. A shift in modal frequency
could induce differermc coupling between structural and acoustic modes. Since
damping mechanism of composite materials are significantly different from
those of metals, it is difficult to assess the equivalence between composites
and metals. The response levels of the inner composite shell are shown in
Fig. 11 for several values of the loss factor g ib . In this case, all the
stiffness moduli are complex with Ei) = Ei ^.(1 + i gib), GiJ = GiJ
(1 + i gij), etc., where Ei^ and Gib are real quantities. 	 These results
indicate that large amount of response reduction can be achieved in a compos-
ite shell for large values of loss factor gib . The loss factor giJ is
function of matrix material, ratio of fibers to material, fiber orientation,
number of laminae layers, etc. The results presented in Fig. 12 correspond to
point loads acting on the interior shell at xi = x2 = L/2, el = - 90 0 and
e2 = 90 0 . The fiber orientation of the three layers (Fig. 2) at the exterior
shell is described in Fig. 12. The fiber orientation for the ten layers of
the interior shell are (A) 0',22.5',45',45'.22.5',0°,90°,90°,90 0 ,90° (B) 90%
6
0',900,0',90',0',90',0',90',0' 	(C) -45',45',-45',450,-450,450,-450,450,-45',
45'. These results show that interior noise is a function of fiber orienta-
tion in a cat ite shell. The interior noise levels might be tailored to
meet specific needs by selecting a suitable fiber orientation. However, in-
terior noise is a function of frequency and only specific frequency bands
might be affected by this procedure.
2.1.5 Double Wall Circular End Plates
Analytical models for deflection response and noise transmission of
double wall circular aluminum plates (Figs. 2,3) to point loads were
developed. The details of the theoretical analysis are given in Ref. 3. The
response levels of the outer and inner plates for point loads acting on the
outer plate are shown in Figs. 13 and 14. The thicknesses of the outer and
the inner plates are identical and equal to 0.25 in. The response levels are
calculated at r - 0 and e - 45 0 . As can ' seen from these results, response
levels are significantly higher at most modal frequencies when the point loads
are located at ri = r2= 10 in. and ei =00 , e2 = 18 0 0 . The sound pressure
levels at x - L/2, r - 23 in. and e = 45 0
 due to noise transmitted through
double wall circular end plates located at x - L, are shown in Fig. 15. The
inputs are two point loads located at rl = r2= 28 in and ei =
 -900,e2 =90o
For conaarison, noise transmitted through a double wall aluminum shell is
included in this figure. It can be seen that interior noise is dominated by
end plate vibrations for frequencies up to 200 Hz and by shell vibrations for
frequencies above 200 Hz. These results indicate that reglecting noise trans-
mitted by the end caps would underestimate interior sound pressure levels for
the low frequency region.
2.2 Experimental Study of Structureborne Noise
The basic objective of the experinerntal study is to assist in the funda-
mental understanding of generation and transmission of structureborne noise.
Tb achieve these goals funds from the Department of Civil Engineering and En-
gineering Mechanics were allocated to construct laboratory facilities and to
purchase additional vibration and noise measurement equipment. The following
equipment and computer programs have been acquired: ZONIC four channel real-
time spectrum analyzer, monochrome and color monitors, dot matrix plotter, 14
channel FM tape recorder, modaly turned hammer, miniature accelerometers and
signal conditioners, acoustic emission equipment, electromagnetic shaker and
power amplifier, MODAL and MODAL MODIFICATION software. Series of preliminary
tests have been conducted utilizing the AereCcrwander aircraft fuselage. The
results shown in Figs. 16 and 17 indicate typical interior noise levels
generated by a shaker excitation to the sidewall (structureborne) and by ex-
terior noise from two speakers (airborne) located at about 3 ft. from the fus-
elage. The results presented in Fig. 16 correspond to interior point located
very close to the vicinity where the shaker excitation is applied. The re-
sults shown in Fig. 17 are for an interior point located at about 8 ft. from
the shaker. In the vicinity of point load excitation, interior noise is do-
minated by the structureborne contribution. Except for the low frequency
region of 4OHz-BOHz, the magnitude of structureborne noise decreases rapidly
with increasing distance from the point of mechanical excitation. For this
aircraft, the natural frequencies of the main structural frame vibrations
aecur in the frequency range of 40 Hz-WHz. These preliminary results tend to
suggest that at sure distance away from source location structureborne noise
is mainly transmitted by low frequency vibrations of the main frame structure.
8
I	 I
3.0 Future Work
We expect to continue the development and improvements of the analytical
models for application to response and noise transmission estimation for space
station applications. In addition, experiments will be conducted on structure-
borne noise generation, propagation and transmission.
4.0 References
1. Vaicaitis, R., and Mixson, J.S., "Review of Research on Structureborne Noise"
26th AIAA/ASME/ASCE/AHS SDM Conference, Paper No. 85-0786-CP, Orlando, Fl.,
April, 1985.
2. Vaicaitis, R., and Bofilios, D.A., "Response of Double Wall Composite
Shells," 26th AIAA/ASME/A.9CE/AHS SDM Conference, Paper No. 85-0604-CP,
Orlando, F1., April, 1985.
3. Bofilios, D.A., "Response and Noise Transmission of Double Wall Circular
Plates and Laminated Composite Cylindrical Shells," Doctoral Thesis, Columbia
University, New York, N.Y., July, 1985.
4. Vaicaitis, R. and Bofilios, D.A., "Noise Transmission Through Double Wall
Composite Shells," 10th Biennial Conference on Mechanical vibration and
Noise, ASME, Volume H-334, Cincinnati, Ohio, Sept., 1985
5. Bert, C.W. and Eagle, M.D., "Dynamics of Composite, Sandwich, and Stiffened
Shell-Type Structures," Journal of Spacecraft and Rockets, Vol. 6, No. 12,
Dec. 1969, pp. 1345-1361.
6. Soedel, W., Vibrations of Shells and Plates, Marcell Dekker Inc., 1981.
9
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24
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Vibrations and structureborne noise in space station

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