Model problem development and analysis continues with the Alternate Resonance Tuning concept. Various topics are presently at different stages of completion and are described. These topics included optimization of panel mass ratios, panel computer code verification and agreement with experimental data, and flexible frame modeling

Bliss, Donald B.

Gottwald, James A.

English

NASA Technical Reports Server

AIRCRAFT INTERTOR NOISE RED1 JCTION 
BY ALTERNATE RES0 NANCE TUNING 
ONE YEAR PROGRESS REPORT 
NASA RESEARCH GRANT NO. NAG-1-722 
Prepared for: Structural Acoustics Branch 
NASA Langley Research Center 
> Prepared by: James A. Gottwald 
Donald B. Bliss 
(hBSA-CB-181530) A I R C R A F X  I N X E R I C R  N O I S E  N88-13005 
E L C U C T I C N  BY A L T E f ; N A T E  R E S C N A b C E  T U N I N G  
Eroqress  Report ( C u k e  Univ.) 14 p CSCL 20A 
U n c l a s  
~ 3 1 7 1  01 70992 
Department of Mechanical Engineering 
and Materials Science 
Duke University 
Durham, North Carolina 27706 
December 1987 
https://ntrs.nasa.gov/search.jsp?R=19880003623 2020-03-20T09:14:20+00:00Z
Existing interior noise reduction techniques for aircraft fuselages perform reasonably well at 
higher frequencies, but are inadequate at lower frequencies, particularly with respect to the low blade 
passage harmonics with high forcing levels found in propeller aircraft. A method is being studied 
which considers aircraft fuselages lined with panels alternately tuned to frequencies above and below 
the frequency that must be attenuated. Adjacent panels would oscillate at equal amplitude, to give 
equal source strength, but with opposite phase. Provided these adjacent panels are acoustically 
compact, the resulting cancellation causes the interior acoustic modes to become cutoff, and 
therefore be non-propagating and evanescent. This interior noise reduction method, called Alternate 
Resonance Tuning (ART), is currently being investigated both theoretically and experimentally. 
This new concept has potential application to reducing interior noise due to the propellers in 
advanced turboprop aircraft as well as for existing aircraft configurations. 
fuselage. A fuselage wall could be constructed of, or lined with, a series of special panels which 
would allow the designer to control the wavenumber spectrum of the wall motion, thus controlling 
the interior sound field. By judicious tuning of the structural response of individual panels, 
wavelengths in the fuselage wall can be reduced to the order of the panel size, thus causing low 
frequency interior acoustic modes to be cutoff provided these panels are sufficiently small. By 
cutting off the acoustic modes in this manner, a significant reduction of interior noise at the propeller 
blade passage harmonics should be achieved. 
individual fuselage panels can be altered. It seems reasonable, therefore, that panel resonant 
frequencies can be manipulated to achieve the ART effect. Application of this concept might involve 
the modification of existing structural panels or development of a new design for fuselage interior 
trim panels. Although complete acoustic cutoff will not be achievable in practice, an approximate 
cancellation should still substantially reduce the interior noise levels at the particular frequency of 
interest. It is important to note that the ART method utilizes the flexibility and dynamic behavior of 
the structure to good advantage, although these properties are not normally beneficial in noise 
control. 
six months of a contract supported by the Structural Acoustics Branch at NASA Langley. 
Considerable progress has been made both theoretically and experimentally as described in the 
following sections. It is important to note that all of the work carried out so far indicates the ART 
concept is indeed capable of xhieving a significant reduction in the sound transmission through 
flexible walls. 
The ART technique is a procedure intended to reduce low frequency noise within an aircraft 
Current noise control treatments have already demonstrated that the mass and stiffness of 
This progress report summarizes the work carried out at Duke University during the second 
Model problem development and analysis continues with the Alternate Resonance Tuning 
concept. Various topics are presently at different stages of completion and will be described below. 
These topics include optimization of panel mass ratios, panel computer code verification and 
agreement with experimental data, and flexible frame modeling. 
2a. W i o O p -  . .  . 
A mid-year progress report presented the basic theoretical model which is currently being 
developed. This model involves sound transmission through a wall of panels of infiiite and 
periodic extent composed of spring-mass-damper elements in various pattems.l Figure la  shows a 
typical result of this model. Note that the design frequencies for this four panel transmission loss 
calculation are varying slightly from the desired ART frequencies of 1,2, and 3 (in nondimensional 
frequency). This is due to the presence of a small but finite amount of damping in the panel system, 
as well as apparent mass due to the loading of the surrounding fluid . The basic panel mass ratios 
(used to calculate the results shown in Figure la) for this four panel system without damping and 
apparent mass were initially derived analytically using MACSYMA, a large symbolic manipulation 
program. These mass ratios were obtained for a given case with certain assumed values for the 
various nondimensional parameters included in the modeling. Initial attempts to find the analytic 
expressions for these mass ratios for the general case (including damping and apparent mass) proved 
unsuccessful due to additional complexity in the governing system equations, and accordingly a 
numerical routine was developed which solved the governing equations for a ratio of transmitted to 
incident pressure (which is then converted to a transmission loss in decibels). A variation in the 
panel computer code has been added which optimizes the mass ratios among the panel elements to 
produce a maximurn ART effect at any desired frequencies. This allows the design attenuation to be 
set at precise frequencies and, as such, the lower harmonics of the desired frequency are now 
precisely located. This additional effect is achieved using a Newton-Raphson scheme in multiple 
dimensions, with appropriate convergence criteria. A typical result is shown in Figure lb. 
Section 2b Derivation of ExDerimental Panel Dvnamic Parameters 
Work is in progress in an effort to correlate data taken during the early part of the ART 
program. A key element in the correlation effort is modeling of the experimental duct 
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apparatus with the panel theoretical model using precise values for panel stiffness, mass and 
damping. Figure 2 shows an apparatus which is being used to isolate an experimental ART panel (a 
four inch passive audio speaker with adjusted suspension and cone mass) far away from any 
interfering surfaces. The voice coil EMF generated by the speakers is conveniently used as a 
monitoring signal. By measuring the change in resonant frequency of the audio speaker (as a result 
of addition of a known mass), the true mass of the speaker cone (and hence, the true mass of the 
experimental passive ART panel) may be determined. Since the panel is rigidly mounted far 
from any surfaces, the apparent mass of the panel (that is, the speaker cone mass plus the effective 
addition to the inertia of the panel by the fluid around it ) may be calculated2 In a similar manner, 
the damping ratio is determined by measuring the panel's dynamic response to a step input. By 
judicious exclusion of the apparent mass in all calculations using the standard relationships, the 
actual panel (speaker) properties may be determined as though the panel were in a vacuum. This in 
turn will permit the use of these parameters in the theoretical analysis in the appropriate manner. 
This work is only partially complete at this time. 
Section 2c Flexible Frame Modeling 
Analytic effort has begun on a flexible frame model which is shown in Figure 3. This 
model will be used to determine the effect of a flexible fiame member on the ART concept. It may 
be possible to use this coupled system to advantage in reducing the first few harmonics associated 
with the blade passage frequency. The frame flexibility and panel-to-panel coupling will change the 
way mass and stiffness must be applied to adjacent panels to achieve the ART effect. A similar 
panel modeling program will be developed to plot transmission losses and determine optimized mass 
ratios for this important case. 
SECTION 3 EXPERIMENTAL WORK 
Section 3a Falloff Measurements Awav from the ART Panel Wall 
In addition to transmission loss plots shown in the previous progress report, the 
effectiveness of the ART concept in the two panel duct configuration was verified experimentally 
with measurement of sound pressure levels as a function of distance away from the ART panel wall. 
Figure 4a shows the sound pressure level falloff in dl3 away from the sound source in the two panel 
duct at various frequencies. Note that this falloff is referenced to a constant interior sound pressure 
ACOUSTIC 
LOADING 
RIGID BASE d 
Figure 2: Test stand to measure ART panel parameters. 

level at the ART panel array. The configuration here included two ART panels with resonant 
frequencies of 100 and 300 hertz. Because of these latter resonances, the falloff in dB at these 
frequencies is fairly low. However, at the design frequency of 200 hertz, a qualitatively exponential 
falloff as a function of distance away from the sound source is observed. This is a true indication of 
the cutoff effect provided by the ART concept. In a similar manner, Figure 4b shows the 
transmission loss across the ART panels as a function of distance away from the sound source. 
This figure indicates the drop in sound pressure level across the ART panel array, or in effect across 
the simulated aircraft fuselage wall. Again at the design frequency of 200 hertz, transmission losses 
of greater than 20 dB are realized for practical distances away from the ART wall. Neighboring 
frequencies show a reduced but nonetheless present transmission loss as well. The latter 
measurement is also an indication of the effectiveness of the ART concept as a function of 
bandwidth. 
SECTION 4. FUTIJRE EFFORT 
Section 4a Upco ming - Analv .tical and ExDe rimental Effort 
Analytical effort will be focused in a number of directions for the fmt half of 1988. The 
E. 
proposed tasks for this next half-year effort are listed below. 
-Continued work on model problems. Primary issues to be addressed with simple 
panel dynamics models include the effect of a non-uniform propagating pressure 
field acting on the exterior panel wall, and the modeling of the aircraft interior 
fuselage as a finite enclosure. 
-Continued theoretical work on the general formulation of the problem. This 
formulation will consider a modal representation of the interior acoustic field 
coupled to a general modal representation of the wall motion. 
-Continued exploration of innovative extensions to the ART concept. One such 
concept is the multiple panel block that allows for ART cancellations at several 
frequencies. Another concept involves higher order panel systems utilizing sets of 
coupled panels to achieve more effective acoustic cancellation and/or provide for 
more multiple cancellations. 
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-Continued laboratory experiments. Set up and use the new data acquisition and 
processing system. Continued experiments on idealized panel configurations. One 
such configuration is shown schematically in Figure 5. This configuration allows 
for two dimensional acoustic modes in the enclosure, rather than simply just the one 
dimensional modes as in the previous experimental work. This latter setup 
will allow the ART concept to be studied under more realistic circumstances. 
By appropriate phasing of the multiple speaker sound source, a propagating incident 
field of the type produced by propellers can be simulated, and the performance of 
the ART panels then can be assessed. 
EXPERIMENTAL APPARATUS FOR SECOND YEAR EFFORT 
‘ARRAY OF SOUND SOURCES 
(amplitude and phase controllable) 
Allows simulation of realistic modal behavior. 
Effect of nonuniform incident fields can be studied. 
Effect of travelling incident fields can be studied. 
Figure 5 :  Experimental apparatus for second year effort. 
REFERENCES 
1. Bliss, D.B., Gottwald, J., and Bryce, J., "Interior Noise Reduction by Alternate Resonance 
Tuning- Six Month Progress Report," submitted to the Structural Acoustics Branch, NASA 
Langley Research Center, August 1987. 
2. Lamb, H., Hydrodynamics. New York: Dover Publications, 1945, p. 139. 
PAPER IN PROGRESS 
Bliss, D. B., and Gottwald, J. A., "Aircraft Interior Noise Reduction by Alternate 
Resonance Tuning." 
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Aircraft interior noise reduction by alternate resonance tuning

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