An experimental investigation of the acoustic response of a single microphone mounted in a flat plate, which was conductedin the anechoic chamber at NASA-Ames, is outlined and discussed in detail. The purpose of this experiment was to obtaindetailed understanding of the acoustic response due to various microphone installations considered for use in multisensorphased arrays. Some of the installation methods used to reduce the flow noise near the microphones in previous studies werefound to cause significant acoustic reverberations, particularly at high frequencies (10-20 kHz). The experiment wasperformed without flow to determine the acoustic effects of installation schemes on Ames wind tunnel multiple-sensor arraysand other microphone installations to identify configurations with the most uniform response. Results of this experiment arepresented in two parts. First, results to determine the acoustic effects of installation schemes on multiple-sensor arrays arepresented. Then, 5-50 kHz data are presented and analyzed for other various microphone installations so as to identifyconfigurations with favorable magnitude and phase response to an external source. (Author)
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AN EXPERIMENTAL STUDY OF THE RESPONSE OF A CONDENSER MICROPHONEINSTALLED IN A FLAT PLATE
Hedayat Hamid* and Cliff Home**NASA Ames Research Center, Moffett Field, CA
Summary
An experimental investigation of the acousticresponse of a single microphone mounted in a flatplate, which was conducted in the anechoic chamberof the National Full-scale Aerodynamic Complex(NFAC) at NASA Ames Research Center, is outlinedand discussed in detail. The purpose of thisexperiment was to obtain detailed understanding ofthe acoustic response due to various microphoneinstallations considered for use in multi-sensorphased arrays. Some of the installation methods usedto reduce the flow noise near the microphones inprevious studies were found to cause significantacoustic reverberations, particularly at highfrequencies (10 - 20 kHz). The experiment wasperformed without flow to determine the acousticeffects of installation schemes on Ames wind tunnelmultiple-sensor arrays and other microphoneinstallations to identify configurations with the mostuniform response.
Results of this experiment will be presented in twoparts. First, results to determine the acoustic effectsof installation schemes on multiple-sensor arrays arepresented. Second, 5-50 kHz data are presented andanalyzed for other various microphone installationsso as to identify configurations with favorablemagnitude and phase response to an external source.
* Aeronautical Engineer, Member AIAA** Aeronautical Engineer, Associate Fellow AIAA
Commercial and military aircraft noise has long beenan important issue for the aerospace industries bothin the United States and in around the world. Theestablishment of stricter regulations on aircraft noisehas challenged many aerospace industries to developquieter engine and airframe components for the nextgeneration of aircraft design, and reduce engine andairframe noise of existing aircraft in order to meetthese new regulations. Such stringent regulationshave increased demands for reliable aeroacousticmeasurements in controlled environments, such aswind tunnels, to characterize and reduce sources ofaircraft noise. Such measurements require bothisolated sensors and multiple-sensor arrays designedto focus on individual noise sources and rejectextraneous noise. This is a challenge for airframenoise measurements, since the model generated noiseis generally lower than the self-noise of aconventional microphone in a wind tunnel. Thispaper does not address the self-noise associated withan isolated sensor. The interested reader is referredto references 1 through 4 for more information on thereduction techniques and analysis of microphoneself-noise.
Recently, broadband multiple-sensor acoustic arrayshave been developed for aeroacoustic measurementsof scaled aircraft models in wind tunnels. Typically,a multiple-sensor array for in-flow acousticmeasurements contains a pattern of sensors ormicrophones mounted in a flat, rectangular metalplate housed in an aerodynamic fairing. Figure 1shows a 40-element phased microphone arraydesigned and developed at the National Full-scaleAerodynamic Complex (NFAC) and the InformationScience Division of NASA Ames Research Center.This array has been utilized during recent airframenoise tests of subsonic and supersonic aircraft modelsto locate and to characterize noise sources on themodels. As can be seen from figure l(a), fortymicrophones are placed within the five spiral arms.The microphones with their protection grids are flushmounted in a flat steel plate, 1.5 inches behind thesurface of the fairing. The fairing is supported abovethe wind tunnel floor by an aerodynamically shapedstrut. The material on the side of the fairing, figurel(b), is an acoustically transparent, 100 MKS Raylstainless steel cloth bonded on a 67% open porousscreen. The air gap between the porous screen and
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the plate is filled with open cell foam with troughscutout at 45 degrees at the microphone locations.Usage of porous screen and open cell foam waschosen to reduce turbulent flow noise and to dampenflow induced vibration of the porous screen.
Arrays for acoustic testing have been routinely usedin wind tunnel testing at Ames for the past few years.Early configurations possessed flush-mounted baremicrophones diaphragms. Because of the transitionto turbulent flow, the leading edge microphonesexperienced a 10 to 20 dB increase in backgroundnoise above the. turbulent flow noise during theseearly wind tunnel tests. This high turbulent flownoise was believed to be a problem for futureairframe noise measurement and reduction since thesound generated by model scale aircraft is generallynear or below the background noise. This, as aconsequence, led to the array developmental study atthe Ames 7- by 10-ft wind tunnel. The microphonerecess behind a transparent, porous screen, asdescribed above, was one configuration whichreduced flow-induced noise near the microphones.
The array development study which led to therecessed configuration revealed some reverberationeffects below 20 kHz. Future acoustic research atAmes will require array systems with bandwidths of50 kHz or higher. For these reasons, an experimentalinvestigation was conducted without flow in theanechoic chamber of NFAC to obtain detailedunderstanding of the wide bandwidth acousticresponse due to the various microphone installations.First, the experiment was performed to determine theacoustic effects of mounting installation schemes onmultiple-sensor arrays used in previous wind tunneltests. Second, acoustic studies for extendedfrequency in the range of 5 to 50 kHz were alsoconducted for other various microphone installationsto identify configurations with favorable magnitudeand phase response to an external source.
The most interesting results of the experiment arepresented and discussed in this paper. A completeand a more elaborate discussion with theoreticalbackground will be presented in a forthcoming thesis.Figure 2 shows those configurations that arecontained and discussed in this paper. Figure 2(b)shows the Ames existing installation scheme. Themicrophone and its protection grid is recessed 1.5inches behind a porous screen. The immediate airgap around the microphone is filled with open cellfoam. Figures 2(c) and (d) show flush-mounted gridconfigurations, with and without the inclusion of theporous screen. Figure 2(e) shows a 1.0 mm recessbare microphone diaphragm configuration. All of theinstallation configurations just described are relatedto a flush-mounted, bare microphone diaphragm
baseline configuration. This relation will be laterdiscussed in detail.
2.0 Experimental Approach
The experiment was conducted in the anechoicchamber at NASA Ames Research Center. Theanechoic chamber has free space 25- by 18- by 11-feet and is anechoic to sound frequencies greater than150 Hz.
2.1 Apparatus, Test Setup and Instrumentation -Two Briiel & KJXT type 4135 1/4-inch condensermicrophones (test and reference) were used in theexperiment. The test microphone was mounted in a48- by 48- by 0.25-inch-thick masonite panel. Thismaterial was selected because of its capability tosimulate an almost perfectly sound reflecting surface.The size was selected to correspond to the size of thewind tunnel array. The frontal (planar) dimensions ofthe panel were several wavelengths larger than thelargest wavelength evaluated, which was at thelowest frequency (5 kHz). The edges were also farenough from the test microphone to minimize edgescattering effects onto the test microphone. Thereference microphone was placed in free space awayfrom the panel and pointed at the source. Figure 3 isa photograph of the experimental setup. Theplacement of the reference microphone in thisexperiment was chosen such that it meets thefollowing conditions:
1. The reference microphone is close enoughto the test microphone so that the coherencefunction between the two microphones is high.This high coherence is required for the signalprocessing method selected for thisexperiment.
2. The reference microphone is placed farenough from other test hardware so that it isunaffected by reflections.
3. The reference microphone's position andorientation do not change throughout thecourse of the experiment.
4. The reference microphone is pointeddirectly at the source. This condition providesthe highest signal response at high frequenciesand thus maximizes the signal-to-noise ratioacross the reference microphone's frequencyrange.
5. Both microphones are positioned at a fixedradial distance of 12 feet from the source.
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The reference microphone's position was 34 inchesbelow the test microphone, 17 inches to the side of it,and 6 inches in front of it. The placement of thereference microphone in this fashion meets all fiveconditions discussed above. The source waspositioned at the same height as the test microphoneat 5.5 feet above the anechoic chamber floor grating.The floor grating in figure 3 was covered with soundabsorbing fiberglass to prevent source reflectionsfrom contaminating the measurements.
Figure 4(a) and (b) shows the experimental setupsused to determine the acoustic effects of mountinginstallation schemes on Ames wind tunnel multiple-sensor arrays. The test microphone, fitted with itsprotection grid, was mounted recessed at 1.5 inchesbehind a 67% open porous screen covered with 100MKS Rayl stainless steel cloth. A 34- by 18- by 1.5-inch foam was used to fill the immediate air gapbetween the porous screen and the array panel nearthe test microphone. This was the mountingarrangement employed in recent tests of the 40-element array in the 40- by 80-ft wind tunnel. A 26-by 2-inch troughs at 45 degrees was cut to allow thepenetration of acoustic waves at the test microphone.The open cell foam in this experiment was testedwith both the vertical (figure 4(a)) and at horizontal(figure 4(b)) orientations. Since the troughs on thefive spiral arms of the 40-element array (seen onfigure l(a)) are situated in a multi-dimensional plane,the above setups simulate both a vertical and ahorizontal component of the spiral.
Figure 5 is an instrumentation diagram for the study.As can be seen from this figure, the white noise inputsignal to the speaker was supplied with a randomnoise generator. The output signals from the test andthe reference microphones were connected to adynamic signal analyzer and a digital oscilloscope foranalysis and display. All data were analyzed using aresolution bandwidth of 125 Hz and 100 averages.
2.2 Analysis Method - The first step in the studywas to select a baseline configuration to which allother configurations are related. Figure 2(a) shows acut-away view of the baseline configuration. The testmicrophone diaphragm was mounted flush with thearray surface and possessed no protection grid.
To determine the response of the microphonemounted in the panel, it was necessary to takeseparate measurements of the baseline configurationand the installation configuration. The measurementswere then related, via the reference microphone, todetermine the various installation effects. Theacoustic differences between microphone installationconfigurations are determined by estimating thecomplex transfer function (magnitude and phase)
between the test microphone in each installationconfiguration and the reference microphone5. Thedifference between the transfer functions of the twodifferent configurations (baseline and installation)results in the transfer function between the twodifferent configurations since the referencemicrophone information cancels itself. The methodused for calculating the magnitude and phase of thetransfer function between two microphone signals(test and reference) with a common input was thecross-spectrum method.
This method takes the cross-spectrum of a two sensorsignal measurement and calculates the transferfunction as
ff(f\- * y ' _ (rf/
Ga(f) G(where H(f) represents the transfer function,G (/) represents the cross-spectrum measurementbetween the reference and test microphone channelsand G^C/) represents the power spectrum of thereference microphone. This method is very accuratesince it eliminates uncorrelated noise at themicrophones.
For a typical test configuration and acousticincidence angle the transfer function of the baselineconfiguration relative to the fixed referencemicrophone was measured as H(f) haseline on a
signal analyzer. Then, for the same incidence angle,the transfer function of the installation configurationrelative to the fixed reference microphone wasmeasured as H(f)ittstallatim on the signal analyzer.
re/Once the two set of measurements were completed,the transfer function of the installation configurationrelative to the transfer function of the baselineconfiguration was formed as the ratio
'dilation
"VJ 'instref
tallationbaseline H(f)
(2)baseline
refNote that the reference conditions from the previousequation have dropped out, leaving only theinstallation effects relative to the baselineconfiguration.
Figure 6(c) shows a typical result. This result wasformed as the ratio of the two previous measurementspresented in figures 6(a) and 6(b). Figure 6(a) showsthe transfer function of the baseline configuration
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baseline at zero degrees incidence angle with then f -
source, including both the magnitude and the phaseof the transfer function. Similarly, figure 6(b) showsthe transfer function for a typical installationconfiguration as H(f)inslallatim, also at zero degrees
«fincidence angle with the source. Figure 6(c),therefore, presents the final result comparinginstallation and baseline configurations. This resultis formed as the ratio of the transfer function of theinstallation configuration with respect to the transferfunction of the baseline configuration as
^••' ^ installation 'baseline
2.3 Measurement Accuracy and ExperimentalError - Reference 5 points out two sources of errorthat occur in the analysis of random data, termedrandom and bias errors. Random errors in estimateof frequency response functions are due to themeasurement noise in the transducers andinstrumentation, and computational noise in thedigital calculations. The resulting random error dueto these sources is directly related to the coherencefunction V (/) calculated between the twomicrophones, and the number of averages nd. Themeasurements in this experiment were each averagedat 100 blocks and at a frequency resolutionbandwidth of 125 Hz. The coherence between thetwo microphones was about 0.95 for the frequencyrange of interest from 5 - 5 0 kHz. From equation(5.52) of reference 5, a coherence of 0.95 willproduce a normalized random error in the estimatedgain factor and a standard deviation in the estimatedphase factor of 1.6% and 0.93 degrees, respectively.These errors are well within the desired accuracy ofthe measurements.
The signal-to-noise ratio in this experiment was about50 dB from low to mid frequencies and 20 dB at 50kHz. For the purpose of this experiment, a 20 dBsignal to noise ratio was deemed adequate.
The other source of error that is mentioned inreference 5 for random data is the bias error. It is asystematic error that will appear with the samemagnitude and in the same direction from one dataanalysis to the next. Early on in the study, it wasfound that a very slight displacement or bending ofthe array panel between the two sets of measurements(baseline and a test configuration) caused asignificant effect on the phase portion of the transferfunction. The effects in the magnitude portion werenegligible. The porous screen imposed a non-elasticbend on the array panel itself. It was shown that avery slight linear displacement of the array panel
from its original position caused a phase shift of morethan 10 degrees, which was outside the desiredaccuracy range. As a result, the 0.25-inch panel wassecured on a 1 and 3/4-inch-thick plywood to keep itfrom being non-elastically bent. The difference wasreadily apparent and the linear phase shift wasbrought to within the desired phase accuracy range of±10 degrees.
One other source that introduced apparent error intothe measurements was a temperature drift thatoccurred between the two sets of measurements. Forinstance, a half degree Fahrenheit change intemperature related to a 104 degrees of phase shift at50 kHz. In order to account for this, the twomicrophones, as discussed in Apparatus, Test Setupand Instrumentation section, were positioned at aconstant radial distance from the source to allow theacoustic waves to propagate and impinge on themicrophones at the same instant of time.
3.0 Results And Discussion
3.1 Effects of Sound-absorbing Foam BehindPorous Screen - Figure 7 shows magnitude andphase of the transfer function for the experimentalsetups of figure 4. The black solid line representsdata for the vertical foam channel and the porousscreen configuration while the dashed line representsdata for the horizontal foam channel and the porousscreen configuration. Results in both cases indicateunacceptably large amplitude and phase oscillations.Results in figure 7 show degradation, relative to aflush-mounted bare diaphragm baselineconfiguration, as much as 8.5 dB peak-to-peak inmagnitude at 36 kHz (vertical foam channel) and 8dB peak-to-peak in magnitude at 35 kHz (horizontalfoam channel). Phase oscillations as much as 45degrees peak-to-peak are also seen for bothexperimental setups. Home, Hamid, and Cooper6
witnessed similar reverberant effects during amultiple parabolic reflector calibration in theanechoic chamber when the same porous screencovering was used to cover the reflector. A gain ofabout 2 dB peak-to-peak magnitude was measuredwith the inclusion of the porous screen in this study.
3.2 Effects of Flush-mount Configurations - The B& K type 4135 condenser microphone can be fittedwith a protection grid for use in data acquisition andwhen calibrating the microphone with a pistonphonecalibrator. This section will present results for twocases where the protection grid was put to use. TheB & K protection grid and microphone diaphragmform an open cavity 1 mm (0.03937 in) deep.
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The solid lines in figure 8 show magnitude and phaseportions of the transfer function for a flush-mountedgrid configuration in the frequency range 5 to 50kHz. Data on this figure are presented for panelposition of 0 degrees incidence angle. Graphicalsetup of this test configuration can be seen in figure2(c). Deviations from a flat transfer function areapparent for both the magnitude and phase. Thegeneral shape of the transfer function show increasedoscillating amplitudes, but decreasing phase withrespect to frequency. As can be seen from the samefigure, the transfer function shows three majoroscillations in the amplitude and in phase.Oscillations occur near 8, 28 and 40 kHz. Theseoscillations range as much as 6 dB peak-to-peak inmagnitude and ±15 degrees in phase near 8 kHz.
The dashed lines in figure 8 show both the magnitudeand phase of the transfer function for a flush-mounted grid behind a porous screen. Data on thisfigure are presented for panel position of 0 degreesincidence angle with the source. Graphical setup ofthis test configuration can be seen in figure 2(d). Theporous screen and the grid were closely spaced in thistest configuration. Deviations from a flat transferfunction are apparent for both the magnitude andphase. The general shape of the transfer functionshows results that are similar to the results obtainedfor a flush-mounted microphone with a protectiongrid. The magnitude portion from the transferfunction is shown to oscillate about the 0-dB line onthe ordinate of figure 8(a). High frequent reverberanteffects, as a result of mounting the porous screen, arealso vivid for these two positions in the lowerfrequency ranges, from about 5 to 20 kHz.
The general shape of the phase portion of the transferfunction, figure 8(b), shows decreasing phase angleswith respect to frequency. One resonance near 8 kHzis apparent.
3.3 Effects of Microphone Depth Variations fromArray Surface - Figure 9 shows magnitude andphase of the transfer function for a 1.0 mm recess,bare microphone diaphragm. Data on this figure arepresented for panel position of 0 degrees incidenceangle with the source. Graphical setup of this testconfiguration can be seen in figure 2(e). Data, bothmagnitude and phase, show improved results withrelatively smooth amplitude and phase. In thefrequency range from 5 to about 13 kHz, the transferfunction shows no major changes with respect to theflush-mounted bare microphone baselineconfiguration. In the frequency range between 13and 20 kHz, the magnitude of the transfer functionincreases by about 3 dB. Beyond 20 kHz, the curveremains relatively constant with frequency. The
phase beyond 20 kHz, figure 9(b), is seen to decreaseto about-65 degrees at 50 kHz.
At the end of this setup, the test microphone wasshifted to a depth of 0.5 mm. This setup wasperformed in attempt to determine the response of thepanel as the test microphone is brought closer to aflush mount. The dashed lines in figure 9 shows bothmagnitude and phase of the transfer function. As canbe seen, this configuration has a highly regularmagnitude and phase response to an external sourceas the bare microphone diaphragm is placed near aflush mount. The results from this configurationsuggests that the magnitude and phase of the transferfunction will approach a flat or ideal response as thetest microphone diaphragm is placed near a flushmount.
4.0 Concluding Remarks
An experimental investigation of the acousticresponse of a single microphone mounted in a flatplate was conducted in the anechoic chamber of theNational Full-scale Aerodynamic Complex (NFAC)at NASA Ames Research Center. The purpose ofthis experiment was to obtain detailed understandingof the acoustic response of various flush-mount andrecessed microphone installations with and without aprotection grid. The experiment was performedwithout flow to simulate mounting installationschemes on Ames wind tunnel multiple-sensor arraysand to study other installations to identifyconfigurations with most uniform response.
Degradation of acoustic response relative to theflush-mounted bare microphone diaphragm baselineconfiguration was seen in all installations. Those testconfigurations which included the use of the soundabsorbing foam in deep recess showed the worstresults with unacceptably large amplitude and phaseoscillations. Large oscillations and detrimentalresonances were also seen in installations whicheither used the porous screen or the protection grid ora combination of the two. The data from the 1.0 mmrecess, bare microphone diaphragm showedimproved results. The results from the latter and the0.5 mm recess suggest that flush-mounted, baremicrophone configurations have the most regularmagnitude and phase response to an external source.
Previous acoustic tests using phased arrays with bothflush-mount and recessed microphone installationshave demonstrated that the recess configurationreduces flow-induced noise near the microphones.However, this study shows that an unacceptableacoustic response results from recessed microphones.Hence, future phased arrays at Ames will incorporateflush-mounted bare diaphragm microphones. Since
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flow noise is uncorreltated between the differentsensors, signal processing will be used to minimizethe flow noise.
References
1. Jaeger, S. M., Alien, C. S., and Soderman, P. T.,"Reduction of Background Noise in the NASA Ames40- By 80-Food Wind Tunnel." First JointCEAS/AIAA Aeroacoustics Conference. June 12-15,1995.
2. Alien, C. S. and Soderman, P. T., "AeroacousticProbe Design for Microphone to Reduce Flow-Induced Self-noise." 15th AIAA AeroacousticsConference, AIAA Paper 93-4343. October 1993.
3. Dassen, T. and Holthusen, H. "Design andTesting of a Low Self-Noise AerodynamicMicrophone Forebody." Second Joint AIAA/CEASAeroacoustics Conference. May 6-8, 1996.
4. Fields, R. "An Experimental Investigation ofFlow-Induced Oscillations of the Briiel & Kjaer In-Flow Microphone." Masters Thesis, CaliforniaPolytechnic University, San Luis Obispo, 1995.
5. Bendat, J. and Piersol, A. "EngineeringApplications of Correlation and Spectral Analysis."Second Edition. John Wiley & Sons, Inc. 1993. Pp.114-115.
6. Home, C., Hamid, H., and Cooper, D. "Designand Calibration of a Multiple Reflector, In-flowMicrophone Array." AIAA 97-0492, AIAA 35thAerospace Sciences Meeting, Reno, NV, Jan 6-9,1997.
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Figure!, (a) Ames 40-element array.(b) An acoustically transparent screen covering the microphones.
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1. Bare microphone diaphragm 5. Microphone protection grid2. Nylon sleeve 6. 1 l/2"-inch-thick foam3. Array panel 7. Porous screen4. White plastic sleeve I (e). 1 mm Recess Bare Microphone Diaphragm Configuration
Figure 2. Summary of experimental configurations.
Figure 3. A pictorial representation of the experimental setup inside the anechoic chamber.
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MasonitcPanel
l/2"-thickFoam
Location ofTest Mic.
Figure 4. Experimental setups utilized to simulate both (a) vertical and (b) horizontal components ofthe 40-element array spiral.
Acoustic Waves
Figure 5. Instrumentation flow-chart for the anechoic chamber experiment.
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CQ•O
20
10
S o
-10
-20
180 , , i , i
1 10" 2 104 3 10" 4 104 5 104
Frequency, Hz
"'/'baseline
(a)-180
0 1 10" 210" 3 104 4104 5 104
Frequency, Hz
20
10
0
-10
-20
-30
-40104 2 104 3 104 4104 5 104
Frequency, Hz
ref
(b)
0 1104 2 10" 3 10" 4 10" 5 10"Frequency, Hz
20
10
«r•ov10I -20
-30
-400 1 10" 2 104 3 104 4 104 5 104
Frequency, Hz
baseline
ISO
90
j=PH
-90
-1800 1104 2104 3 104
Frequency, Hz4 104 5 10"
Figure 6. Typical transfer function data.(a) magnitude and phase from the transfer function of the baseline configuration(b) magnitude and phase from the transfer function of the installation configuration(c) ratio of the transfer function of the installation configuration with respect to the transfer function of the baseline configuration.
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