AIAA 2003-3195 Performance Analysis of a Cost-Effective Electret Condenser Microphone Directional Array William M. Humphreys, Jr. Carl H. Gerhold Allan J. Zuckerwar Gregory C. Herring Scott M. Bartram NASA Langley Research Center Hampton, VA 23681-0001 9th AIAA/CEAS Aeroacoustics Conference & Exhibit 12-14 May 2003 / Hilton Head, SC For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, Virginia 20191 PERFORMANCE ANALYSIS OF A COST-EFFECTIVE ELECTRET CONDENSER MICROPHONE DIRECTIONAL ARRAY William M. Humphreys, Jr.* Carl H. Gerhold† Allan J. Zuckerwar‡ Gregory C. Herring§ Scott M. Bartram¶ Aerodynamics, Aerothermodynamics, and Acoustics Competency NASA Langley Research Center Hampton, Virginia 23681-0001 Abstract microphones via laboratory testing and calibration, (2) evaluate the beamforming Microphone directional array technology capability of the electret-based LADA using a continues to be a critical part of the overall series of independently controlled point sources instrumentation suite for experimental in an anechoic environment, and (3) demonstrate aeroacoustics. Unfortunately, high sensor cost the utility of an electret-based directional array in remains one of the limiting factors in the a real-world application, in this case a cold flow construction of very high-density arrays (i.e., jet operating at high subsonic velocities. The arrays containing several hundred channels or results of the investigation revealed a more) which could be used to implement microphone frequency response suitable for advanced beamforming algorithms. In an effort directional array use over a range of to reduce the implementation cost of such arrays, 250 Hz - 40 kHz, a successful beamforming the authors have undertaken a systematic evaluation using the electret-populated LADA to performance analysis of a prototype measure simple point sources at frequencies up 35-microphone array populated with commercial to 20 kHz, and a successful demonstration using electret condenser microphones. An ensemble of the array to measure noise generated by the cold microphones coupling commercially available flow jet. This paper presents an overview of the electret cartridges with passive signal tests conducted along with sample data obtained conditioning circuitry was fabricated for use with from those tests. the Langley Large Aperture Directional Array (LADA). A performance analysis consisting of Nomenclature three phases was then performed: (1) characterize the acoustic response of the D free expansion jet nozzle j ________________ diameter, ft *Research Engineer, Advanced Measurement and eˆ array steering vector, Diagnostics Branch, Senior Member AIAA. see eqn. (4) †Senior Research Scientist, Aeroacoustics Branch, f frequency, Hz Member AIAA. ‡Senior Research Scientist, Advanced Measurement Gˆ cross spectral matrix and Diagnostics Branch. k acoustic wave number, ft-1 §Research Scientist, Advanced Measurement and K number of steering locations Diagnostics Branch. ¶Engineering Technician, Advanced Measurement M total number of array and Diagnostics Branch. microphones This material is declared a work of the U.S. N number of data blocks Government and is not subject to copyright protection in the United States. 1 American Institute of Aeronautics and Astronautics P(eˆ) array output power for steering potential of this technology. For instance, the next generation of array processing algorithms vector eˆ, see eqn. (5) being developed will potentially require the P RMS pressure of fundamental F acquisition of significantly more acoustic plus harmonic frequencies, Pa information than is presently done, in some cases P RMS pressure of harmonic H requiring acquisition of acoustic boundary frequencies, Pa conditions over the entire sphere of source St Strouhal number & directivity. DA systems involved in these types ro,ro distance from source to x of tests will employ hundreds or thousands of m c and mth microphone, ft individual microphones. Traditional array hardware, typically composed of externally- r,r distance from steering location m & biased condenser microphone / preamplifier / to x and mth microphone, ft power supply combinations for a number of c R normalized array output of acquisition channels, is in many instances too k reference LADA at kth expensive to implement a high-density steering location microphone system. Therefore, it is desirable to THD total harmonic distortion, identify devices and hardware which can be used percent, see eqn. (1) to implement a system exhibiting measurement T normalized array output of performance comparable to the current k ECM LADA at kth generation of arrays, but at much lower cost. steering location Several alternate microphone devices have been U free expansion jet exit velocity, described in the literature which exhibit j ft/sec characteristics suitable for DA use and at & & economical cost. Among these are MEMS- W(k,x,xo) ideal array response, based devices described by Sheplak8, Saini9, and see eqn. (7) Arnold10, and thin-film electret devices described W&s hamming window weighting by Hsieh et.al.11 x& Cartesian coordinate, ft Over the last few years, the rapid pace of x array phase center, ft micro-fabrication technology development has c X (f) kth FFT data block for ith yielded several promising new commercial ik microphones. Given the increasing interest at microphone, see eqn. (3a) &(cid:2) Langley Research Center and elsewhere in (cid:2)x(cid:2)m distance vector from steering building higher density array systems, the location to mth microphone, ft authors initiated a systematic search for a ε weighted RMS array response suitable low-cost commercial off-the-shelf RMS error (COTS) microphone which could be used for ω angular frequency ( = 2πf ), this purpose. After examining several types of radians / second microphones currently available (including externally biased condenser, MEMS, and electret condenser devices), the authors chose to conduct Introduction a detailed performance analysis of a prototype DA system populated with COTS Electret Aeroacoustic testing capability in both Condenser Microphones (ECM’s). The anechoic and hard-walled facilities has grown objectives of the performance analysis were tremendously over the last several years, with threefold: successful collection of noise source location and directivity data in a variety of experiments. One 1. Characterize the acoustic response of a of the reasons for the success of these suitable ECM via laboratory testing and measurements has been the development of both calibration, microphone directional array (DA) 2. Evaluate the beamforming capability of an instrumentation and associated robust data ECM-based directional array using a series processing algorithms applied in the analysis of of independently controlled point sources in the acquired data.1-7 While the gain in test an anechoic environment, and productivity resulting from the use of DA 3. Demonstrate the utility of an ECM-based systems has been significant, several challenges directional array in a real-world application. remain to be overcome before fully realizing the The authors chose as a demonstration to 2 American Institute of Aeronautics and Astronautics apply the ECM array to measurement of driver on the output stage and can operate at a source strength and directivity in a cold flow maximum supply voltage of +10V, with a jet at high subsonic and low supersonic maximum current draw of 0.5 mA. The nominal velocities. sensitivity of a raw WM-60A cartridge when driven by a +5V source through an 8.2 kΩ load Note that the cost and performance of the resistor is 15 mV/Pa. Figure 1b illustrates the sensors chosen to implement high-density DA typical response of the microphones, which are systems are critical parameters determining the designed to exhibit a flat magnitude response viability of such arrays, but they are not the only over the audio frequency range of 20 Hz – 20 parameters to consider when designing such kHz. systems. Microphone package size, data Equalizing Circuit / Packaging: While a typical handling efficiency (i.e., efficiency of the data WM-60A cartridge operates with a flat response acquisition system), and array mobility (i.e., over the audio frequency range, its use in number and size of cables, power connections, aeroacoustic testing requires that the bandwidth array mounting hardware, etc.) also determine of the device be extended to a significantly the ultimate success of the system. While higher value. Above 20 kHz the microphone microphone package size and construction are exhibits a first order sensitivity roll-off of discussed subsequently, array data handling and approximately 6 dB per octave relative to its mobility are beyond the scope of this paper - audio range sensitivity. To compensate for this reference 10 provides a more detailed discussion roll-off, the authors designed a passive circuit to of these issues. Rather, it is the purpose of this equalize the broadband frequency response of paper to present an overview of the performance the microphone, in essence trading sensitivity for analysis conducted on the prototype ECM-based bandwidth. Figure 2 shows a schematic of the array constructed for this study. The authors ECM cartridge with the equalizing circuit. The believe this DA system represents a “small circuit was fabricated using surface mount scale” version of what can be achieved using devices on a small printed circuit board and economical microphones. mounted along with the ECM cartridge in a 0.32-inch outside diameter aluminum cylinder Microphone Construction approximately 2 inches in length. An 84-inch, four-conductor cable was attached to the rear of the cylinder via a gold-plated screw connector. The ECM’s employed for this work were The other end of the cable terminated in a quick “hybrid” devices composed of COTS disconnect coupler which was attached to a microphone cartridges coupled with passive power and signal distribution patch panel. A signal conditioning circuitry and packaged to total of 38 microphones were fabricated in this enable easy integration into a DA system. The fashion, at an average cost per microphone of details of the packaging are described below. approximately $40. Figure 3 shows a representative finished microphone. ECM Cartridge: Several commercially-available ECM’s were investigated for use in construction ECM Calibration of the prototype array system. After considering candidate devices, the authors chose to use Four different types of calibration data were Panasonic WM-60A ECM cartridges as the sensing element.* Figure 1a illustrates the obtained – the 250-Hz sensitivity of each packaged microphone (raw cartridge coupled construction of the ECM cartridge, which houses with equalizer), the broadband frequency a recessed 0.25-inch diaphragm protected by a response (magnitude and phase) of each cap containing a central access hole packaged microphone, the background noise approximately 0.13 inches in diameter. The level for a sample of raw ECM cartridges, and microphone incorporates a field-effect transistor the total harmonic distortion for a sample of raw cartridges. The various calibration procedures __________________ and representative data obtain from them are * Specific manufacturer’s names are explicitly described subsequently. mentioned only to accurately describe the research performed. The use of manufacturer’s names does not imply an endorsement by the U.S. Government Sensitivity: Figure 4 shows a histogram of the nor does it imply that the specified equipment is the 250-Hz sensitivity distribution for the ensemble best available. of 38 packaged microphones used in this study. 3 American Institute of Aeronautics and Astronautics The sensitivity of each microphone was representative microphones. The magnitude determined using a calibrated pistonphone response of each device is referenced to its generating a regulated Sound Pressure Level baseline 250-Hz sensitivity. As can be seen in (SPL) of 94 dB (relative to a reference pressure the figure, the ensemble of magnitude responses of 20 µPa). The sensitivity of an individual (with one exception) matched to within ±5 dB at microphone is defined here as the calibration any one particular frequency while the phase value (specified in units of mV/Pa) which when responses matched to within ±10 degrees at a applied to the output of the excited microphone particular frequency over the range of 250 Hz allows it to indicate an SPL of exactly 94 dB at through 16 kHz. The matching of phases within 250 Hz. As can be seen in Figure 4, the the range shown in Figure 5 implies that a sensitivities show a partial bimodal distribution directional array populated with these centered around 6.25 and 7.75 mV/Pa, with a microphones should yield accurate beamformed 6.5 mV/Pa average sensitivity exhibited for the estimates of noise strength without the need for a entire ensemble of microphones. The decrease phase correction. The limits over which array in the sensitivity of the packaged microphones phase matching needs to be performed are versus the raw WM-60A cartridges is due to the discussed at length by Mosher13. introduction of the equalizing circuitry. Note that the ensemble distribution is dominated by Broadband Frequency Response – Ultrasonic variations in the sensitivities of individual raw Spectrum: Magnitude and phase responses for ECM cartridges only. Given the low-cost of each microphone for frequencies above the audio these cartridges, if one desires a matched spectrum (16 – 40 kHz) were obtained using a response among the microphones incorporated substitution method freefield calibration. This into a DA system, it is a simple matter to calibration method conformed with the measure the sensitivities of a large sample of International Electrotechnical Commission (IEC) cartridges and then choose those with similar standard for calibration of microphone characteristics. However, this step was not equipment.14 Each fabricated electret performed for this study. Rather, the individual microphone was mounted in an 8-foot by 12-foot measured sensitivities of the packaged by 6-foot high anechoic chamber directly in line microphones were accounted for during analysis with a series of 1.0-inch diameter ultrasonic of all collected data. ceramic transducers, as shown in the sketch in Figure 6. Each transducer was positioned Broadband Frequency Response – Audio 74 inches from the microphone. The shortest Spectrum: The broadband pressure response of microphone to chamber wedge-tip distance was each packaged microphone covering the audio approximately 24 inches. A custom mounting frequency spectrum (250 Hz to 16000 Hz) was bracket with attached guide bar, shown in obtained using a standards-traceable Bruel and Figure 7, allowed correct alignment of each Kjaer 4226 multifunction calibrator which had microphone in the chamber such that all been modified to allow access to the transducer diaphragms occupied as closely as possible the driving signal. A special nylon coupler was same axial location with respect to the source. fabricated to allow insertion of the microphones Freefield calibration data were obtained at into the calibrator with minimal acoustic loss. frequencies of 23.176, 29.976, and 39.576 kHz The calibrator generated a constant SPL of 94 dB by operating each of three different ceramic and was driven by an external precision signal transducers at its resonant frequency (to generator operated over the range of 250 Hz – eliminate higher-order source diaphragm 16 kHz in 250-Hz steps. The driving signal from vibration modes). The transducer driving signal the generator and the microphone output signal and the microphone output signal were recorded were recorded by a two-channel, PC-based by the same two-channel PC-based transient data transient data recorder with a per-channel recorder used for the audio spectrum calibration, sampling rate of 100 ksamples/sec. To obtain at a per-channel sampling rate of the magnitude and phase response of each 100 ksamples/sec. Upon completion of microphone, the transfer function between the acquisition of calibration data using the three driving signal and microphone output were ceramic transducers, the electret microphone was computed using Welch’s averaged periodogram replaced with a standards-traceable Bruel and method.12 Kjaer 4136 reference microphone and a repeat Figure 5 shows the pressure responses set of calibration data was acquired using the obtained using the calibrator for 10 three transducers. The freefield magnitude and 4 American Institute of Aeronautics and Astronautics phase response of the electret microphone at the was applied when using these devices in a DA resonant frequency of each transducer was system at frequencies above 20 kHz. Given that obtained using Welch’s method in a similar only three freefield calibration frequencies were manner to the audio spectrum calibration. acquired (due to the limited availability of One challenge encountered during the ceramic transducers at ultrasonic frequencies freefield calibration process concerned the below 40 kHz), a linear interpolation procedure determination of the proper corrections to apply was employed to provide the narrowband phase to the freefield phase response. Such corrections correction at frequencies other than the resonant were required to account for offsets introduced in ceramic ones. the line of sight distance between the ceramic transducer and the microphone when installing Background Noise Measurement: The different devices in the anechoic chamber, since background noise level of a raw ECM cartridge the guide bar on the mounting bracket shown in was determined using a vacuum-isolation Figure 7 provided a necessary, but not sufficient chamber mounted on a vibration-damped table.15 alignment. The preferred method for computing The vacuum-isolation chamber consisted of an the phase correction was to generate a very short inner chamber maintained at ambient conditions duration resonant frequency chirp using the into which the cartridge was placed. The inner ceramic transducer and then measure the time for chamber was then suspended within a vacuum the sound wave to travel from the source to each chamber to provide a high degree of acoustic reference or test microphone. Unfortunately, the isolation from the surrounding environment. ceramic transducers employed for the test did not Figure 9 illustrates the background noise produce a chirp signature suitable for accurately autospectrum for a typical cartridge for three determining the transit time. Thus, an alternate different vacuum pressures. As can be seen in procedure was developed to obtain the phase the figure, classic 1/f noise dominates the corrections using a high frequency paper-cone response at the lower end of the frequency tweeter in place of the ceramic transducers. The spectrum. The background noise characteristic tweeter provided a much cleaner chirp signature exhibited in Figure 9 is typical for electret-type for determining small transit time variations, and microphones. care was taken to not move the tweeter when installing different devices in the mounting Total Harmonic Distortion: The total harmonic fixture. Using an acquisition sampling rate of distortion (THD) for a series of raw ECM 300 ksamples/sec, sufficient to resolve ≈3 µsec cartridges was determined using an externally time increments, the distance offset for each of driven Whittaker PC-125 acoustic calibrator the 38 microphones when installed into the excited at a frequency of 1 kHz for a range of anechoic chamber was adequately determined sound pressure levels spanning 100 – 140 dB. via a time series cross covariance analysis. Note The THD for a microphone is defined as the root that this process assumed that the reinstallation mean square (RMS) value of the total harmonics of a microphone into the mounting bracket using of a measured calibration signal, divided by the the guide bar resulted in an additional offset of RMS value of the fundamental plus harmonics of no more than a few thousandths of an inch, the signal. This can be expressed in percentage corresponding to sub-microsecond travel time form as adjustments. As will be seen in the phase data shown subsequently, this correction procedure P worked very well in practice. THD= H x100% (1) P Figure 8 shows the freefield responses for ten F representative packaged microphones obtained using the procedure described above. The where P and P are defined as H F freefield data (converted to pressure response) is overlaid with the Figure 5 audio spectrum pressure response data. As can be seen in the P = P2 + P2 +(cid:22)+ P2 (2a) H 2 3 n figure, the ultrasonic magnitude responses were matched to within ±5 dB at a given frequency while the phase responses were matched to P = P2 + P2 + P2 +(cid:22)+ P2 (2b) F 1 2 3 n within ±25 degrees. Since the high frequency phase response deviation between microphones greatly exceeded ±10 degrees, a phase correction 5 American Institute of Aeronautics and Astronautics measured over the first n harmonics. It was the “reference LADA”), and (2) fabricated and determined empirically that the fundamental and packaged electret microphones (hereafter first three harmonics were sufficient to obtain an referred to as the “ECM LADA”). The reference accurate estimate of the THD for the ECM LADA microphones were powered by a bank of cartridges tested. regulated DC supplies, one for each microphone. Figure 10 shows the THD measured on ten The ECM LADA was powered by a single representative cartridges. Indicated on the plot is regulated DC power supply, since the current the average SPL level at which the THD reaches draw from each electret microphone was no four percent. Sound pressure levels encountered more than 0.3 mA using a supply voltage of during aeroacoustic testing in ground facilities +5 V and a load resistor of 8.2 kΩ. In all cases (anechoic chambers, treated wind tunnels, etc.) the output signals from each microphone were typically do not exceed 120 dB; therefore, the conditioned using high-pass elliptic filters at a maximum expected THD when using these frequency of 300 Hz. Although not required, the microphones in a DA application is electret microphone outputs were additionally approximately 1-2%. amplified by a factor of 10 after filtering. Directional Array Evaluation System Data Acquisition: The data acquisition system consisted of a 64-channel transient data recorder Array Construction: The performance analysis controlled by a PC. All data channels were objective of testing the packaged electret simultaneously recorded with a 16-bit dynamic microphones in a prototype DA was facilitated range at a sampling rate of 100 kHz. The analog by utilizing an existing array mounting system. bandwidth of the system at this sampling rate For this purpose, the authors chose to utilize the was 40 kHz based on the settings of the Langley Large Aperture Directional Array anti-aliasing filters. Typical acquisition run (LADA).16 A photograph of the LADA is shown times ranged from 5 to 10 seconds with the data in Figure 11. A four-foot diameter fiberglass stored in raw binary format on the PC hard drive. panel provided a flat surface to flush mount all microphones. The panel was attached to a pan- Data Reduction: Post processing of acquired tilt unit secured to a rigid tripod support. This LADA data began with the computation of an allowed precise alignment changes in the M x M cross-spectral matrix for each ensemble elevation and azimuth of the face of the array.& A of data, where M is the total number of small laser diode pointer was place at x , microphones in the array. The computation of c corresponding to the phase center of the array (in the individual matrix elements was performed this case the center of the fiber glass panel), to using Fast Fourier Transforms (FFT) of the aid in array alignment. The LADA incorporated original data ensemble. This was done after 35, 0.32-inch diameter microphone mounting converting the raw data to engineering units. The holes arranged in a two-dimensional pattern time data were segmented into a series of non- consisting of logarithmic spirals. This overlapping blocks each containing 4096 sam- microphone layout, shown in Figure 12, was ples. Using a Hamming window, each of these based on a design by Underbrink et.al. at Boeing blocks of data was Fourier transformed into the (see chapter 3 in reference 7). It consisted of frequency domain with a frequency resolution of five spirals of seven microphones each with the 24.4 Hz. The individual cross spectrum for inner-most microphones lying on a 1-inch radius microphones i and j was computed via and the outer-most on a 17-inch radius. This design resulted in acceptable beamwidth and G (f)= 1 ∑N[X*(f)X (f)] (3a) minimal sidelobe height over an operational ij NWs k=1 ik jk frequency range of 2 - 30 kHz. The micr&ophone mounting coordinates, referenced to x , are c listed in Table I. where Ws is the Hamming window weighting constant, N is the number of blocks of data, and Array Population: The LADA was populated X is an FFT data block. The full matrix was with either of two types of sensors – formed as (1) commercial, standards-traceable, 0.25-inch externally-biased Bruel and Kjaer 4136 condenser microphones (hereafter referred to as 6 American Institute of Aeronautics and Astronautics where the superscript T denotes a complex ( ) transpose of the matrix, and Peˆ is a mean- G G (cid:22) G 11 12 1M squared-pressure quantity. The division by M2 G (cid:23) serves to reference the array output level to an Gˆ = 22 . (3b) equivalent single microphone output level. (cid:25) (cid:23) References (16) and (17) provide greater detail G concerning additional processing performed on MM the LADA data. The lower triangular elements of this Hermitian ECM LADA Evaluation matrix were computed by taking the complex conjugates of the upper triangular elements. All Test Configuration: The beamforming capability cross spectral matrix elements were employed in of the ECM LADA was evaluated in the Langley subsequent processing, with no modification of Anechoic Noise Research Facility (ANRF). The the diagonal terms. Note that for in-flow ANRF is an anechoic chamber measuring 27.5 ft applications of the array, the diagonal terms can x 27.5 ft x 25.6 ft in which static or wind tunnel be removed to improve the spectral dynamic testing can be performed. The facility is range by subtracting off self-noise dominated primarily employed for engine nacelle noise auto-spectra during the beamforming process studies. To assess the capability of the ECM (see chapter 1 of reference 7). However, this LADA to identify discrete incoherent noise additional step was not required for the data sources, independently controlled speakers were acquired as part of this study. installed in the facility. These speakers were A classical delay and sum beamforming attached to mounting platforms positioned at approach was used for the analysis of the array various locations with respect to the center of the data. This approach assumed distributions of array. Figure 13 illustrates one such monopoles for the measured sources. In delay configuration where a single source (consisting and sum beamforming, the array is electronically of a piezo-electric cone tweeter) was located “steered” to a series of chosen source locations. 8 feet directly in front of the array. Both pure For each selected steering location, a steering tone and white noise signals were recorded by vector eˆ containing a retarded time phase the array microphones and processed using the adjustment for each microphone in the array was data analysis procedures described previously. defined as All calibrations performed on the ECM LADA were repeated for the reference LADA, using the & same calibration frequencies, source strengths, r & 1 exp[j(k ⋅x )] and source locations. r 1 c eˆ = (cid:23) (4) Performance Metrics: Two quantitative metrics rm exp[j(k&⋅x& )] were used to assess the accuracy of the r m beamformed output when using the ECM LADA c to characterize point sources in the ANRF. The first metric consisted of computation of a &(cid:2) &(cid:2) weighted RMS error for the beamformed output where (cid:2)k(cid:2) is the acoustic wave vector, (cid:2)x(cid:2)m is the of the ECM LADA compared with similar distance vector from the steering location to each output from the reference LADA. Using an microphone m, ω is the angular frequency, and expression similar to that presented by Arnold, the ratio (r /r) is included to normalize the et.al.18, the weighted RMS error summed over all m c distance related amplitude to that of the phase steering locations measured by the array can be center of the array. The output power spectrum expressed as (or response) of the LADA at the steering location was obtained from ∑K T (R −T )2 k k k P(eˆ)= eˆTGˆeˆ (5) εRMS = k=1 K (6) M2 7 American Institute of Aeronautics and Astronautics where T is the normalized measured response by very similar between the theoretical response, k the ECM LADA, R is the normalized measured ECM LADA output, and reference LADA k response by the reference LADA, and K is the output. The lower right portion of each figure total number of measured steering locations. shows a horizontal slice through each contour The second performance metric consisted of plot (normalized to the reference LADA output) comparing the beamformed output of the ECM and further illustrates the uniformity of the LADA with its theoretical response. Assuming a sidelobe structure when comparing the two simple monopole source, the LADA ideal array arrays. response can be expressed as Table II lists the weighted RMS errors for frequencies spanning the range of W(k,x&,x&o) ≡ ∑M ro ejk[(ro−r)−(rmo−rm)] (7) 3n1or5m Hazli z- e2d0 kHEzC, Mco-m puatnedd usirnegfe reeqnnc.e -(L6A) DfoAr ro m=1 m beamform outputs similar to those shown in Figures 14 - 16. The error values are plotted in & where x is an arbitrary Cartesian location in Figure 17. It is clearly seen in this figure that the weighted RMS errors are higher for very low space to which the array is electronically steered, & beamform frequencies, but approach a xois the source location, ro and r oare the & m frequency-invariant average value of 0.0046 distances from the source to x and the mth above 2500 Hz. This is a remarkable result and c indicates that for the data obtained using a single microphone, respectively, and r and r&m are the point source in the ANRF, the beamform results distances from the steering location to x and the c when using the reference and ECM LADA’s are microphone, respectively. The response shown extremely consistent above a frequency of in eqn. (7) is normally expressed in decibels & 2500 Hz. Interestingly, this frequency also referenced to the level obtained at xo : corresponds closely with the theoretical low frequency limit of the array microphone pattern & & for the LADA. & W(k,x,xo) dB(x) = 20log & & (8) 10W(k,xo,xo) Cold Flow Jet ECM LADA Demonstration Along with the beamforming evaluation using Eqn. (8) is plotted as a color cont&our map with distributions of point sources, it was also desired contour level proportional to dB(x), computed to demonstrate the utility of the ECM LADA in a over the complete ensemble of measured steering real-world application. The authors chose as a locations. Color contour maps generated using demonstration to apply the array in a series of eqn. (8) over a range of beamform frequencies simple measurements of source strength and were compared with their experimental directivity in a cold flow jet operated at high counterparts to assess the response fidelity of the subsonic exit velocities in the ANRF. Note that ECM LADA. it is beyond the scope of this paper to present a detailed study of jet noise using the array. Sample Results: Figure 14 illustrates the Rather, the goal of the demonstration was to theoretical and experimental array responses for apply and evaluate the ECM LADA in a rich the ECM and reference LADA’s measuring the sound field containing broadband noise and high 5-kHz narrowband component of a generated background levels. white noise source. The tweeter source (shown The jet which was chosen for the in Figure 13), was located with respect to the demonstration was a 2-inch diameter, Mach 1.0 center of the LADA at (0.0, 0.0, 96.0) inches. contour convergent nozzle which had been The theoretical response was computed using previously used for measurements of mean eqns. (7) and (8), while the experimental pressure and farfield acoustics.19 Figure 18 beamformed output of the LADA was obtained shows a photo of the nozzle mounted in the using eqn. (5). Figures 15 and 16 illustrate ANRF with the ECM LADA shown in the similar responses for the 10-kHz and 20-kHz background. The majority of the nozzle shown in narrowband component of the white noise the photo was covered with acoustic foam to spectrum, respectively. As can be seen in these prevent sound reflections. The jet was operated figures, the spatial distribution of sidelobes is at high subsonic exit velocities spanning Mach 8 American Institute of Aeronautics and Astronautics numbers from 0.50 to 0.99 to provide generated Panasonic WM-60A electret cartridge were noise which was then measured by the array. fabricated for use with the Langley Large Noise data were acquired at several orientations Aperture Directional Array. The results of the of the array with respect to the jet exit plane (i.e., investigation revealed a fabricated microphone various azimuth angles with respect to the jet frequency response suitable for directional array axis) to obtain a measure of the jet directivity. use, with minimal magnitude and phase response Figure 19 shows a sample noise location map for variations in the audio frequency spectrum and a narrowband frequency of 5 kHz obtained with moderate variations at higher frequencies. Also, the LADA positioned 8 feet from the center of the results using an ECM-populated LADA to the nozzle, pointing directly at the exit plane measure point source noise in the Langley (azimuth angle = 90 degrees). ANRF revealed excellent agreement between One useful characteristic of a DA system reference array data, theoretical array response, when measuring free jet flow noise is the array’s and the LADA beamformed output. Finally, a ability to extract from the beamformed data the successful demonstration was conducted using variation of the axial location of the jet peak the ECM LADA to measure properties of the source strength as a function of frequency. As noise field generated by a cold flow jet. Thus, discussed by Lilley20, the axial location data economical COTS electret microphones show should collapse to single curve when the great promise for array work, especially for frequency parameter is expressed as the Strouhal lower-frequency applications such as the jet number noise demonstration presented here. Additional fD studies similar to this one are planned for COTS St = j (9) MEMS microphones which have recently U become available. j where f is the acoustic frequency, D is the jet Acknowledgments j diameter, and U is the jet exit velocity. j Figure 20 shows the axial location of the jet peak The authors gratefully acknowledge the source strength as measured by the ECM LADA contributions of Mick Hartzheim of the for three different jet exit velocities spanning Raytheon Company in the construction of the Mach numbers from 0.5 to 0.99. Also shown in packaged electret microphones, and of Beverly the figure are two theoretical curves presented by Anderson of the NASA Langley Aeroacoustics Lilley in reference 20 which were obtained from and Hypersonics Propulsion Support Section in calculations based on Lighthill’s acoustic the construction, installation, and operation of analogy. The lower curve is based on the the cold flow jet. assumption of a zero initial thickness for the jet mixing region. The upper curve is based on the References assumption that the mixing region becomes fully turbulent a defined distance downstream from 1. Gramann, R.A., and Mocio, J.W., the jet exit. These two curves bracket the ECM “Aeroacoustic Measurements in Wind LADA results, indicating a good ability of the Tunnels Using Adaptive Beamforming array to extract useful information from the jet Methods”, Journal of the Acoustical Society noise data. of America, Vol. 97, Number 6, pp. 694- 701, 1995. Summary 2. Watts, M.E., Mosher, M., and Barnes, M.J., Directional array technology will continue to “The Microphone Array Phased Processing be an integral part of experimental aeroacoustics. System (MAPPS)”, AIAA Paper 96-1714, However, cost remains a limiting factor in the 2nd AIAA/CEAS Aeroacoustics Conference, construction of future high-density array systems State College, PA, 1996. which will be needed to implement advanced beamforming algorithms. In an effort to reduce 3. Meadows, K.R., Brooks, T.F., Humphreys, the construction costs of future arrays, the W.M., Jr., Hunter, W.W., Jr., and Gerhold, authors have undertaken a systematic C.H., “Aeroacoustic Measurements of a investigation of one class of economical sensing Wing-Flap Configuration”, AIAA Paper 97- device, the electret condenser microphone. A 1595, 3rd AIAA/CEAS Aeroacoustics series of customized microphones based on the Conference, Atlanta, GA, 1997. 9 American Institute of Aeronautics and Astronautics