1. *Specificmanufacturer'snamesareexplicitly mentioned onlytoaccuratelydescribetheresearch performed. Theuseofmanufacturer'snamesdoes not implyan endorsement bytheU.S.Government nor doesitimplythatthespecifiedequipmentisthe bestavailable. The sensitivity of each microphone was determined using a calibrated pistonphone generating a regulated Sound Pressure Level (SPL) of 94 dB (relative to a reference pressure of 20 µPa). The sensitivity of an individual microphone is defined here as the calibration value (specified in units of mV/Pa) which when applied to the output of the excited microphone allows it to indicate an SPL of exactly 94 dB at 250 Hz. As can be seen in Figure 4, the sensitivities show a partial bimodal distribution centered around 6.25 and 7.75 mV/Pa, with a 6.5mV/Pa average sensitivity exhibited for the entire ensemble of microphones. The decrease in the sensitivity of the packaged microphones versus the raw WM-60A cartridges is due to the introduction of the equalizing circuitry. Note that the ensemble distribution is dominated by variations in the sensitivities of individual raw ECM cartridges only. Given the low-cost of these cartridges, if one desires a matched response among the microphones incorporated into a DA system, it is a simple matter to measure the sensitivities of a large sample of cartridges and then choose those with similar characteristics. However, this step was not performed for this study. Rather, the individual measured sensitivities of the packaged microphones were accounted for during analysis ofallcollected data. Broadband Frequency Response - Audio Spectrum: The broadband pressure response of each packaged microphone covering the audio frequency spectrum (250 Hz to 16000 Hz) was obtained using a standards-traceable Bruel and Kjaer 4226 multifunction calibrator which had been modified to allow access to the transducer driving signal. A special nylon coupler was fabricated to allow insertion of the microphones into the calibrator with minimal acoustic loss. Thecalibrator generatedaconstant SPLof94 dB and was driven by an external precision signal generator operated over the range of 250 Hz - 16 kHz in 250-Hz steps. Thedriving signal from the generator and the microphone output signal were recorded by a two-channel, PC-based transient data recorder with a per-channel sampling rate of 100 ksamples/sec. To obtain the magnitude and phase response of each microphone, the transfer function between the driving signal and microphone output were computed using Welch's averaged periodogram method.12
2. Broadband Frequency Response - Ultrasonic Spectrum: Magnitude and phase responses for each microphonefor frequencies above theaudio spectrum (16 - 40 kHz) were obtained using a substitution method freefield calibration. This calibration method conformed with the International Electrotechnical Commission (IEC) standard for calibration of microphone equipment.14Each fabricated electret microphonewas mounted in an 8-foot by12-foot by 6-foot high anechoic chamber directly in line with a series of 1.0-inch diameter ultrasonic ceramic transducers, as shown in the sketch in Figure 6. Each transducer was positioned 74 inches from the microphone. The shortest microphone to chamber wedge-tip distance was approximately 24 inches. A custom mounting bracket with attached guide bar, shown in Figure 7, allowed correct alignment of each microphone in the chamber such that all diaphragms occupied as closely as possible the same axial location with respect to the source. Freefield calibration data were obtained at frequencies of 23.176, 29.976, and 39.576 kHz by operating each of three different ceramic transducers at its resonant frequency (to eliminate higher-order source diaphragm vibration modes). The transducer driving signal and the microphone output signal were recorded bythe same two-channel PC-based transient data recorder used for the audio spectrum calibration, at a per-channel sampling rate of 100 ksamples/sec. Upon completion of acquisition of calibration data using the three ceramictransducers, the electretmicrophonewas replaced with a standards-traceable Bruel and Kjaer 4136 reference microphone and a repeat set of calibration data was acquired using the three transducers. The freefield magnitude and phase response of the electret microphone at the resonant frequency of each transducer was obtained using Welch's method in a similar manner totheaudiospectrum calibration.
3. Total Harmonic Distortion: The total harmonic distortion (THD) for a series of raw ECM cartridges was determined using an externally driven Whittaker PC-125 acoustic calibrator excited at a frequency of 1 kHz for a range of sound pressure levels spanning 100 - 140 dB. The THD for a microphoneis defined as theroot mean square (RMS) value of the total harmonics of a measured calibration signal, divided by the RMS valueofthefundamental plus harmonics of the signal. This can be expressed in percentage formas
4. Figure 10 shows the THD measured on ten representativecartridges. Indicated on theplot is the average SPL level at which the THD reaches four percent. Sound pressure levels encountered during aeroacoustic testing in ground facilities (anechoic chambers, treated wind tunnels, etc.) typically do not exceed 120 dB; therefore, the maximum expected THD when using these microphones in a DA application is approximately1-2%.