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This paper presents the theory, design, and validation of a microphone calibrator used to calibrate multiple microphones simultaneously. This work was done in conjunction with the development of an acoustic energy density probe, which was used to validate the calibrator. The probe uses multiple microphones to acquire the data needed to compute the acoustic energy density. The probe microphones are 0.006 m diameter electret microphones which typically do not have an ideal “flat" response over a wide range of frequencies as compared to precision condenser microphones. The probe microphone characteristics prompted the need for simultaneous, multi-microphone magnitude calibration. The idea behind the calibration process was to simultaneously subject each microphone on the probe to the same known acoustic pressure over the frequency range of the probe (0-2 kHz). This is done using equal-length small-diameter tubes connected to a single source at each microphone. The calibrator was modeled using and equivalent circuit model. The model results are presented and compared to measured results. The calibrator was validated to result in the same pressure along individual paths to each microphone simultaneously. Test results show that the calibrator can calibrate each probe microphone within ± 0.5 dB up to 2000 Hz, and within ± 1 dB up to 4900 Hz with a confidence level of 95%. Directivity tests were performed to further characterize the probe. Using a correction factor the probe is shown to be directionally independent.

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This thesis describes the phase and amplitude correction of 12.7 mm diameter, Type-1 microphones for three frequency bands, including within the infrasound regime, and its application to acoustic measurements. Previous data stem from acoustic intensity measurements using two-dimensional, four-microphone probes, which emphasized the requirement of having the acoustic phase and amplitude difference be much greater than the interchannel mismatch. Although correcting the amplitude/phase is well-known, obtaining the necessary transfer functions in the infrasound regime is challenging because (1) signal-to-noise ratios are often poor, (2) long measurement times are required for averaging, and (3) microphone responses vary significantly across these low frequencies. In this paper, a convenient infrasound source previously studied for infrasound adverse effects on humans is intended for performing a relative calibration. This work also seeks to elaborate recommendations for probe spacing, averages or length recordings, and instrument mismatch. The last two chapters show the PAGE method application in infrasonic sources. Those chapters have the intensity measurements using free-field microphones with larger separation distances than commercial intensity probes but compact compared to state of the art in infrasonic arrays.