Aerodynamic impact sound from colliding spheres for calibration of microphone systems

Tentative damage-risk criteria for hearing and material design standards require measuring the peak values of impulsive sounds. However, use of current or proposed ANSI and IEC standards provides only the magnitude calibration in response to sinusoids, and does not ensure accuracy in measuring the peak values of impulsive sounds. We have used digital transient capture and FFT techniques with a 3.2-mm-diam microphone system calibrated in magnitude and phase to measure peak sound pressure levels and spectra of impact sounds produced by a prototype source. This source uses an essentially identical pair of colliding steel spheres of either 25.4- or 44.5-mm diameter to produce peak sound pressure levels within the range 100-120 dB re 0.00002 Pa with precisions (ranges of peak amplitudes of nominally identical impacts) of 0.5 dB or better, and with spectra exhibiting fairly broad maxima within the range 3-10 kHz. For some conditions, at least, the waveform of sound pressure is approximately diphasic, and of duration <0.5 ms. The results demonstrate the feasibility of a simple, robust sound source suitable for use in moderately noisy, nonanechoic environments to check the free-field response of microphone systems, and subsequent instrumentation, to impulsive sounds.