Avoiding the audio range: Frequencies very high and very low

Abstract

Condenser measurement microphones have been an essential component of acoustic measurements for many years. Although used most often in the “audio range” (20 to 20 000 Hz), they are also capable of quality measurements both well above and well below that range if their limitations are respected and if their outputs are processed carefully. For example, below 20 Hz, the natural low-frequency roll-off provides an automatic pre-whitening for ambient spectra, thereby reducing the dynamic range required, and the roll-off can be compensated by a simple pole-moving filter in post-processing. Well above 20 kHz, diffraction can be significant, the characteristics of an analog-to-digital converter's anti-aliasing filter can be important, and transit time across the surface of the microphone's membrane may not be negligible. Nowhere are these characteristics more important than in measurement of the rise time of acoustic shock waves. If accurate reproduction of the time-domain waveform is important, then any compensation schemes must account for the system phase response as well as the magnitude response. A diffraction-compensation filter can be effective in minimizing waveform over-shoot and a carefully designed baffle can delay the onset of diffraction effects if leading-edge phenomena like peak pressure and rise time are important.