Extraction of infrasonic waveforms from conventional condenser microphones

There are several commercial measurement-grade condenser microphones that are suitable for measurements down to about 0.1 Hz; however, these one-half-inch microphones reach those low frequencies with an in-line attenuator that compromises the overall dynamic range. In a previous paper [T. M. Marston and T. B. Gabrielson, J. Acoust. Soc. Am. 119, 3378 (2006)], a process was described for digital reconstruction of the lowest frequencies that avoids the attenuator and the sacrifice in dynamic range. This work has been extended to evaluate a reconstruction process for a one-quarter-inch condenser microphone, which has significantly higher low-frequency roll-off than the one-half-inch infrasonic microphones. One reason for using the one-quarter-inch microphone is its higher peak pressure limit. By modeling the preamplifier input-impedance bootstrapping and the pressure-equalization leak, extension to a few tenths of a hertz can be performed. However, the pressure-equalization leak characteristic frequency varies from microphone to microphone and with temperature. Consequently, there is a single free parameter that requires a measurement of the actual very-low-frequency response or measurement of a physically constrained reference waveform (like the N-wave from a clean sonic boom). [Work supported by the FAA/NASA/Transport-Canada Center of Excellence for Aircraft Noise and Aviation Emissions Mitigation.]