Calibrating infrasound reference microphones

Low-frequency commercial measurement microphones can be used for infrasound measurement well below their nominal low-frequency roll-off as long as the magnitude and phase of the infrasonic response is known. Three methods are used at Penn State for laboratory measurement of this response: (1) an “absolute calibration” in a piston-driven chamber designed for accurate determination of internal pressure based on piston displacement, (2) a “comparison” calibration in the same chamber where the response is compared to the output of a dc-coupled piezoresistive pressure transducer, and (3) a “hydrostatic” calibration based on the pressure change associated with a sinusoidal variation in elevation. For the first technique, a full thermo-viscous acoustic model for the chamber is used to bridge the adiabatic-to-isothermal transition and to convert piston volume velocity to pressure. The second technique depends on the calibration of the dc-coupled reference transducer. The third technique depends on the elevation-dependence of hydrostatic pressure. The first and second techniques are practical from 0.001 to 25 Hz; the third technique has a smaller practical frequency range—0.05 to 0.5 Hz—but permits an independent check on the other techniques. [Funded by the US Army Space and Missile Defense Command.]