In‐situ calibration of infrasound array elements.

Abstract

A typical infrasound array element consists of an infrasonic pressure sensor connected to a multiple‐pipe or porous‐hose system for reduction of wind noise. While the frequency response of the sensor itself may be known, that response is modified by the wind‐noise reduction system. One approach to measuring the frequency response of the complete system is to perform a comparison calibration using ambient noise and a co‐located reference sensor with sufficiently low self‐noise and well characterized frequency response. In the technique discussed here, the reference sensor is a virtual reference constructed by summing the outputs of two or three calibrated microphones; symmetric placement of the microphones places the phase center of the virtual reference at the geometric center of the pipe system. Proper combination of auto‐ and cross‐spectral averages over a several‐hour period produces an estimate of the response of the infrasound system relative to that of the virtual reference. Measured coherence and the consistency between the magnitude and the phase of the response provide quality checks on the process. This approach has been demonstrated at infrasound monitoring sites in Washington, Alaska, Manitoba, Austria, and Antarctica. [Funded by the US Army Space and Missile Defense Command.]