Calibration and characterization of the response of infrasound sensors to environmental factors

An infrasound calibration system has been developed at the National Center for Physical Acoustics. The calibration tank is comprised of a 1 in. cylindrical shell 40-in. in diameter, 40-in. long, with 40 in. hemispherical end caps. The interior volume of the tank is approximately 1.8 cubic meters. Up to eight normal-sized infrasound sensors can be enclosed in the volume for one measurement session. Pressure and temperature in the interior and exterior of the tank are also monitored. The pressure source is a 10-in. driver, which is calibrated using multiple barometers that are placed internal to the tank. The measurement paradigm is to drive the tank at a constant amplitude (typically 10-Pa peak-to-peak) and known frequency (typically 0.5 Hz), and to track the variation in the measured response of the test sensor with respect to multiple reference sensors in order to characterize the effect of manipulation of the environment (static pressure, temperature, and seismic motion) on the test sensor. For sensors with very low nonlinearity, allowing the static pressure to change while the driving amplitude of the speaker was held constant was found to be necessary for assessing the effect of nonlinearity on these sensors. Variations in sensitivity on the order of 50 ppm can be measured in a 10-s interval.