An infrasound calibration system for characterizing the environmental effect on sensor sensitivity and noise floor

Abstract

An infrasound calibration system has been developed at the National Center for Physical Acoustics. The calibration tank is comprised of a 1" cylindrical shell 40" in diameter, 40" long, with 40" diameter hemispherical end caps. The interior volume of the tank is approximately 1.8 cubic meters. Each hemisphere has a 10" punch-out with sealing gasket that allows either a speaker assembly or an “end cap” to be attached to each end. Normal assess is through an end cap attached to one end, with a speaker assembly attached to the other. The end cap allows rapid switching out of sensors. The speaker assembly consists of a 10" subwoofer with a sealable back volume designed to equalize the static pressure in the interior of the tank to that of the speaker back volume. The subwoofer is able to generate pressures up to 10-Pa in the interior of the chamber, with excellent isolation from external sound. Using 2 subwoofers, by playing a different frequency tone through each speaker and measuring their associated intermodulation distortion in the transduced signal, the linearity of the sensor can be accurately assessed. The measured leak constant of the tank is longer than one week, permitting the characterization the sensor response to ambient pressure to be measured. The large interior volume of the calibration chamber allows the interior be heated or cooled returned slowly to ambient conditions, so that the effects of changing temperature on the sensor can be assessed.