Abstract
A microphone system has been designed and tested for use in measuring VLF atmospheric-pressure variations. The system is a cylinder of over-all dimensions 5 in. in diam and 6 in. long, containing an inlet tube, two volumes connected by a bypass tube, the microphone, and its electronics. Essentially, a capacitor microphone (with very low self-noise electronics) has an acoustically transparent back electrode. Each surface of the membrane is exposed to an acoustic low-pass filter, i.e., a tube and a volume. Fixed parameters are: the 1-in.-diam 0.25-mil-thick stainless-steel diaphragm that resonates around 1 kHz; the low-frequency volume of 1000 cm3, and the high-frequency volume of 50 cm3. The variable parameters are the tube diameters (10 to 20 mil) and lengths (up to a few inches) that serve to establish the bandpass range. The system has been pistonphone calibrated at LTV Research Center and UCLA, with a nominal sensitivity (adjustable) of −58 dB re 1 V/μbar. The operational range can be selected within 0.001 to 100 Hz. The system has been packaged in a weather-resistant configuration.
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