This paper shows that a sealed-box loudspeaker can be used as the sensor for a very high-performance infrasound microphone. Since the cone displacement essentially responds directly to infrasound pressure, the velocity-induced loudspeaker output must be electronically integrated to give a flat response. The undamped resonance peak of the loudspeaker is avoided by feeding the short-circuit voice coil current into the virtual ground input of an integrator op-amp. Design equations are given and a complete response analysis is presented. A prototype is compared with a conventional microphone used for infrasound measurement, showing the improved performance of the sealed-box loudspeaker design.
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