Abstract
This paper describes the design of a high-sensitivity low-frequency microphone. It has two basic components, a microphone element and a preamplifier. The microphone element is of the condenser type, using an edge-clamped stretched-Mylar diaphragm. The condenser microphone is used as one arm of a capacitance bridge which is furnished excitation by an rf oscillator. The bridge output is an rf signal modulated by the variations in microphone capacitance caused by pressure changes in the incoming signals. After suitable amplification, the modulated signal is processed in a peak detector. The resulting low-frequency signals are directly coupled into an emitter follower output stage. This pressure-sensitive transducer provides a useful output at pressure levels from 0.01 to 40 d/cm2 and frequencies of 0.1 to 100 cps. The frequency response of the microphone can be varied to suit system requirements by minor adjustments of either the microphone element or the preamplifier. Differences of phase shift between two or more microphones can be held to less than 1° at 20 cps. (The work reported herein was supported in part by a U. S. Army Signal Corps contract.)
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