Abstract
The thermal noise of condenser microphones was measured in an instrumentation system incorporating a novel vacuum‐isolation vessel (see preceding abstract), at an isolation pressure of 10−5 Tort, and an FFT spectrum analyzer, covering a frequency band from 1 Hz to 25.6 kHz. The contributions to the thermal noise include the mechanical Johnson noise (due to the Brownian motion of air molecules impinging on the microphone membrane), preamplifier noise (1/f and 1/f2 components), and background noise from the analyzer itself. Measurements on a dummy microphone, having the same capacitance as the corresponding measurement microphone, permit the mechanical Johnson noise to be isolated. The experimental results are compared to a theoretical analysis by Tarnow [B&K Tech. Rev. 3‐1972].
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