An Interferometer Microphone
The Brownian or thermal noise limit has been reached up to frequencies of about 50 kHz in air using an axially symmetrical two-beam interferometer whose pellicle beamsplitter bulges under sound pressure. Frequency spectra of the Brownian noise observed in hydrogen, air, and CC12F2 are shown to be correlated to cavity resonant modes (or echoes) within the microphone.
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