Photoacoustic response of condenser microphones

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Abstract
The membranes of BK 4166 half-inch and BK 4145 1-in.-diameter condenser microphones were illuminated with a chopped light beam of a 5-mW He-Ne laser and the occurring microphone signal detected as a function of the modulation frequency. The response curves provide new information on the frequency dependence of a background noise light absorption by the microphone membrane produces in photoacoustic cells. The signal from the (1)/(2) -in. microphones was found to decrease monotonically with increasing frequency, and the signal phase had a limiting value of −π/2 rad at high frequencies. The corresponding magnitude curves of the 1-in. microphones showed a sharp minimum in the 200–340-Hz region, and the limiting phase was π rad. A complex thermoacoustic signal generation mechanism is proposed to explain some of these results.