High‐temperature fiber‐optic lever microphone

The design and construction of a fiber‐optic lever microphone, capable of operating continuously at temperatures up to 538 °C (1000 °F), are described. The design is based on the theoretical sensitivities of each of the microphone system components, namely, a cartridge containing a stretched membrane, an optical fiber probe, and an optoelectronic amplifier. Laboratory calibrations include the pistonphone sensitivity and harmonic distortion at ambient temperature, and frequency response, background noise, and optical power transmission at both ambient and elevated temperatures. A field test in the Thermal Acoustic Fatigue Apparatus at Langley Research Center, in which the microphone was subjected to overall sound‐pressure levels in the range of 130–160 dB and at temperatures from ambient to 538 °C, revealed good agreement with a standard probe microphone.