Field tests of a high-temperature fiber optic lever microphone

The operating principle of a high-temperature fiber optic lever microphone (FOLM), rated at 538 °C (1000 °F) continuous service, is based on intensity modulation of light reflected from a vibrating membrane. The frequency response of the microphone, calibrated in the laboratory at ambient temperature and 538 °C by the electrostatic actuator method, is typically 60–70 kHz. Applications to high-temperature field testing include the thermal acoustic fatigue apparatus at Langley Research Center, an advanced short takeoff and vertical landing model at Lewis Research Center, the Mach 22 helium tunnel at Langley (ambient temperature only), interior mixer-ejector surfaces at GE Aircraft Engines, Cincinnati, and a high temperature acoustic impedance tube at Langley. In most of these applications the FOLM made possible high-temperature, high-frequency acoustic data to be taken for the first time. Typical test results will be discussed.