A Thermal Transducing Microphone

Bevan Diprose

New Zealand Acoustics, 16(4), pp.14- 27 . (2003).

As sound waves propagate through the air, adiabatic compression and expansion occurs in the air due to the longitudinal travelling wave1. These small fluctuations in air pressure generate a small local change in temperature of the air medium. By utilising the speed of sound dependence on temperature, a high frequency ultrasonic sound wave travelling across an audio frequency wave is phase modulated on its arrival at the receiver. A system has been developed which generates a 630 kHz ultrasonic wave in air and the resulting phase modulation of the received signal is demodulated by a circuit based on a standard FM receiver chip.