Passive Acoustic Thermometry

Conventional acoustic thermometry is usually performed by exciting a resonator with a sound source and identifying the resonant frequencies of the system from which the speed of sound can be determined and from that the temperature. Alternatively a sound source and receiver separated by a short distance can similarly be used to determine the speed of sound, and hence temperature, by measuring the time delay between source and receiver. Described in this paper is an alternative approach to acoustic thermometry which we refer to as "Passive Acoustic Thermometry" because it does not require a dedicated sound source. Instead, background ambient noise is used to excite a resonator containing a microphone and an additional microphone outside the resonator replaces the sound source. It is demonstrated in the paper how the frequency response function between the two microphones can be used to determine resonant frequencies of the resonator and how this is used to determine air temperature. In principal the same device could potentially be used to also measure relative humidity and this, together with some potential applications is discussed.