Passive acoustic measurements of air temperature at various altitudes

Acoustic thermometry is a fast, noncontact temperature measurement method that does not require heat exchange and, thus, is suitable for real-time monitoring of changes in air temperature at high altitudes where the thin air is not conducive to establishing thermal equilibrium. In this work, real-time measurements of air temperature at altitudes of up to 5200 m were achieved using a passive acoustic thermometer, which is an acoustic Fabry-Perot resonator (AFPR), consisting of an electret condenser microphone and an acoustic waveguide. The resonant frequency (fR) of the AFPR as a linear function of the mode order number (m) is measured using ambient white noise instead of external sound source, and the air temperature is determined based on the slope of the fR versus m curve. The surface air temperature changes in Beijing and the Kashgar Plateau were measured in real time over more than 15 h using the AFPR. By mounting the AFPR on a tethered balloon, the continuous monitoring of air temperature during liftoff and descent of the balloon was tested. The average deviation between the results simultaneously measured with the AFPR and commercial electronic thermometer was less than 0.5 °C, which verified the reliability of the AFPR-based passive acoustic thermometry.