A research team from the Aerospace Information Research Institute of the Chinese Academy of Sciences has achieved real-time atmospheric temperature measurements at altitudes of up to 5,200 meters using a new acoustic thermometer mounted on a tethered aerostat. Their findings were published as a cover article in The Journal of the Acoustical Society of America.

Monitoring atmospheric temperature at high altitudes is essential for meteorological forecasting and climate change research. Traditional electronic thermometers can be unreliable due to the effects of solar radiation and exhibit slow response times in thin air. Infrared thermometers, on the other hand, are not suitable for directly measuring gas temperature.

Acoustic thermometry presents a high-accuracy alternative by directly detecting atmospheric temperature with a wide dynamic range and robust performance. However, existing methods depend on precise distance control between a microphone and speaker to measure sound speed related to air temperature, which limits their versatility and suitability for use on aerostat platforms.

To tackle these challenges, the research team developed a passive acoustic temperature measurement method based on the acoustic Fabry–Perot resonator (AFPR). The AFPR consists of a tubular acoustic waveguide and an electret condenser microphone (ECM) with its head inserted into the waveguide. An acoustic FP cavity is formed between the ECM diaphragm and the open port of the waveguide, and its resonant modes can be excited by ambient white noise. This design allows for rapid air temperature measurement by determining the resonant frequencies of various modes without the need for a separate sound source.

The AFPR is a low-power, non-contact, and rapidly responsive passive acoustic thermometer. Its applicability has been demonstrated in an anechoic room with very weak white noise (–9.2 dB relative to 20 μPa). The measurement error of the AFPR in this controlled environment is less than 0.1 °C compared to readings from a commercial electronic thermometer.

Furthermore, the researchers conducted several high-altitude passive acoustic temperature measurements in various locations in China, including Kashgar and Hengdian, by mounting the AFPR on a tethered aerostat. During the ascent, pause, and descent of the aerostat, the frequency response curves of the AFPR were measured at intervals of 1.25 seconds. From each curve, a linear relationship between resonant frequencies and resonant mode order was established, allowing the researchers to deduce air temperature at corresponding altitudes.

Test results confirmed that the AFPR can accurately measure air temperature variations with altitude, showing a difference of no more than 0.5 °C compared to the electronic thermometer used. Experimental measurements at different altitudes and under various climatic conditions validated the high stability and environmental adaptability of the AFPR.

Due to its simple structure, high sensitivity, and low power consumption (including wireless data transmission of less than 2.5W), the AFPR is well-suited for deployment on unmanned aerial vehicles for remote sensing applications. Its potential applications span atmospheric science research, environmental monitoring, high-altitude, and near-space detection, among others. Additionally, the AFPR-based passive acoustic thermometry can integrate with sound source localization, acoustic wind measurement, and other technologies to establish a multi-parameter high-altitude acoustic monitoring network.

This work was funded by the National Key R&D Program of China and the National Natural Science Foundation of China.