Non-contact air temperature estimation based on ultrasonic propagation time measured by a laser Doppler vibrometer

Abstract There is increasing demand for measuring and monitoring air temperature in various fields of science and technology. This study proposes a method to estimate air temperature by measuring ultrasonic propagation time using a laser Doppler vibrometer (LDV). The LDV detects the fluctuation of refractive index of air induced by ultrasound through optical interferometry, enabling non-contact sensing of ultrasonic propagation crossing the laser beam of the LDV. Sound velocity is calculated from ultrasonic pulse arrival time and converted to temperature using an empirical relation. Experimental results in a temperature-controlled chamber showed agreement within ±0.5 °C compared with thermocouples ( R 2 = 0.96) where ultrasonic pulse were generated using a speaker unit with center frequency at 40 kHz. The method also exhibited faster response to rapid temperature changes, overcoming delays inherent to contact sensors. These results indicate the potential of the LDV-based ultrasonic sensing for real-time temperature monitoring in environments where conventional sensors are impractical, such as cleanrooms, smart buildings, and environmental applications.