Visualization of airborne ultrasound field using thermal images

Thermoacoustics, the interaction between heat and acoustic waves, is used in a wide range of advanced technological applications. Conversion of ultrasound waves into heat is employed in medical applications, including tumor ablation. However, thermal responses to airborne ultrasounds have not been extensively studied. Herein, we show that thermal responses to high-intensity airborne ultrasounds above 1000 Pa can be used for acoustic measurements. Thermal images on a mesh screen enabled the real-time visualization of three-dimensional acoustic fields. Based on the temperature distribution on a surface that reflects ultrasound waves, it was possible to determine the ultrasound focus position with a resolution much finer than the wavelength. Our results demonstrate that thermography observations based on ultrasound-temperature conversion can be used to visualize acoustic fields, which are conventionally difficult to observe. Our findings will advance technologies that use strong airborne ultrasound, such as mid-air haptics and acoustic levitation, and provide new scientific and technical tools for detecting surface properties and composition, including cells and fragile materials.