  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.
