Visualization of sound source positions using pinpoint nonlinear secondary emission by ultrasound focus scanning

This study proposes a method for visualizing sound fields utilizing midair nonlinear acoustic phenomena in a spatially localized manner. Conventional microphone-array-based sound field visualization method requires multi-channel synchronous signal processing that handles phase information of the observed waveforms, which inevitably hinders production of cost-effective recording devices. Additionally, the inserted microphones themselves can disturb the measured sound field, and artifacts owing to the spacing between microphones may arise. To address these issues, the study introduces a measurement method that involves scanning a focal point of converging ultrasonic beams in the target sound field. The ultrasonic focus generates secondary parametric waves via frequency modulation of the target sound field only near the focal point due to the acoustic nonlinear effect. The visualization of the target field is completed by demodulating these waves measured with a single immobilized microphone located outside the field. This technique achieves spatial selectivity of recording via steering of the ultrasonic focus serving as a parametric probe, allowing the target sound field information to be reconstructed from a monaural recorded signal. This approach of sound field visualization ranging over hundreds of millimeters is based on a single-channel recording, where no recording elements densely arranged in the target sound field are required.