Measurement of sound pressure sensitivity of high-frequency microphone using multiple harmonics in intense ultrasonic beams

• Published in: Acoustical Science and Technology Vol. 43; no. 5; pp. 269 - 275
• Main Authors: Kamakura, Tomoo, Hayashi, Hideo, Yasuno, Yoshinobu, Nomura, Hideyuki
• Format: Journal Article
• Language: English
• Published: Tokyo ACOUSTICAL SOCIETY OF JAPAN 01.09.2022; Japan Science and Technology Agency
• Subjects: Cellular polypropylene; Diameters; Frequency response; Harmonics; KZK equation; Microphone sensitivity; Nonlinear harmonics; Resonant frequencies; Sensitivity; Sound pressure; Waveforms
• ISSN: 1346-3969; 1347-5177
• DOI: 10.1250/ast.43.269

Intense ultrasonic beams suffer progressive waveform distortion due to the nonlinearity of the air, causing numerous harmonics to be generated in the beam. Since these harmonic pressures can be theoretically predicted with sufficient accuracy by the Khokhlov-Zabolotskaya-Kuznetsov model, it is possible to obtain the pressure sensitivity of a microphone at high frequencies. To generate intense ultrasonic waves, a planar aperture source of 118 mm in diameter and with resonant frequencies of 40 and 63 kHz was used. A specialized microphone developed and tested for receiving high-frequency ultrasonic waves was fabricated from a single cellular polypropylene (CPP) sheet of 50 μm in thickness and of a sensing area approximately 13 mm2. Using at least the first nine harmonics, i.e., those in the range of 40 to 567 kHz, it was demonstrated in this study that the frequency response of the CPP microphone has a sensitivity of −70 to −80 dB re. 1 V/Pa at frequencies below 400 kHz.