Broadband ultrasound emission from a three-layer thermo-acoustic transducer

A thermophone generates sound with temporal variation of Joule heat which leads to expansions and contractions of a small volume of air near the surface of the metallic film. Consequently, the SPL of the sound is proportional to the input power and the emitted sound is the second harmonic of the input signal.1 Here, the fundamental characteristics of a three-layer thermophone (200-nm-thick Pt film, 5-μm-thick glass heat-insulating layer, and 500-μm-thick Si heat-releasing layer) were investigated. The device (10×12 mm2, 2.32 Ω) showed a broadband frequency response up to 140 kHz, and its SPL level at 50 mm was nearly constant (61.5±3 dB) from 20 to 140 kHz at 1.8 W power consumption. The conversion efficiency was 2.68×10-5%, which is 150 times the efficiency of the authors’ previous prototype, in which the heat release was less efficient (594-μm-thick alumina layer). Furthermore, by applying both AC and DC currents to the thermophone, it emitted the fundamental and the second harmonics of a 50-20 kHz FM sound, and hence a broadband (100-20 kHz) compound signal. These results and this device’s simple manufacturing process suggest that it may be used as a high resolution ultrasound sensor. 1Shinoda et al. (1999), Nature, 400(6747), p.853