地下埋設物探査用電磁誘導形送波器

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An electromagnetic induction type sound source is a useful sound source for detection of the bodies buried at depth of less than 2 or 3 meters under the ground surface by the pulse-echo method due to radiating an intense impulsive wave of good reproducibility. In this study, theoretical and experimental analyses about the radiation characteristics of this sound source placed on the ground surface were performed. This sound source is composed of a flat spiral coil cemented on a bakelite plate, an aluminum diaphragm contacting with the coil and a weight set on the bakelite plate as shown in Fig. 1. Equation (8) expresses sound pressure of an impulsive sound wave radiated from the sound source and transmitting towards the direction just below the sound source. Fig. 2 shows a waveform of radiated sound pressure calculated by Eq. (8). Eq. (8) indicates that the peak pressure of the impulsive sound is proportional to the frequency of driving current fe and the input energy N, but independent of the radius of diaphragm a. Power-spectrum and directional characteristics of the radiated sound are expressed by Eq. (11) and Eq. (12) and shown in Fig. 3 and Fig. 4 respectively. Experiments in the laboratory and in the field were carried out in order to justify the theoretical expression. In the laboratory semi-cylindrical acrylite sand bath shown in Fig. 5 and a small sound source were used. The directional characteristics and the peak pressure as functions of the input energy were measured. The results are shown in Fig. 4 and Fig. 6 respectively. In the field test, a sound source shown in Fig. 7 was used. As shown in Fig. 8, the radiated sound pressure was measured with a piezoelectric microphone (sensitivity in water:-101. 3 dB at 8 kHz) buried at depth of 1 m in sand just below the sound source, and the distance from the sound source to the microphone was varied from 1 m to 20 cm by digging a hole. In this experiment, the frequency of driving current was increased from 200Hz to 1200Hz by decreasing condenser capacitance from 180μF to 5μF, and the input energy was kept at 250 joule by increasing the charging voltage as the capacitance is decreased. The power-spectrum and the peak pressure as functions of the driving current frequency were measured. The results are shown in Fig. 9 and Fig. 3 respectively. In Table 1, the measured values of the peak pressure are compared with the theoretical values obtained by Eq. (8) or Eq. (9) using n=350, L=3mH. Cp=340m/s, d=0. 2m and N=250 Joule. In Fig. 10, a waveform of microphone output in case of 20 cm in distance and 200Hz in driving current frequency is shown. From the fact that the measured values agree with the theoretical ones with reasonable accuracy, the equation (8) was proved to be appropriate as a theoretical expression of the sound pressure radiated from the electromagnetic induction type sound source.
社団法人日本音響学会の論文
1973-07-01

地下埋設物探査用電磁誘導形送波器

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