PURPOSE: To secure a high-efficiency selection of the low frequency to be used for the system of the underwater acoustic wave transmission, by applying the electric signal amplitude-modulated via the low frequency desired for wave transmission from the high frequency to the nonlinear cable transmitter.

CONSTITUTION: High frequency H and (a) of a fixed amplitude undergo amplitude modulation M via low frequency acoustic wave L and (b). As a result, the amplitude-modulated voltage waveform (c) emerges at the terminal of cable transmitter T, and at the same time the current of the similar waveform flows to T. Thus, a large amount of vibration amplitude is obtained to the driving force of positive direction of T, and the opposite nonlinearity is obtained in the negative direction. In this case, acoustic wave (d) is transmitted with its average value (1-dot broken line) featuring nearly the same waveform as original low frequency (b). The high frequency component of this transmitted wave (d) shows a large amount of attenuation during the underwaver propagation. On the other hand, the low frequency component features a small amount of attenuation and reaches a remote distance. In this way, the acoustic wave of an extremely low frequency can be transmitted via the miniature cable transmitter which resonates to the high frequency.