A new acoustical imaging by means of frequency-differential hologram matrix

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In the conventional multifrequency holography, a continuous carrier with frequency f is transmitted and the phase and amplitude of the received signal is measured to get the frequency response H(f) associated with transmitting and receiving transducer arrays. As a new scheme of CW sonar, however, we have introduced the concept of the FDHM ( frequency-differential hologram matrix) D(f). The scheme makes it possible to compensate the propagation loss, so that a weak signal from a far-region target can be enhanced.To measure the FDHM, we transmit a carrier frequency-modulated by a sinusoidal signal, and the received signal is first phase-detected and then product-detected using the sinusoidal modulation signal. Making up such transmitter and receiver, We constructed a computercontrolled system for measuring the FDHM. We then measured the FDHM at 256 different carrier frequency and the data was Fouriertransformed to get a one-dimensional image of a target. As a result, several merits of the method were experimentally proved, such as canceling of unwanted effect due to coupling between transmitter and receiver, improvement in signal to noise level by about 15 dB, compared with the result by the frequency response method, and the compensation of the propagation loss by which a weak signal from a target in the far region can be enhanced.Next, for designing a two-dimensional imaging system by means of the FDHM with respect to a linear array, a computer simulation has been carried out. It was shown that the FDHM measured with an array with 32 transducer elements and 32 carier frequencies gives a clear image of a target. We also propose to use the secon-order FDHM, because it gives an image of targets with uniform intensities.