In designing a holographic side-looking sonar it is necessary to make clear the relations among the beat frequency f_b the pulse repetition frequency f_p and the shape of scanning beam of CRT, which affect the quality of reconstructed image. In this paper these relations are discussed and examined experimentally. Table 1 (a) and (b) show the relation between f_p and f_b which makes sure the reconstruction, and the effects of the shape of scanning beam, f_b and f_p on the reconstructed image are shown in Fig. 8. The results obtained here may be useful for designing a side-looking sonar.

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The measurement of noise source contributions is to determine the amount of acoustic power contributed to the field at a given observation point by each of several noise sources. K. W. Goff presented an approach to this measurement problem that, by the use of a correlation technique, yields the desired results without requiring any control of the sources involved. However, the practical application of this method is limited by the severe restricting conditions imposed. This paper proposes a new approach to the problem using a cross-spectral density function of the acoustic signals. This method of analysis in the frequency domain eliminates those restrictions inherent in the correlation technique. Referring to Fig. 1, x(t) and y(t) are the acoustic signals at the source in question and the observation point, respectively. G(f) is assumed to be the frequency transfer function between these two points. Not only pure delays dut any linear transfer characteristic can be assumed for G(f). The signal z(t) is the component of y(t) due to the source in question. The acoustic signals origination from the other sources are represented together by n(t). Although the signal z(t) is not directly observable, it is possible to estimate its power spectrum using the spectra of the other signals as shown by Eqs. (2), (3) and (4), in which Ф indicates the spectra of the signals designated by the respective subscript indices. The average power of z(t) is obtained by integrating the estimated power spectrum Ф_zz(f) over the frequency range of interest. Therefore, the power contribution desired is given by the ratio of this integrated value to that measured power spectrum Ф_yy(f) as in Eq. (5). Fig. 4 is an experimental result obtained with the arrangement shown in Fig. 3. The solid line in Fig. 4(b) is the power spectrum of the total acoustic signal at the observation point P and the dotted line represents the estimated power spectrum of the component due to the speaker A. The correlation technique fails to give the correct result in this case because the two peaks of the cross-correlation function corresponding to the direct and reflected acoustic signals overlap as shown in Fig. 4(a). However, the cross-spectral method is applied without any difficulty, yielding the measured contribution of 45. 3%. This agrees well with the directly measured value of 45. 0% obtained by silencing the speaker B. In order to verify the applicability of the cross-spectral method to highly periodic noises, an attempt was made to separate the exhaust noise of an automobile from the engine noise. A microphone for collecting the total acoustic signal was placed 5 meters from one side of the car and a second microphone, for picking up the sound at the source, was placed next to the exhaust outlet just out of the gas stream. In Fig. 5, the large peak of Ф_yy(f) in the frequency range below 50 Hz is the power of the wind noise produced at the structure of the microphone. The peaks at 65Hz and 130 Hz are mainly due to the exhaust noise, while the power of the engine noise, except for the fan noise component of 160 Hz, disperses widely to higher frequencies.

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An H-type resonator vibrating transversely can be used as a low frequency resonator similarly to a tuning-fork or a uniform bar which has been used for electro-acoustical devices. Because, the H-type resonator has following merits. (1) In a limited mode, the resonant frequency can be lowered as compared with that of the tuning-fork or uniform bar, (2) the resonant frequency is decided by adjusting not only dimensions of arm but also those of connecting coupler. And, it is possible to obtain lower frequency by forming stepped-arm (or adding concentrated mass to the tip of the arm). Such resonator is called the stepped H-type resonator (Fig. 1). In this paper, analyses of the resonant frequency (Chapter 3), vibration mode (Chapter 4) and equivalent circuit constants (Chapter 5) of the stepped H-type resonator are carried out by using an equivalent mechanical network (Fig. 3) corresponding to a quarter part of the resonator (Fig. 2) and taking symmetry of the vibration mode in consideration. The connecting coupler is assumed as pure bending stiffness, because it is a slender bar as compared with the arm of the resonator. First, relation between the resonant frequency and dimensions of the stepped H-type resonator is discussed (Fig. 4, 5, 6), and a convenient chart for design is presented (Fig. 6). And, theoretical values of the resonant frequency are compared with experimental values (Fig. 4, 6). Secondly, the vibration mode of the arm is calculated for some values of stiffness determined by the dimensions of coupler (Fig. 8). Furthermore, the equivalent circuit constants of the resonator, i. e. equivalent inductance, capacitance and capacitance ratio, are calculated by taking impedance of bonded piezoelectric ceramics in consideration (Fig. 9), on the basis of the vibration mode. As the results, it is found that the theoretical values are in approximate agreement with experimental values (Fig. 10, 11). Finally, the optimum dimensions of piezoelectric ceramics giving the minimum value of the capacitance ratio are shown (Fig. 12, 13).

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