The frequency responses of TA-type hydrophones consisted of the electro-acoustic transducers made of barium titanate are described. The physical structures of the barium titanate trancducers of the TA-type hydrophones are short and thin hollow cylinders and they receive the acoustic energy of the underwater, only from their external surfaces and not from their internal and upper and lower surfaces of the cylinders. The theoretical responses of the TA-type hydrophones should have the flat characters between the values near to ω_2 and ω_1, where ω_2 is the natural angular velocity of the transducer while ω_1 is the angular velocity decided by ω_1R_2C_0=1, R_2 being the load resistance of the electric terminal of the hydrophone, and C_0 the static capacitance of the transducer. The results of the measurements, however, show that the frequency characteristics are apt to deviate from their theoretical values by the various defects of the constructions of the hydrophones, and there are many difficulties to obtain the flat responses especially on the rod shape TA-hydrophones
Lately, K. Kikuchi and K. Fukushima presented a method of determining the stepwise velocity distribution on a circular membrane in an infinite screen which will produce an appropriate directional characteristic corresponding with an ideal one, that is assumed to be 1 in the regionχ_b≧χ≧0(χ_b:sine of the cutoff angle, χ:sine of deviation angle from the axis of a circular membrane), and zero in the remainder. The procedure of this method is thus; firstly, χ_b and the number of steps of distributed velocity (N) are selected, and the resultant directivity pattern is calculated. Usually, this is not the same as the ideal directivity and the deviation is not so small. And if the resultant pattern is not acceptable for the present purpose for the designer, χ_b and N are changed and the calculation is continued until the acceptable pattern is obtained. Finally the velocity distribution is decided. So the selection of χ_b and N is important. In the present paper, the author wishes to propose some charts which may facilitate the selection. The performed studies are mainly as follows:(1)The characteristic of convergency of the formulas after K. Kikuchi and K. Fukushima is examined, and some numerical examples concerned with the convergency are represented. (2)The resultant directivity patterns are calculated in the case of 0. 5≧χ_b≧0. 1, 10≧N≧2. The value and the angle of the first secondary maximum (refered to primary lobe's maximum) and the specified diminishing angles of directivity pattern are shown as functions of χ_b taking N as the parameter. (Fig. 8〜Fig. 13)By the use of these figures, χ_b and N which will produce an acceptable resultant pattern may be decided. (3)An approximation chart which may be used when N>10 is given in Fig. 14. (4)Frequency characteristics of directivity patterns are also discussed. And it is found that the mimimum of N which is necessary to suppress the secondary maxima which will appear when frequency is increased below the first secondary maximum is 7(or 6 if the slight excess is neglected).
The formula which gives the distribution of sound energy along a corridor radiated by a simple source of sound located in it, has been derived in connection with ratio of the height H and the breadth B and the absorption coefficients (α_ω1, α_ω2, α_c and α_f)of its walls, ceiling and floor. The results of numerical calculation of the formula were also presented for the case of H/B: 1, 1. 5 and 2, α_f:0. 03, α_ω:0. 2〜1. 0 and α_c:0. 2 〜0. 8. The calculated distribution shows good agreement with the observed distribution.
In order to perform the reciprocity field calibration in water, it is convenient to prepare three kinds of auxiliary transducers. The first kind to be used only as a projector must have high sensitivity as a projector and flat in frequency response. The second kind is to be used only as hydrophone with high sensivity and flat frequency response. The third kind to be used as both projector and hydrophone must have adequately high sensitivities as projector and as hydrophone together with flat frequency responses for both cases. The author derives the theoritical sensitivities both as projector and as hydrophone for the transducers employing cylindrical barium titanate shells as active elements and shows that these are suitable transducers for the above mentioned use. A projector, a hydrophone and four transducers used as both projector and hydrophone were designed and built. The projector has a cylindrical barium titanate shell of radius R =32. 5 mm and thickness h=5 mm, with the sensitivity as projector of 31±3 dB re 1 μbar per v at 1 m distance in the frequency range of 15 to 160 kc. The hydrophone has a shell of R=5. 4 mm and h =0. 8 mm, and the sensitivity as hydrophone is about -110 dB re 1 v per μ bar in the frequency range of 10 to 140 kc. Each of the four transducers used as both projector and hydrophone has a shell of various radius, and the both frequency responses as projector and hydrophone of each transducer are fairly flat near each resonant frequency (about 30, 40, 70 or 100 kc). Appling these transducers, calibrations in the frequency range of 15 to 140 kc have become easy.