階段状速度分布を有する円形振動膜の指向性合成について

抄録

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&gt10 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).