1971 年 27 巻 10 号 p. 507-514
In order to design n artificial mouth measured for loudness, the knowledge of a property of diffraction coefficient of variously obstacles combined with the human mouth and with a spherical sound source is important. Therefore, we measured diffraction coefficients of various obstacles combined with the human mouth and with the spherical sound source, a detailed comparison of the diffraction coefficients of both methods is made. The diffraction coefficient of the obstacle as shown in Fig. 1 is represented as Eq. (4), where D_0 denotes the diffraction coefficient of the obstacle which is replaced with a rigid body, Z_L denotes input impedance viewed from the surface of the obstacle, and Z_r denotes radiation impedance of the obstacle combined with the sound source. The obstacle used in experiments to determine whether Eq. (4) is good or not was a telephone transmitter of the Japanese commercial type. The measured values of Z_r combined with the human mouth and the spherical sound source are shown in Fig. 3 and an equivalent circuit of transmitter is shown in Fig. 4. As numerical results of 20 log_(10)|Z_L/(Z_L+Z_r)|, which are shown in Fig. 5 by the solid line and the dotted line, agree with the experimental results of D/D_0 as shown in Fig. 5 by the experimental points, it becomes evident that Eq. (4) is good. Diffraction coefficients of variously shaped rigid obstacles placed in front of the human mouth at a distance of 2 cm are shown in Figs. 6〜10 by the marks (・) represented as mean values, and diffraction coefficients of the same obstacles placed in front of the spherical sound source (20 cm in diameter with a vibrating part 5 cm in diameter) at a distance of 2 cm are shown in Figs. 6〜10 by the solid line. On the basis of these experiments, it seems that diffraction coefficient depends on the front shape of obstacle, since it is large for a flat shaped obstacle and small for a spherical shaped obstacle. Although the diffraction coefficients are very different according to the shape of the obstacle, the difference between diffraction coefficients combined with the human mouth and with the spherical sound source is obtained with better accuracy than with 2 dB. (Fig. 11)Diffraction coefficient D_0 of the telephone transmitter of the Japanese commercial type is shown in Fig. 12. The difference between diffraction coefficient combined with the human mouth and with the spherical sound source as shown in Fig. 11 by the marks(□) agrees with the average value of the previous variously shaped obstacles. When the distance between the sound source and the obstacle is varied, and the obstacle is inclined, the diffraction coefficients D_0 of a circular disc and the telephone transmitter are shown in Figs. 13〜16. When the distance between the sound source and the obstacle is 2 cm, the diffraction coefficient D_0 below 2 kHz does not change of the obstacle is inclined at an angle of 45゜. On the basis of these results, it becomes evident that the system of the human mouth and the telephone transmitter is approximated by the system of the spherical sound source and the circular disk.