球状標準音源用球かくの内部インピーダンスとその制動法

抄録

Some acoustical characteristics of the spherical cavity of the sound source used as a standard for free field measurements have been investigated. This sound source consists of a spherical shell and of a diaphragm vibrating at a constant rate at the circular window on the surface of the spherical shell. In general the medium in the cavity makes the back load of the diaphragm, and its impedance which affects the vibration of the diaphragm can not be neglected;so the necessary conditions for designing the standard sound source should be investigated. A number of papers on the spherical sound source have been reported;most of them, however, have been described on the radiated sound field, and only a few have been concerned with the sound field in the spherical cavity. To design a standard sound source of greater stability and reliability, the back load impedance of the diaphragm which is caused by the sound field in the cavity should be made clearer. In this paper the acoustic load of the diaphragm is analyzed and experimentally verified, and the sound pressure distribution in the cavity is also calculated. It has been shown that the frequency characteristics of acoustic impedance which changes over a wide range are not desirable for the formation of a standard sound source. To avoid these undesirables sound absorbing materials have been inserted into the cavity and the acoustic impedance resulting from two different packing methods have been analyzed. As a result, it has been shown that the undesirable phenomena can be eliminated and that the values of acoustic impedance can be limited within a narrow range.