吸音性障壁による騒音減衰 : 障壁のよる騒音制御に関する研究 その2
1976 年 32 巻 7 号 p. 430-435
詳細
1976 年 32 巻 7 号 p. 430-435
This paper presents a method to estimate excess attenuation of noise by an absorptive barrier, i. e. a barrier covered with sound absorbing materials. The approximate theory of the diffraction by an absorptive barrier is derived by the procedure that the second term of the theoretical solution of diffraction by a hard barrier is multiplied by the complex sound pressure reflection coefficient, because the solution of diffraction by a hard barrier is composed of two terms, the first term presents the combined fields of an incident wave and a diffracted wave caused by the incident wave and the second term presents the combined fields of a reflected wave and a diffracted wave caused by the reflected wave (see Eqs. (1), (2) and (3)). "Effect of Absorption" [EA], i. e. the increase of the excess attenuation caused by the absorbing treatment of the barrier, is defined as the difference of sound pressure level in the shadow zone resulting from the replacement of the hard barrier by the absorptive barrier (see Eqs. (5p) and (5s)). It is found by the numerical calculations of Eqs. (5p) and (5s) that the effect of absorption for a plane sound wave _p [EA] is not so much different from that for a spherical sound wave _s [EA](see Figs. 2, 3 and 4). A single chart, which may be very convenient for quick estimation of the effect of absorption in the practice of noise control, is offered under the conditions that the imaginary part of the acoustic admittance ratio of the absorbing materials is neglected and that source point and receiving point are symmetric with respect to the barrier (see Fig. 9). The validity of the method presented here is confirmed by comparing the estimated values with the measured values, consequently, this method may be useful to estimate excess attenuation of noise by a barrier with absorbing treatment in the shadow zone of the barrier (see Figs. 10, 11 and 12). It is also discussed that the excess attenuation by an absorptive barrier may be calculated for an incoherent line source (see Eqs. (8) and (9)).
