Abstract
As an effective sound absorbing wall for anechoic room, a flat type sound absobing wall and two types of its modifications, having minimal thickness and simple construction, were examined. Characteristics of the sound absobing wall were evaluated by the normal absorption cofficient using acoustic standing wave tube. The results obtained are as follows; (1) Absorbing properties of the flat type sound absorbing wall of multilayer having different density are as efficient as those of the wall having the taper type sound absorbing element, and the double layered walls are practically sufficient. (2) If we replace the absorbing materials near the hard wall with air space, the amount of the absorbing materials may be saved, but one should take care that the thickness of the air space is not too large. (3) Resonance absorber is not recommended for the sound absorbing wall of an anechoic room because it may impair the absorbing properties except in resonance frequency. On the basis of these results, an anechoic room lined with the double layer flat type absorbing wall was constructed in the Electrical Communication Laboratory N. T. T. . Density of the glass fibre used as absorbing material was 12kg/m^3 in the inner layer and 24kg/m^3 in the outer layer. The inverse square law properties of this room ware measured and presented in Fig. 13. Furthermore, the inverse square law properties of this room ware compared with those of an anechoic room lined with wedge type absorbing wall in natural frequency(Fig. 14), in frequency normalized with the lower limiting frequencies (Fig. 15), and in effective free field area (Fig. 16). From these results, it was verified that this anechoic room as compared with the anechoic room lined with wedge type absorbing walls is superior in taking advantage of the space and by no means inferior in acoustic characteristics.
