残響測定について

概要

論文の詳細を見る
Reverberation time is generally used in the estimation of the acoustical properties of a room. However, the decay curve representing a room shows complex irregularities according to the nature of the source of sound and the writing speed of the recording system. To find the effects of the source and the writing speed of the recorder on the reverberation curve, the author obtained the reverberation curves of several halls of differernt sizes and has set forth a tentative standard method for the determination of the reverberation time. When we use white noise as the source of sound, the amplitude fluctuations of the source appear on the slope of the decay curves; several such curves obtained at a given position in the room will not appear identical, due to the fact that white noise has a random distribution of amplitudes and the source is cut off at arbitrary instants. (Examples are shown in Fig. 1). In curves where a warbling tone is adopted as source a similar fluctuation is observed that depends on the frequency of the tone being emitted when the source is cut off. The decay curves obtained with a pistol shot source and those with white noise of short duration such as 30 m. s. , 50 m. s. , and 100 m. s. , were compared with those resulting from white noise lasting 1 second and from a warbling tone. To get relatively high levels at low frequency, the pistol shot was fired in a small felt covered box, the energy being radiated from a small hole on the box. The fluctuation of the decay curves are influenced also by writing speed of the recorder: the experimental results with a white noise pulse of 100 milliseconds are shown in Fig. 5. Letting the decay constant of the sound be β and that of the recording system α, we obtain theoretically the decay curve (x/x_0)=<α/α-β^e>^-<βt>-<β/α-β^e>^αt where x__0: maximum ampliteude of the signal, x: amplitude at time t. Calculated examples are shown for the cases of writing speeds of 70, 140 and 300 db/sec, and for reverberation times, of 0. 7, 1 and 1. 5 secs. From this it will be seen that the reverberation time must be determined from the tangent of the courve, particulary when the writing speed is not adequate in relation to teh decay rate of the sound. The shape of the decay curve is affected by the directional properties of the source also, a steep decay in the initial part is often observed at high frequencies. It has nothing to do with the properties of the room and this part must be excluded in determining the reverberation time. For the study of the acoustical properties of a room, the detailed structure of the decay curve must be investigated, and the author has proposed a method of annotating irregularities in the charts representing the frequency response of the reverberation time.
社団法人日本音響学会の論文
1954-12-30

著者

五十嵐壽一
東京大学理工学部研究所

残響測定について

スポンサーリンク

概要

著者

関連論文

スポンサーリンク