A method was developed for measuring the propagation velocities and the attenuation constants of the acoustic waves in porous materials. This method is based upon the detection the resonance frequency and the frequency width of the half value in the resonance curve of the specimen subjected to impinging acoustic waves. Some measurements on the propagation velocities and the attenuation constants were performed in the case of the circular tube of 2. 0 cm radius. The results obtained are in good agreement with the Kirchhoff's theory.
In this paper, the accuracy of reverberation time measurements was discussed in relation to the irregularity and the level range of reverberation decay curves recorded. The statistical relation was found between the unbiased estimate u of standard deviation of measured reverberation time T and the irregularity s, newly defined, of the decay curve. When s is small enough and recorded level range of the curve is constant, u/T is proportional to s. When s is lager than the critical value, u/T approaches to some constant value independent to s. Personal difference in either mean value or standard deviation of measured reverberation time was found to be negligible. It became clear that the recorded level range S/N, defined as the level difference between steady state sound pressure produced by signal sound source and background noise in a room, must be larger than 30 dB in order to prevent the systematic error of reverberation time. s varies without question to some extent when either the kind of sound source or the writing speed of recording apparatus is changed. Their effects on u were therefore examined. The above results are concerned with the case of the high speed level recorder which is widely used for this purpose. The accuracy were also examined for the case of utilizing the reverberation time meters of cathode ray oscillograph type. From the results obtained, it was also considered as to what kind of sound source and the measuring apparatus are preferable and how many records of reverberation curves for a given condition must be taken in order to get the desired accuracy.