Abstract
Results of the 3rd Round Robin Test in Japan have shown relatively large deviations in measured absorption coefficients. To investigate the diffuse condition in a reverberation room, which was supposed to be one of the important factors for the precision of measurements, directional diffusitivity during decay process was introduced as the index of diffusitivity for the middle and high frequency regions. From the measurements of this index, the sound fields of all rooms shown in Fig. 17 could be regarded as the same order of diffusitivity, when the total area of suspended diffusing plates in each room would reach up to 80 percent of the floor area of the room. The important factors for the disagreement of the absorption coefficients in the 3rd Round Robin test would have to be found in other items. To investigate the fluctuations of absorption coefficient related to the reverberation time measurements, experimental works were conducted in the following ways. (1) Reference decay curves were sent to 13 institutes and were read by 25 persons. (2) The absorption coefficient of identical test materials was measured by using the same instruments in 11 reverberation rooms. The reverberation time was read by only one person from the recorded decay curves. (3) To check the effect of dynamic characteristics of a high speed level recorder, decay curves were recorded by each level recorder in respective institutes. These records were read at first by only one person and then by respective persons in each institutes. From these results, it was concluded that the personal tendency for reading the decay curves and the dynamic characteristics of a high speed level recorder would play an important role for the precision of measurements of sound absorption coefficients in a reverberation room. The 4th Round Robin test, where the specifications for measurements were derived from this investigation, showed fairly good results.
