残響室法吸音率の測定精度に関する研究(I) : 第3回協同比較試験と音場拡散条件の検討

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Precision measurement of the reverberant sound absorption coefficient has always been drawing attentions in the history of architectural acoustics. In Europe and North America, the round robin tests were carried out repeatedly to standardize the method of measurement. Results of these investigations were collected in the ISO Recommendations or ASTM Standards. However, it seems that some essential problems remain unsolved, such as diffusivity in the reverberation room, edge effect of test materials and so on. In Japan, the 3rd round robin test was carried out from 1965 to 1966 by the cooperation of 13 research laboratories. Details of reverberation rooms used for this round robin are shown in table 1. Glasswool board (50 mm thick, 25kg/m^3) was used as test materials. Test areas and the number of suspended diffusing plates were varied in nearly the same manner as in the European round robin (1959). Other Specifications of measurement conformed to the ISO Recommendation R 354. Test results are illustrated in Fig. 1〜Fig. 7. Deviations of measured absorption coefficients were fairly large. Thus, investigations into the precision measurement of absorption coefficient have been pursued for these two years. In general, measurement of sound absorption coefficient in a reverberation room assumes a diffuse sound field in the room. Diffusivity in a room is connected with the room volume, room shape, various diffusers put in the room, area of test materials and so on. Among these factors, the room volume would have influence mainly upon the diffusivity in the low frequency region. Test materials used in this investigation have relatively small absorption coefficient in low frequencies and it would be inadequate to discuss the requisite volume of reverberation room only through this investigation. Thus, the primary object of this research was to obtain the diffuse conditions in the middle and high frequency region. In the European round robin test, the diffuse condition in a room was specified by the ratio of the total area of suspended diffusing plates to the floor area of the room. However, this specification does not determine uniquely the sound field for different shapes or volumes of rooms. It would be desirable to introduce the index of diffusivity which can be measured directly in the room. Various methods and quantities have been proposed, such as fine structures of decay curves, directional distribution of sound energy, cross correlation in the sound field and so on. Through preliminary investigations, elevational diffusivity or ratio of horizontal to vertical mean energy during decay process was adopted for the evaluation of the sound field. So far as measurement of the absorption coefficient is concerned, it was concluded that the diffusivity in the reverberation room is sufficient, if the ratio of horizontal to vertical meanenergy during decay process is less than 3. 0 (Fig. 14). From the measurement of this ratio, the sound fields of all the rooms shown in Table 1 (except 13) could be regarded as of the same order of diffusivity, when the total area of suspended diffusing plates in each room attains about 80 percent of the floor area of the room. Important factors in the disagreement of the absorption coefficients in the 3rd round robin test would have to be sought in other items.
社団法人日本音響学会の論文
1968-11-30

残響室法吸音率の測定精度に関する研究(I) : 第3回協同比較試験と音場拡散条件の検討

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