DEPENDENCE OF SOUND ABSORPTION COEFFICENTS UPON THE AREA OF SUSPENDED DIFFUSER AND ACOUSTICAL MATERIAL

The diffusivity of the sound field in a reverberation chamber, which can be improved by a certain amount of randomly placed and orientated suspended diffusers, usually has a significent effect on the test results for sound absorption coefficient. From our experimental results we found that the diffusivity would be sufficient when the total area of suspended diffusers reaches 60-70% of the floor area in a small chamber, say about 100 m³. But we also found that for some acoustical materials such as perforated soft fibertile, the sound absorption coefficients were independent on di’ffusivity. It can be concluded that the acoustical property of such material has only a little variation for different angles of incident wave, hence they would not be suitable for the determination of the diffusion improvement of sound field.
Test results of many commonly used acoustical materials show that when test specimen area changed from 10m² to 6m², provided that the ratio of length to width was controlled between 0.7 to 1.0, the deviation of the sound absorption coefficient obtained would be rather small, say, less than 0.02 for common acoustical materials or less than 0.06 for certain highly absorbent materials. So a minimum test area of 6 m² could be used with sufficient accuracy to meet the needs for engineering practice.