混响室中悬挂扩散板和试件面积对吸声测量结果的影响

混响室中的扩散程度通常对材料吸声系数测量值有较大影响,悬挂无规取向的扩散板可改善声场的扩散性能。我们的经验是在100m³左右的小混响室内,扩散板占地面面积的60—70％即可达到足够的扩散。但是这里也发现如软质纤维板（不论其表面处理及穿孔情况如何）之类材料的吸声系数与扩散条件关系不大,可解释为它们随入射角不同只有很小变化,因此不适宜用来判断声场扩散的改善程度。
多种常用吸声材料的测试结果表明,如果试件长宽比在0.7—1.0之间,试件面积从10m²改变至6m²对吸声系数测量值的影响并不太明显,其差异约小于0.02,个别吸声系数很高的材料,其差异也不超过0.06,与一般测试误差相接近。因而可以把最小试件面积规定为6m²,以便利测试,也能满足一般工程要求的测试精度。

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.