Evaluation of receiving room diffusivity and the effect on low frequency impact insulation class

The Impact Insulation Class (IIC) test standard is widely used in the building noise control community. This method is governed by ASTM E989 and ASTM E492. Low frequency response (100 Hz – 200 Hz third octave bands) of the floor-ceiling structure is often the limiting factor in IIC determination. The recorded sound pressure level in the reverberant receiving room in these third octave bands is highly sensitive to diffusion in the room. This means that microphone placement and volumetric averaging are crucial to calculating an accurate IIC value. In this paper, room diffusion is examined numerically and experimentally in the low frequency IIC third octave bands. It is shown that even for large receiving rooms, the diffuse field assumption may not be valid in these bands if small volume sampling is used. The effects of poor diffusion and/or volumetric averaging on IIC values is discussed.