ISO 354:2006 Results discussion
A topic that generates much confusion in acoustics is why one can obtain absorption coefficients greater than one when measured in a reverberant chamber. Given the absorption coefficient is a ratio between the total incident energy and the absorbed energy it is difficult to understand why an absorption coefficient can be greater than unity as it implies that the sample under test is generating energy, which goes against the laws of physics.

Cox and D’Antonio (2004) summarised the reasons why the absorption coefficient can exceed unity, and these reasons include:

1) Edge diffraction:
The diffraction from the edges at low frequencies causes the reflected wave to no longer be planar, and so diffraction produces the edge effect whereby substantially more absorption happens near the edges of an absorber than at its centre.

2) Non-diffuseness: In order to apply the Sabine formulation a statistical approach is assumed to define the sound field in the room. Therefore it can be assumed that the time and space distribution of the sound pressure level is even across the room, which is typically not the case in real situations. In fact, the ISO 354:2006 requires the room to be diffuse which usually demands the installation of stationary suspended diffusers. With all, the reverberation time needs to be measured at several locations with different positions of microphones and loudspeakers in order to later average the reverberation time of the room.

3) Sabine formulation:
It is well understood that the Sabine (1922) formulation should not be applied when the mean absorption value is higher than 0.4. Various authors propose the use of the Norris-Eyring or Millington formulae to compensate for this.
(Cox and D’Antonio, 2004)