On Some Important Practical Aspects Related to the Measurement of the Diffuse Field Absorption Coefficient in Small Reverberation Rooms

The use of small reverberation rooms for the measurement of the Diffuse Field Absorption Coefficient (DFAC) is common practice in the automotive industry. Such practice brings with itself a few issues, related to the limited size of the measurement environment. Some of these issues (e.g. measurements’ repeatability and reproducibility) have already been thoroughly investigated in articles published at past SAE NV Conferences. This paper intends to focus on some other “minor” aspects related to the measurement of DFAC in small reverberation rooms that so far have received little attention but that can, anyhow, have a non-negligible influence on the measurement results, in particular when they have to be compared to target curves. These “minor” aspects involve, for example, the use of metal frames to seal the edges of test samples, the effect of using correlated or un-correlated signals for the excitation loudspeakers, the importance of taking into account the environmental conditions (like temperature and humidity) and the effect of microphones’ size and orientation. Experimental results obtained in one of the most widely used small-sized reverberation rooms (the so-called Alpha Cabin) are presented that show how the effect of all these practical aspects on DFAC results is far from being irrelevant, thus proving how important it is to consider and document them carefully when producing and comparing DFAC results.