Some notes about low frequency measurements in a reverberation room

The modal field can influence reverberation time measurements under the Schroeder frequency, as it is well known, but the reverberation room is an extreme environment that deserve some further considerations. Indeed the Schroeder frequency depends on the mode distribution, which in turn depends on the peculiar mode shapes, due to the room shape. For a typical reverberation room the boundary frequency separating the modal regime from the statistical one may extend up to three times the Schroeder frequency. Moreover, the reverberation time in the modal regime depends on the proper excitation of the sound field; this is the reason for which some authors use the 'modal reverberation time' for each mode instead of the average on a frequency band. For these reasons, the recent ISO 354 draft recommends exciting the sound eld by placing the sound sources in the corners. The present work shows some practical experiences concerning the use of various kind of sound sources, changing the loudspeakers size, the distance from the corners, the excitation techniques. The results show how a proper sound source, whose electroacoustic characteristics are optimised for the ISO 354 frequency range, may improve the sound absorption mea-surements techniques and may in uence some metrics related to the sound field diffusion.