Directional sound field decay analysis in a reverberation room

The measurement of sound absorption in reverberation rooms is subject to a poor interlaboratory reproducibility. Among other factors, this is caused by a non-isotropic sound field during the procedure. This study investigates the directional properties of the energy decay. The analysis method is based on capturing directional energy decay curves calculated from a plane wave decomposition of the sound field. Estimators for isotropy and axial symmetry are used to quantify the distributions of incident energy during the decay process. Experimental results are presented for a reverberation room in four configurations: with and without panel diffusers and with and without an absorbing specimen. Anisotropic energy distributions are found when an absorbing specimen is mounted. Especially if none or insufficient diffusing elements are mounted, the sound field isotropy is time dependent, decreasing with time, whereas the axial symmetry increases. The phenomenon is associated with a multi-exponential decay, resulting from weakly damped axial modes and strongly damped modes at grazing incidence on the absorber. It is further supported by a strong correlation between the estimated isotropy and temporally varying instantaneous energy decay rate. Results show that the isotropy estimator's temporal evolution cannot reliably identify a regression interval for accurate absorption coefficient estimation.