Analysis of non-exponential sound decay in an enclosure composed of two connected rectangular subrooms

Acoustically coupled rooms have recently been drawing more and more attention in the architectural
acoustics community. Determination of decay times in these coupled rooms from
computer-based models has become increasingly significant. In computational modelling of
coupled enclosures the geometrical acoustics methods do not apply at lower frequencies, and
models must account for the wave-nature of sound fields, especially modal effects. In this
investigation a combination of modal analysis with finite-difference algorithm was used to
predict a decay in the sound field energy in an enclosure composed of two connected rectangular
subrooms. Random incidence absorption coefficients inside subrooms have been modified
in this study to create different configurations of absorbing materials on subrooms walls.
When one of these subrooms was much more absorbent than the other one, calculation results
have shown that, at any frequencies, there existed both the initial sudden and the late slow
decrease in the sound field energy. This effect was characterized by two metrics: early decay
time (EDT) and late decay time (LDT) determined from a sound level decay from −50 to
−60 dB. The analysis of numerical data confirmed that the most influential factor in realizing
the non-exponential sound decay is a phenomenon of eigenmodes localization.