Examination of low-frequency sound decay in two connected enclosed spaces having different sound absorption on boundary surfaces

In this paper, a sound decay in two connected enclosed spaces was investigated. The research concerned the low-frequency range, because in this situation the sound decay is predominantly non-exponential. Numerical simulations were performed for a system of two connected rectangular subrooms. The larger subroom was highly reverberant, while the smaller subroom had walls that provided moderate sound damping. Simulation results showed that after switching off the harmonic source, a non-exponential behavior of sound decay is usually observed. This is because many eigenmodes are involved in the sound decay, hence this process strongly depends on both the source frequency and its position. Reverberant properties of the system were also determined using energy decay curves calculated by the reverse-time integration of squared room impulse response. Simulations showed that the effect of the significant difference in sound absorption in subrooms is a wide range of modal reverberation time values, i.e. it is large for eigenmodes localized in a highly reverberant room and small for eigenmodes localized in a damped room. The analyzed room system is therefore characterized by an interesting feature resulting from the localization of eigenmodes. When the source is located in the larger subroom, slowly decaying modes localized in this subroom dominate the sound decay process. However, when the source is located in the smaller subroom, the sound decay in the larger subroom is much faster since non-localized modes with a shorter modal reverberation time are excited within this subroom.