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The reverberation time in a room with unevenly distributed sound absorbers, such as a room having an absorptive floor and/or ceiling, is often observed to be longer in the middle- and high-frequency ranges than the values obtained using the Sabine/Eyring formula. In the present study, this phenomenon was investigated through a scale-model experiment and three-dimensional wave-based numerical analysis. The reverberation time in a room having an absorptive floor and/or ceiling was verified to be longer in the middle- and high-frequency ranges, and the arrangement of absorbers was found to affect the frequency characteristic of the reverberation time. The increase in the reverberation time is caused by the slow decay of the axial and tangential modes in the horizontal direction. The reverberation time is longer in the high-frequency range (in which the wavelength is sufficiently shorter compared with the height of the ceiling) than in the low-frequency range, even when the frequency characteristics of the absorption coefficients of the absorbers are flat. As a means of improving such an uneven reverberation time in a room, both the placement of diffusers in the vertical direction and the use of inwardly inclined walls (in rooms with highly absorptive floors) have been found to be effective.

Experimental and numerical studies on reverberation characteristics in a rectangular room with unevenly distributed absorbers