It is known that seat rows have a considerable effect on the sound transmission in auditoriums. When the direct wave propagates over the audience seats at near-grazing angle, excess attenuation appears in the low frequency range of 100-300Hz because of the interference between the direct wave and the scattered wave from the seat rows arranged periodically. The integral equation derived from normally derived Helmholtz-Kirchhoff’s formula (NDF) is used to solve this problem. In numerical calculation, it is assumed that the seats are composed of rigid plates of negligible thickness and the potential is uniform in each divided surface element. Sound fields calculated were compared with 1:10 scale-model tests and showed good agreement.

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