Acoustics 08 Paris 309 Walls and ceilings are often designed to suppress undesirable echoes. For that purpose, walls and ceilings are given absorptive surfaces which absorb sound energy or uneven surfaces which provide wave diffusion. While it is not difficult to predict the degree of echo suppression effects in the case of absorptive surfaces, it is generally difficult to do that in the case of uneven surfaces. In addition, if the surfaces have periodicity, unintended effects called coloration

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... can distort tonal characteristics of the sound field. Thus, many acoustic designers would hesitate to employ this kind of periodic-type diffusers. However, there are some cases where it is necessary to suppress echoes without any energy loss. Periodic-type diffusers have the advantages of easy estimation of diffusion properties, simple method of design, and ready availability. In this study, a subjective experiment with simulated stimuli was carried out to clarify the echo suppression effect of the periodic-type diffusers from the viewpoint of reflected energy level. Although coloration interfered subjective judgments in some cases of the experiment, it is shown that the periodic-type diffusers have echo suppression effect if coloration does not occur. This study is also devoted to discussion about the cause of the coloration.