This study proposes an in situ method for measuring the sound absorption coefficient of architectural finishing materials using a one-sided cosine-directional microphone array. Unlike conventional reverberation room methods that rely on changes in reverberation time before and after material installation, the proposed method allows direct estimation of absorption based on the energy balance between incident and reflected waves. To separate these two wave components, a linear microphone array is configured with directionally weighted coefficients that realize a one-sided cosine directivity pattern. The interference between the incident and specularly reflected waves, as well as evanescent fields near the material surface, are suppressed through the design of the weighted coefficients. Experimental validation was conducted using fiberglass in an anechoic chamber with normal incidence and a reverberation room with diffuse fields. The measured absorption spectra showed good agreement with the conventional reverberation room data within specific frequency ranges, although deviations were observed in some frequency bands. In particular, increased error was observed below 250 Hz and in the high-frequency region. Potential causes include array size, microphone spacing, directivity degradation, and edge effects related to specimen dimensions. These aspects are considered in the discussion of possible error sources.
Shinichiro Koyanagi (Wed,) studied this question.