Evaluation of sound absorption characteristics of material with multi-channel sound field reproduction system

Multi-channel sound field reproduction system is utilized as a measurement system of absorption characteristics of acoustical material. The used system named "Sound Cask" has ninety six channel evenly distributed loudspeakers in around 8 cubic meters enclosure and works with boundary surface control principle that reproduce any sound field by replicating the pressure distribution at the surface of arbitrary enclosed region. The reproduced field in Sound Cask is around 0.4 m diameter sphere inside the Cask. The random incidence condition is aimed in this study. Uncorrelated noises that are processed with inverse filter matrix are used as measurement signal. This condition is thus equivalent that the acoustical material is exposed to uncorrelated noises coming from various directions without undesired effect of the enclosure. First, the degree of isotropy in the reproduced field is measured by the index called UAD, uniformity of arrival directions, that evaluates the uniformity of directions of sound intensities. The results indicate the superiority of proposed method with inverse filter compared with conventional reverberation chamber especially at low frequency. The absorption coefficients of small sized fiber glass (e.g., 0.3 m x 0.3 m) are then measured by in-situ two microphones method. The results show reasonable values compared with typical coefficients obtained by conventional measurement method. These indicate effectiveness and possibility of proposed method. Additionally, the proposed method can reproduce any desired conditions of sound incidence to the material such as oblique and simultaneous incidence from plural directions. Stable measurement with such specialized conditions would be expected with proposed method.