Sound diffraction materials11/10/2023 The way this works can be understood with the aid of Huygens' Principle: Every point illuminated by a wave can be considered as a spherical-wave source. This also causes an effect similar to that of a larger area of absorption. (2) Edge diffraction causes sound crossing an edge of an absorber to change direction. This makes near-grazing waves strike the absorber and be partially absorbed. When a sound field encounters the edge of a piece of absorptive material, the acoustical impedance near the material changes, causing wave refraction, with the result that the material can appear as much as a quarter wavelength larger in each dimension than it actually is. (1) The statistical assumptions upon which the Sabine theory is based are violated in practice. Still remaining to be explained is “edge effect”: What physical processes are involved? The answer seems to be twofold: Many labs have now installed plate diffusors to improve this characteristic. One physical mechanism that has been proposed to explain the difference in measured α among different labs is the actual diffuseness of the sound field in the reverberation chamber used for testing. While true, and perhaps comforting, this statement may create as many questions as it solves. Leo Beranek states that a properly done measurement of α is accurate for the particular piece of material measured, in the specific room and location in which the measurement is performed. However, we are left with the questions of why the absorption of a particular block of material apparently varies with size and shape, and why different values are measured in different labs. This new definition validates the possibility of α > 1, although such values of α cannot be used with computerized acoustical modeling software. Therefore, we must define α as a constant of a particular material which, when obtained using the standard measurement techniques and the Sabine equation, allows prediction of RT with some accuracy, given certain known limitations. However, as we discussed in audioXpress' April 2016 Sound Control article, using an α of unity in the Sabine equation does not give zero RT, although if α were indeed the fraction of incident sound absorbed by a material, having an α of unity would leave no sound in the reverberant field, and thus zero RT. If α is indeed, as Wallace Clement Sabine stated, the fraction of sound absorbed by a surface, then by definition, it cannot exceed unity. The physical reasons behind this apparent anomaly are often lumped into the category of “edge effect” - along with the observation that equally competent test labs using identical standard procedures can measure different absorption coefficients (α) that sometimes exceed unity! This latter situation has been the cause of much speculation throughout the years, with some experts stating that any α greater than unity is erroneous.
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