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Figure 7.20 Geometry of the Diffuse Field Absorption Coefficient Calculation (Cremer et al., 1982)

Figure 7.20 Geometry of the Diffuse Field Absorption Coefficient Calculation (Cremer et al., 1982)

Calculated Diffuse Field Absorption Coefficients

In Eq. 7.63 we saw that we could write the absorption coefficient as a function of the angle of incidence, in terms of the complex impedance. Although direct-field absorption coefficients are useful for gaining an understanding of the physics of the absorption process, for most architectural applications a measurement is made of the diffuse-field absorption coefficient. Recall that a diffuse field implies that incident sound waves come from any direction with equal probability. The diffuse-field absorption coefficient is the average of the coefficient a6, taken over all possible angles of incidence. The geometry is shown in Fig. 7.20. The energy from a uniformly radiating hemisphere that is incident on the surface S is proportional to the area that lies between the angle 6 — A6/2 and 6 + AO/2. The fraction of the total energy coming from this angle is dE 2 n rsin 6 rdO 1 . „

and the total power sound absorbed by a projected area S cos 6 is n/2

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