Difference between locally-reactive and extended-reactive boundary conditions in a non-diffuse sound field with unevenly-distributed sound absorbers

摘要

In wave-based numerical analyses, locally-reactive boundary conditions are often adopted, whereas their applicable range is not sufficiently clarified. The difference between locally-reactive and extended-reactive conditions may be clearly observed in a non-diffuse sound field with unevenly-distributed sound absorbers, because the incident angle of many sound waves in such a field is far from the normal of the boundary surfaces. In the present paper, the difference between both of the conditions in a rectangular room with unevenly-distributed sound absorbers is investigated in detail by a wave-based numerical analysis using the fast multipole BEM (FMBEM). A porous-type sound absorber on a rigid wall is assumed for absorptive surfaces. For locally-reactive cases, real- and complex-number surface impedances are adopted, in the latter of which the phase is considered. Extended-reactive cases are analyzed using the domain decomposition method. Main conclusions are summarized as follows: (ⅰ) Many results with complex-number impedances have the same tendency as those with extended-reactive conditions, even in a non-diffuse sound field, (ⅱ) one can judge whether an extended-reactive surface can be treated with a complex-number impedance, from the theoretical dependence of the absorption coefficient on the incident angle and relationship between the absorber thickness and the analysis wavelength.