Excess sound absorption due to periodically arranged absorptive materials

Abstract

Discontinuities, of any kind, in the surface impedance have a tendency to cause the effect of excess absorption. A well-known example of this phenomenon is the so-called ‘‘edge effect,’’ defined as the dependence of sound absorptivity of a test material on its area size. In relation to this problem, the excess sound absorption of periodically arranged flat surfaces composed of different materials is investigated both theoretically and experimentally. The analyses show that excess absorption, having a specific trend of absorption characteristics, clearly occurs over the full frequency range of interest. As a result of parametric survey conducted on the performance of a surface arranged periodically with rigid and porous materials, it is found that flat surfaces with desirable absorption characteristics can be made by adjusting the parameters suitably. The parameters are the period of the surface, width ratio of the two materials, flow resistivity, and thickness of the porous material. Also, it is shown, as an important matter, that the absorptivity is considerably high and smooth over the wide frequency range.