Methods to characterize the acoustic properties of periodic surfaces
Sound striking a periodic surface is absorbed and scattered depending on the surface properties. In comparison to non-periodic surfaces, periodic surfaces cause a higher sound absorbtion, due to their nearfield: If the nearfield is large, the scattered soundfield will be small and the amount of the absorbed sound high. If the scattered sound field is large, the nearfield is becomming smaller and the periodic surface is less absorbing because more sound energy is reflected. The mechanisms of sound absorption and acoustic scattering by periodic surfaces consisting of bands of alternating impedance are demonstrated first by means of a numerical model. Subsequently, measuring methods are presented to determine the acoustic properties of periodic surfaces at normal, diffuse and directed sound incidence. These measuring methods are assessed with regard to accuracy, informational value and practicability and compared to the theoretically calculated values. It is shown that it is possible (with varying accuracy) to determine the structural absorption at normal, diffuse and directional incidence. The geometrical reflection can be separated from the scattered field at diffuse and directional sound incidence. None of the measuring methods presented allows the separate examination of the near and farfield. The measurements essentially confirm the calculated values.