Strips, slits, surface waves and slit-based MPP

Abstract

Maa Dah-You was a pioneer of research into Microperforated Plates (MPP). A thin rigid-porous layer containing infinitely-long slits of small (1 mm or less) width may be regarded as a special case of MPP. Surface waves are created by near grazing sound propagation from a point source over either a thin rigid-porous layer or a (slightly) rough surface. Measurements have been made on surfaces composed from parallel periodically-spaced 1 m long rectangular strips (width 0.0126 m, height 0.0253 m) on an acoustically-hard surface. The edge-to-edge spacing has been varied between 0.003 m and 0.06 m. When the spacing is substantially smaller than the strip height these surfaces behave as locally-reacting rigid-framed hard-backed slit-pore layers with an effective depth slightly larger than the strip height. When the spacing is equal to or greater than the strip height the surfaces behave as periodically-rough surfaces. Surface wave magnitude is relatively unaffected by the strip spacing but the main frequency content is lowered as the mean spacing is increased. These data suggest that the surface waves have a similar physical origin. Predictions of absorption coefficient spectra for MPP containing infinitely-long slits are compared with those for MPP with slits of finite length.