GROUND EFFECT OVER HARD ROUGH SURFACES

Laboratory and outdoor measurements are reported of the relative sound pressure level spectrum over hard surfaces containing either random o r periodically spaced arrays of 2-D roughnesses. The resulting data ha ve been compared with predictions obtained analytically and with numer ical predictions of a boundary element code. Effective impedances of t he rough surfaces have been calculated from the boss theory developed by Twersky. A classical asymptotic approximation for propagation near grazing incidence from a point source over an impedance boundary has b een modified, heuristically, to allow for diffraction grating effects. The resulting predictions are found to be in tolerable agreement with the data except for close and random packing. The boundary element co de is found to give superior results for larger roughnesses, but compu tational restrictions on element size reduce its usefulness for roughn esses with small width or height. (C) 1998 Acoustical Society of Ameri ca. [S0001-4966(98)01809-8]