Ground effect over hard rough surfaces

Abstract

Laboratory and outdoor measurements are reported of the relative sound pressure level spectrum over hard surfaces containing either random or periodically spaced arrays of 2-D roughnesses. The resulting data have been compared with predictions obtained analytically and with numerical predictions of a boundary element code. Effective impedances of the 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 been 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 code is found to give superior results for larger roughnesses, but computational restrictions on element size reduce its usefulness for roughnesses with small width or height.

Using natural means to reduce surface transport noise during propagation outdoors

“…there is significant coherent scattering and reflection) there can be a significant change in ground effect. Many laboratory experiments have shown that the influence of small scale roughness on propagation over hard and soft surfaces can be considered in terms of effective surface impedance [38][39][40]91,92]. Particularly if the surface is acoustically-hard, roughness induces a surface wave.…”

“…The latter corresponds to effectively making a surface acoustically softer [38][39][40] (see also Section 4.3). Starting from a symmetric trapezoidal berm (15 m wide, 4 m high, with a 3-m wide top), consisting of compacted earth (represented by a flow resistivity of 300 kPa s/m 2 ), positioned near a 4-lane road with mixed traffic, various roughening approaches were numerically studied in Ref.…”

Exploiting ground effects for surface transport noise abatement

“…There is considerable laboratory data showing that the effect of adding small scale roughness to a surface, whatever its original impedance, is to modify its apparent impedance [2,[28][29][30][31][32]. If the original smooth surface is acoustically-hard then the effective impedance of the roughened surface is finite.…”

“…there is significant coherent scattering and specular reflection) there can be a significant change in ground effect. Many laboratory experiments show that the influence of small scale roughness on propagation over hard and soft surfaces can be considered in terms of effective surface impedance [2,[28][29][30][31][32]. Also, particularly if the surface is acoustically-hard, roughness induces a surface wave.…”

A Wavenumber Independent Boundary Element Method for an Acoustic Scattering Problem

“…This problem has attracted much attention in the literature (see, for example, [17,30,31,16,21,6,48]), both in its own right and also as a model of the scattering of an incident acoustic or electromagnetic wave by an infinite rough surface [8,47,36,7]. In the case in which there is no variation in the acoustical properties of the surface or the incident field in some fixed direction parallel to the surface, the problem is effectively twodimensional.…”