Models and measurements of sound propagation from a point source over mixed impedance ground

Abstract

Measurements of the excess attenuation of sound from a point source over mixed impedance ground in an anechoic chamber are compared with predictions obtained from models based on (a) the semiempirical theory due to De Jong, (b) Nyberg’s theory, (c) a Fresnel-zone approximation, and (d) a boundary element code. The impedance discontinuities studied in this work are perpendicular to the source–receiver line. When there is a single discontinuity between acoustically hard and finite impedance surfaces, the De Jong semiempirical model, the Fresnel-zone model and boundary element code are found to give satisfactory agreement with measured data. The frequency of the first maximum in attenuation is found to be highest at approximately 70% hard surface cover rather than at the 100% expected. It is argued that this is a result of edge diffraction. When extended to multiple impedance discontinuities, the De Jong semiempirical model performs poorly. However, both Nyberg’s theory and the boundary element code give good agreement with measured results, and the Fresnel-zone model gives qualitative agreement with the measured data. The measurements confirm that, under certain constraints given by Nyberg’s theory, the ground effect due to mixed impedance may be determined from that predicted by using the area-average impedance.

Using natural means to reduce surface transport noise during propagation outdoors

“…The De Jong model uses semi-empirical modifications of analytical expressions for diffraction by a rigid half-plane. Boulanger et al [88] have shown that De Jong's model gives good agreement with laboratory data for propagation over a single impedance discontinuity; however it fails if there are multiple impedance discontinuities. Lam and Monazzam [89] observed that the De Jong semi-empirical model, derived initially for a hard-to-soft discontinuity (during propagation away from the source) fails to satisfy reciprocity and modified it accordingly.…”

“…A Fresnel-zone method is used to account for discontinuous terrain in the HARMONOISE prediction scheme [80]. As has been noted elsewhere [3,88,91], a Fresnel-zone approximation, while potentially satisfactory for predicting overall broadband levels, is not useful for detailed predictions over multiple discontinuities.…”

Exploiting ground effects for surface transport noise abatement

“…[22] for a single discontinuity. Boulanger et al [23] show that De Jong model gives good agreement with laboratory data for propagation over a single impedance discontinuity; however it fails if there are multiple impedance discontinuities. Lam and Monazzam [24] observe that the De Jong semiempirical model, derived initially for a hard-to-soft discontinuity (during propagation away from the source) fails to satisfy reciprocity and modified it accordingly.…”

“…A Fresnel zone method is used to account for discontinuous terrain in the HARMONOISE prediction scheme [4]. Boulanger et al [23] propose a modification to the Fresnel zone scheme used by Hothersall and Harriot [17]. Instead of the linear interpolation of two excess attenuations, Boulanger et al [23] use a linear interpolation between the two pressure terms.…”

Ground effects in FAA's integrated noise model

“…The approach decided upon was very similar to that implemented within the F ederal Highway Administration's Traffic Noise Model (FHWA TNM ®) 28,29 and is based on the work of Boulanger. 32 Specifically, the acoustically soft -ground and hard-ground effect were apportioned based on a distance-weighted coefficient. This coefficient was computed based on the ground distance associated with the acoustically hard and acoustically soft portion of the ground contained within the so -called Fresnel Ellipsoid.…”