The sound levels were analyzed to obtain the average sound pressure levels per ⅓‐octave band. Predictions of the ray- based approaches show excellent agreement with those of the relatively computationally intensive BEM formulation.Traffic noise attenuated by a barrier alongside a highway was measured with microphones positioned above an asphalt surface 15 and 30 m behind the barrier. Indeed, Muradali and Fyfe compared the use of the Maekawa chart, the Kurze- Anderson empirical equation and the Hadden-Pierce analytical formulation with a wave-based boundary element method (BEM). Consequently, the empirical formulations described in Section 9.3 can also be used to compute the amplitudes of the diffracted rays. The spherical wave reflection coefficient Q K is used for P 6 and P 8 as the reflection is assumed to take place at the receiver side but Q s should be used instead if the reflection happens at the source side.Īlthough we can use any of the more accurate diffraction formulae described in Section 9.2 to compute P,-P 8, a simpler approach, following Lam and Roberts, is to assume that each diffracted ray has a constant phase shift of л/4 regardless the position of source, receiver and diffraction point. The contributions from the two vertical edges E, and £, are Where P, - P 4 are those given in Equations (9.33a-9.33d) for diffraction at the top edge of the barrier. The total field is given byįigure 9.14 Eight ray paths associated with sound diffraction by a finite length barrier. Both possibilities are illustrated in Figure 9.14. These rays will either reflect at the source side or on the receiver side of the barrier depending on the relative positions of the source, receiver and barrier. The reflection angles of the two diffracted-reflected rays are independent of the barrier position. The two rays at either side are, respectively, the direct diffracted ray and the diffracted-reflected ray from the image source. This is a reasonable assumption if the ground is acoustically soft. In principle, there are reflected- diffracted-reflected rays also which reflect from the ground twice, but these are ignored. In addition to the four‘normal’ ray paths associated with diffraction at the top edge of the barrier (see Figure 9.7), four more diffracted ray paths from the vertical edges, including two ray paths each from either side, have been identified. įigure 9.14 shows eight diffracted ray paths contributing to the total field behind a finite-length barrier. The accuracy of this approach has been verified. In this section, a more practical method is described. There are other diffraction theories for the vertical edge effects which have met with varying degrees of success when compared with data. Sound fields due to rays diffracted around the two vertical edges of a finite- length barrier. 5.1 and 5.2) with effective flow resistivity 9000 Pa s nr 2 and (known) layer thickness of 0.015 m).The source is 0.355 m from the 0.3 m high barrier and 0.163 m above the ground.The receiver is 0.342 m from the barrier and 0.198 m high. The MacDonald solution, (see Section 9.2.2) has been adapted to include theįigure 9.13 Comparison of laboratory data (points) with predictions obtained from the Hadden and Pierce solution (solid line) and the Lam and Roberts approximate scheme (broken line) for a barrier place on an impedance ground (Delany and Bazley hard backed layer (Eqns. Noise Reduction by a Finite-Length BarrierĪll outdoor barriers have finite length and, under certain conditions, sound diffracting around the vertical ends of the barrier may be significant.
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