Reduction of surface transport noise by ground roughness Shahram Taherzadeh a,⇑ , T. Hill b , S. Taherzadeh a , K. Attenborough a , M. Hornikx c a Engineering and Innovation, The Open University, Milton Keynes MK7 6AA, UK b Sonatest Ltd, Dickens Rd, Milton Keynes MK12 5QQ, UK c Technische Universiteit Eindhoven, Eindhoven, Netherlands article info Article history: Received 4 June 2013 Received in revised form 5 March 2014 Accepted 12 March 2014 Keywords: Insertion loss Transport noise Parallel walls Lattice abstract Measured insertion losses due to the ground effects associated with low configurations of loosely stacked household bricks on a car park are reported. A particularly successful design has the form of a two brick high square lattice which is found to offer a similar insertion loss to regularly-spaced parallel wall arrays of the same height but twice the total width. Part of the insertion loss due to the roughness configurations is the result of transfer of incident sound energy to surface waves which can be reduced by introducing wall absorption or material absorption in the form, for example, of shallow gravel layer. Predicted finite length effects have been explored using a Pseudo-Spectral Time Domain Method, which models the com- plete 3D roughness profile. It is concluded from measurements and predictions that the lattice design has less dependence on azimuthal source-receiver angle than parallel wall configurations. These predictions are supported by measurements of level difference spectra as a function of azimuthal angle. A 2D Bound- ary Element Method gives predictions that agree well with data for parallel wall arrays up to 16 m long and it is used to investigate the potential insertion loss of longer configurations up to 0.3 m high. It has been found possible also to make predictions of the insertion loss due to infinitely long 3D lattices using the 2D BEM with the lattice represented by the surface impedance derived from fitting short range data with a slit-pore impedance model. The insertion losses of recessed configurations are predicted to be approximately 3 dB less than those of embossed configurations of the same size. Outdoor experiments also show that pathways can be made through such roughness configurations without significantly affecting their insertion loss. It is concluded that artificial roughness configurations could achieve sub- stantial noise reduction along surface transport corridors without breaking line of sight between source and receiver, thereby proving useful alternatives to noise barriers. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Noise barriers are a traditional method of reducing traffic noise. These obstruct the direct path between the source and the receiver thereby causing the sound to diffract over the top of the barrier. The performance of noise barriers depends on the inverse ratio of incident sound wavelength to the difference between the lengths of the direct path between source and receiver and that from source and receiver via the top of the barrier. The greater this ratio, the greater is the attenuation. To be most effective a barrier should be placed as close as possible to source or receiver. If the barrier is positioned close to the source, its effectiveness decreases as the receiver moves further way. The acoustical benefits afforded by noise barriers have to be compared with the facts that they obscure the line of sight between source and receiver, have a significant visual impact, divide communities and interfere with wildlife corridors. Gaps in a noise barrier reduce its effectiveness and the presence of a barrier reduces any pre-existing ground effect by increasing the mean path height to the receiver. Ground effect includes destructive and constructive interfer- ence between direct and ground-reflected sound and leads to a frequency-dependent attenuation in excess of that due to wave- front spreading and atmospheric absorption [1]. Even the category of ground known as ‘grassland’ involves a wide range of ground effects [2,3]. Hitherto recognition that ground effects may contrib- ute to traffic noise attenuation, have been restricted to outdoor surfaces that are porous. Deliberate exploitation of ground effects has been restricted to the development of porous road surfaces which inhibit traffic noise generation as well as propagation. Less attention has been paid to exploitation of the finite impedance induced by the presence of roughness on an otherwise acoustically-hard ground. This is the result of multiple scattering by roughness elements [2,4]. http://dx.doi.org/10.1016/j.apacoust.2014.03.011 0003-682X/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +44 1908653320. E-mail address: shahram.taherzadeh@open.ac.uk (S. Taherzadeh). Applied Acoustics 83 (2014) 1–15 Contents lists available at ScienceDirect Applied Acoustics journal homepage: www.elsevier.com/locate/apacoust