The combined eects of porous asphalt surfacing and barriers on trac noise G.R. Watts a, *, S.N. Chandler-Wilde b , P.A. Morgan b a Transport Research Laboratory, Crowthorne, Berkshire, RG45 6AU, UK b Department of Mathematics and Statistics, Brunel University, Uxbridge, Middlesex, UB8 3PH, UK Received 22 August 1997; received in revised form 24 June 1998; accepted 9 July 1998 Abstract There is considerable interest in the use of porous asphalt (PA) surfacing on highways since physical and subjective assessments of noise have indicated a signi®cant advantage over con- ventional non-porous surfaces such as hot rolled asphalt (HRA) used widely for motorway surfacing in the UK. However, it was not known whether the bene®t of the PA surface was aected by the presence of roadside barriers. Noise predictions have been made using the Boundary Element Method (BEM) approach to determine the extent to which the noise reducing bene®ts of PA could be added to the screening eects of noise barriers in order to obtain the overall reduction in noise levels. # 1998 The Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. Keywords: Porous asphalt; Noise barriers; Boundary Element Method (BEM). 1. Introduction On high speed roads the predominant noise is that produced by vehicle tyres as they roll over the road surface [1]. Porous asphalt (PA) surfacing which had been initially developed at TRL to reduce spray, was also found to produce signi®cantly lower vehicle noise [1±3] when compared with a conventional HRA (hot rolled asphalt) surfacing. However, measurements of the noise bene®ts of this type of sur- face have been restricted to the roadside locations or where propagation is over ¯at grassland and there is no reliable information on the combined acoustic eects of PA and roadside noise barriers. Since an indication of the likely bene®ts of PA was needed for a wide range of situations it was decided to model the combined eects of PA and barriers. Applied Acoustics 58 (1999) 351±377 www.elsevier.com/locate/apacoust 0003-682X/99/$ - see front matter # 1998 The Transport Research Laboratory. Published by Elsevier Science Ltd. All rights reserved. PII: S0003-682X(98)00045-0 * Corresponding author. Tel.: +44-(0)1344-770414; fax: +44-(0)1344-770918; e-mail: grwatts@e.trl. co.uk