Effects on roadside noise levels of sound absorptive materials in noise barriers

There is considerable interest in the use of sound absorptive materials for reducing noise reflected from noise barriers although there is little reli able information on their effectiveness in practice. The primary object was to determine, under carefully controlled measurement conditions, the effec ts on roadside noise levels of applying sound absorptive materials to the t raffic face of noise barriers. Two sites were chosen where a modular type o f noise barrier had been erected. At one site the road ran in a shallow cut ting and 3.7 m high barriers had been erected on both sides of the road wit h a separation distance of 34 m. At the other site a single 3 m high barrie r had been erected adjacent to an eight lane highway. At both sites the pan els were of identical manufacture and were highly sound absorptive on the t raffic face but reflective on the reverse side. Measurement of traffic nois e were made close to the roads both behind and opposite the barriers with t he panels in the normal position and then measurements were repeated after the panels had been reversed such that the reflective side faced the traffi c. It was found that at both sites there was an increase of noise of genera lly less than 1 dB when the barrier face was changed from sound absorptive to reflective. (C) 1999 Transport Research Laboratory. Published by Elsevie r Science Ltd. All rights reserved.