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Pacific Highway study provides growing insight on truck engine compression brake noise.
The NSW Roads and Traffic Authority (RTA) engaged Heggies to review noise emissions and community impacts following the opening of an upgraded section of the Pacific Highway in northern NSW. The local community expressed concern regarding the levels of noise emission that seemed to have increased substantially since the upgrading works were completed. The reported increase in noise emissions came despite the RTA's use of a 2.5 m high noise wall along the upgraded highway. Residents have particularly targeted the noise from compression brakes on semi-trailers (especially B-Doubles) as a source of disturbance. Key members of the community were consulted and a comprehensive programme of investigation was jointly developed to address the major areas of concern. Other specific factors evaluated and assessed include the effectiveness of the noise walls, the total daily and nightly traffic noise "doses" due to the relatively continuous traffic flows - as well as the effects on sleep of the higher peaks of noise associated with the louder trucks and their braking systems. The influences of different weather conditions also form part of the study.Some truck engine compression brake noise has characteristic, raspy "sputtering" characteristic - the "level" of the audible noise emitted during compression braking is modulated at a rate associated with engine speed. It appears that the depth (and rise-time) of the modulation correlates closely with the subjective assessment of "disturbance potential".A suggested methodology for evaluating this component of compression brake noise has been put forward by researchers working on behalf of the National Road Transport Council. Heggies' study is one of the first applications of the proposed methodology to a real situation. During extensive night-time surveys of noise emission from the upgraded roadway, Heggies' field personal gathered operator-attended digital recording of truck pass-by noise signals for later analysis using the NRTC methodology and other techniques.It is hoped that further studies will embrace community members to judge the "acceptability" of engine compression brake noise. These ongoing studies will assist in further development of noise impact assessment methodologies that better address the special characteristics of engine and exhaust brake noise from trucks.
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