Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The Reduction of engine noise, one of the primary noise sources in trucks and busses, is essential in order to fulfill various noise emission regulations such as ISO 362 . At the same time, it is equally important to meet market demands in order to attract new customers while competing with other brands to lower overall noise levels.
Sound power is a convenient descriptor of noise emissions when compared to sound pressure it is not dependent on the distance from the source and the surrounding environment. A number of standards for sound power measurement exist, requiring different methods, tools and environments. In engine development at Scania the sound power level is measured for different engine types for noise level determination and comparison purposes. Additionally, attempts to reduce noise through modifications of engine parts require many iterations in which sound pressures recorded at specific microphone positions are of primary interest. The necessity of running each engine at different speeds and load conditions with various modifications during development (combined with time restrictions) narrows down the choices to ISO 3745 which involves measuring sound power with stationary microphones.
Despite ISO3745 apparent ease of use, prerequisites such as the number of microphones, the distance limitation of the microphones and free field conditions often pose a practical challenge. In Scania’s anechoic chamber it is impossible to meet these requirements due to limitations inherent to the room design such as size, poor absorption and limited space on the underside of the engine. This thesis comprises engine acoustic simulations in Nastran together with various measurements. Based on these, guidelines for power calculation have been developed taking into account the level of uncertainty and correction factors.
2012. , 87 p.