Reducing engine noise with an enclosure using micro perforated plate

Ship- and power plant combustion engines are complex technical systems that have been spared from noise reducing measures. Since these types of technical systems are commonly placed in sound-insulated rooms or at locations where the emitted sound does not affect people, little effort has been made to reduce the level of noise. The optimal way to reduce noise is at the source, where the sound creating forces are the target of the measure. However, the complexity of an engine makes this a very time consuming project. Described in this paper, the sound is reduced at the sound path, with an enclosure. For an engine, this is a very effective and feasible solution, which also is relatively fast to design and applicable to many different models. The purpose of this enclosure is to reduce sound power from the engine top with 10 dB(A). One part of this project was to construct a mathematical model for the design of an enclosure prototype. The model simulates a three layer wall, consisting of a micro perforated plate (MPP), an absorber and a solid plate and gives as results the absorption and the transmission. The result was confirmed with measurements. By then using classical room acoustics, it is possible to predict the insertion loss of an enclosure. To facilitate the usage of the mathematic model, a stand alone (C++) design application was created. A prototype was manufactured which was tested on the W6L34SG engine. It was tested with loudspeakers and under real condition. This paper describes the mathematical model of the project and also gives result between calculated and measured acoustic quantities.