|Title:||Noise reduction using double wall enclosure technique|
Hong Kong Polytechnic -- Dissertations
|Department:||Department of Civil and Structural Engineering|
|Pages:||1 v. (various pagings) : ill. ; 30 cm|
|Abstract:||The study on the noise reduction using double wall enclosure technique has put great emphasis on the characteristics of low frequencies noise reduction which are commonly generated by compressors or diesel engines. Three types of materials were used in the construction of the enclosures : plywood, acrylic sheets and galvanised sheets. The major testing element was the acrylic panels, which was constructed in double walls to form the enclosure. Thickness of the acrylic and also the air gap were varied and the acoustic parameters were measured. Other enclosures made of wood and Galvanised sheet were tested for comparison with the performance the acrylic enclosures. The relatively higher frequencies noise which is beyond the Mass Law regions were not studied. The performance parameters of the enclosure such as Insertion Loss and Transmission Loss were deduced from the measurements. The experiment indicates that the enclosures with 1mm and 34mm air gap and also the enclosure made of galvanised sheets were relatively less affected by the resonant frequency when compared with other enclosures made of acrylic sheets. The wooden enclosure was effective in reducing noise at frequencies below 250Hz. It was revealed that the text book constant C used to deduce the power increase due to the diffused field inside the enclosure does not correctly predict the power increase and thus leads to a discrepancy to the prediction of the transmission loss. It is suggested that for small enclosures the sound field inside are not perfectly diffused and the near field effect or directly field have great contribution to the power increase. It was further revealed that the performance of the enclosure differ greatly from the prediction from Mass Law. However, if the enclosure were treated as its lowest resonance frequency and the loss were determined based on the stiffness-control characteristic of the enclosure, the equation (equation 2.43 on page 24,) used to predict the Insertion Loss of the panels shows a better agreement with the test results and differs by roughly a constant. The equation, though based on a one dimensional wave, is a better approximation to the performance of the enclosures.|
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