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To identify the optimum structure of noise-proof cover with apertures, the insertion loss of three noise-insulating-absorbing-structures ("bare single plate", "absorber inserted double plate", "single plate with inside absorption") are calculated by transfer matrix method and acoustical energy balance between input and output. The calculated insertion loss were compared from the viewpoint of build-up effect and total transmission loss. The result shows that the insertion loss of the absorber inserted double plate is larger than the others between two frequencies fc1 : the lower frequency of the advantage of double plate and fc2 : the upper frequency of the advantage of double plate. At frequencies lower than fc1, the insertion loss of absorber inserted double plate is lower than that of single plate with inside absorption because of the resonance effect between the double plates. At frequencies higher than fc2, the insertion loss of absorber inserted double plate is also lower than that of single plate with inside absorption because of the build-up effect caused by low absorption of double plate. When the open area ratio becomes larger, fc2 becomes lower, but fc1 almost remains. When the open area ratio becomes larger than a value about 0.5 %, fc1 becomes lower and fc2 almost remains in contrast to above. Therefore, the frequency range of the advantage of double plate; fc2 fc1 is the narrowest at an open area ratio about 0.5 %.

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Dependency of open area ratio on noise reduction for cover structure with apertures (Comparison between absorber inserted double plate and single plate with absorption)