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

Abstract

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 f_c1 : the lower frequency of the advantage of double plate and f_c2 : the upper frequency of the advantage of double plate. At frequencies lower than f_c1, 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 f_c2, 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, f_c2 becomes lower, but f_c1 almost remains. When the open area ratio becomes larger than a value about 0.5 %, f_c1 becomes lower and f_c2 almost remains in contrast to above. Therefore, the frequency range of the advantage of double plate; f_c2 - f_c1 is the narrowest at an open area ratio about 0.5 %.