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The present study focuses on a silencer built within the thick portion of a door edge and reports on the results of evaluating silencers by determining sound transmission loss via theoretical analysis and experiments on three types of silencers. The theoretical analysis involved determining the calculated values of sound transmission loss obtained using the transfer matrix method. The change in cross-sectional shape was analyzed by elemental division of the transfer matrix. Using the above, simulations were performed with respect to the optimum shape of the silencer. These theoretical analyses were then compared with the measurement results. Furthermore, the study includes the results of an experiment that attempted to restrain the dip in sound transmission loss by adding a non-woven fabric to the opening of the silencer. In a side branch silencer with an increasing shape wherein the longitudinal cross-section is a linear or an exponential function, the peak of the transmission loss was shifted to the lower frequency side when compared with that in the case of a rectangular side branch silencer. Furthermore, in comparisons between the two, the sound attenuation peak frequency was lower in the case of the exponential shape. The resonance of the side branch was suppressed by adding a non-woven fabric to the opening of the side branch silencer. As a result, the peak and dip of sound attenuation were alleviated, and the sound attenuation characteristics could be adjusted.

Study of arrayed side branch silencer for ventilation door (Improvement of attenuation characteristic using of the linear expanded shape side branch tube) AbstractIn this article, we examined several longitudinal-sectional sizes of a silencer whose volume was limited by the thickness and aperture ratio of the ventilation door. We report the results of improvement in the sound attenuation properties of these shapes. As an evaluation method for the silencer, we measured transmission losses using an impedance measurement tube and calculated them on the basis of a theoretical analysis. The following results were obtained from the evaluation of the silencer using the measurement and calculation results and from the verification of the improved shape of a silencer and silencer array. For a side branch tube silencer with a 1/3 aperture ratio, assuming that it is installed into the ventilation door, we observed a side branch tube with an linear expanded longitudinal-sectional shape by improving the shape using the theoretical analysis of transmission losses. In addition, as a result of the experiment with a sample on the basis of the theoretical analysis, both the experimental and the theoretical analysis were in good agreement. Furthermore, the expanded shape of the longitudinal-section of the side branch tube decreased the frequency of the sound attenuation peak. It was found that a silencer with a combination of four of these silencers can be arrayed side-by-side in a ventilation door; this results in a flat transmission loss of more than 5 dB over a broad band.