Abstract
The peak in sound transmission through a single isotropic panel at the critical frequency causes a drop in transmission loss (TL) of that panel and of composite walls constructed with such a panel as a face sheet. In investigations on a composite wall with plywood face sheets and Styrofoam core, such a coincidence dip was not observed. The aim of this study is to investigate the disappearance of coincidence notch and the probable causes for such a phenomena. The bending stiffness of a single ply of plywood changes gradually from the softest direction to the stiffest direction. Therefore, the bending wave phase velocity and sound transmissibility change with direction. It is hypothesized that for a group of sound waves randomly incident on the panel, this change in transmissibility eliminates or lowers the peak and, therefore, the coincidence notch smoothens. A plywood panel consists of plies laminated with their grains in different directions. These laminations of the plys may reduce orthotropy in thick plywood panels. A comparison of theoretically redeicted and experimentally observed TL of plywood sheets supports the hypothesis that antisotropy is the reason for a lack of coincidence transmission for some plywood sheets.
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