Modal sound transmission loss of a single leaf panel: Effects of inter-modal coupling

Sound transmission through a single leaf panel has mostly been discussed and explained by using the approaching wave concept, from which the well-known mass law can be derived. In this paper, the modal behavior in sound transmission coefficients is explored, and it is shown that the mutual modal radiation impedances in modal sound transmission coefficients may not be ignored even for a panel immersed in a light fluid. By introducing the equivalent modal impedance which incorporates the inter-modal coupling effect, an analytical expression for the modal sound transmission coefficient is derived, and the overall sound transmission coefficient is simply a modal superposition of modal sound transmission coefficients. A good correlation is obtained between analytical calculation and boundary element method. In addition, it is found that inter-modal coupling has noticeable effects in modal sound transmission coefficients in the subsonic region but may be ignored as modes become supersonic. It is also shown that the well-known mass law performance is attributed to all the supersonic modes.