Low Frequency Sound Transmission of Double Plates in an Impedance Tube

In this paper, low frequency STL (Sound Transmission Loss) of single and double plates installed in an impedance tube is discussed using an analytic method. The vibration motion of the plates and sound pressure field are expanded in terms of the infinite series of modal functions. Vibration mode is assumed as a product of the beam modes corresponding to the crosswise directions. Under the plane wave condition, the low frequency approximation is derived by including the first few symmetric modes, where anti-symmetric modes do not contribute to wave propagation. Comparison of the STL done by the proposed method with the FEM results shows an excellent agreement. It is found that the peak frequencies of the double plates coincide with those of each single plate. When the two plates are identical, the STL becomes zero at the natural frequencies of the single plate. However, when the two plates are not identical, the STL is always greater than zero. The location and amplitude of the dips are investigated using an approximate solution when the cavity depth is very small.