SOUND POWER RADIATED FROM RECTANGULAR PLATES WITH UNCONSTRAINED DAMPING LAYERS

Abstract

Application of damping layers has been widely used to reduce noise and structural vibration, in particular for thin-walled structures. Damping layers that exhibit viscoelastic properties can be applied to the surface of a structure to dissipate vibration energy. This technique reduces the vibration response at resonance or increase the decay rate which can consequently be used to decrease sound radiation [1]. In this work, a numerical method to estimate the sound power radiation is applied to rectangular plates having an unconstrained damping layer treatment. Fluid loading is neglected so structural and acoustic analysis can be considered separately. Vibration analysis is based on the finite element method [2] and Ross-Ungar-Kerwin (RUK) theory [3], while acoustic radiation is treated by means of a lumped parameter model instead of the classical Rayleigh's integral formula [4,5]. Mechanical properties of a two-layer plate may be represented by an equivalent plate accounting for mass, stiffness and viscoelastic damping added on the plate by means of the RUK theory. Then, natural frequencies and vibration velocity distribution of the composite plate are easily obtained. In addition, the lumped parameter model and its associated radiation matrix provide quite efficient numerical computation of the sound power radiated. Numerical results show that the thickness and stiffness of the unconstrained damping layer have significant effects on the sound power radiated from the plate. It is also found that the sound power levels radiated from the plate with the coating treatment are not always lesser than those with the untreated plate. The relative stiffness and thickness of the unconstrained damping layer and the base plate must be taken into account when the system is designed to achieve low vibration and noise radiation levels. The method presented here appears quite useful for rapid design purposes. In addition, the matrix equation obtained for the sound power radiated may be used as part of an optimization scheme in which the sound power could be minimized. Thus, reusing of stored acoustic resistance matrix may lead to significant reduction in the CPU time.