Damping of double wall panels including a viscothermal air layer

This paper deals with the dynamic behaviour of double wall panels, with emphasis on damping and sound radiation. It will be shown that a narrow air layer separating the two plates of a panel significantly alters the mentioned quantities by its viscothermal properties. Numerical and experimental results will be presented for a double wall panel excited with a point force. Numerical results are obtained using a fully coupled vibro-acoustic finite element model. The air between the plates is modelled by special viscothermal elements. A very efficient frequency response method is used to determine the structural response to harmonic excitations. The radiated power is calculated with the help of socalled radiation modes. Experimental results are obtained using a special designed set-up. An electrodynamic shaker excites one panel and the excitation power is measured. The radiated sound power is measured with a sound intensity meter. The dissipated energy is determined by considering an energy balance. Typical parameters such as the distance between the plates and the ratio of plate thicknesses are varied. Numerical and experimental results agree fairly well. From the results it can be concluded that for narrow air layers, i.e. when the so called ’shear wave number’ is low, a large amount of energy is dissipated by viscothermal effects in the air layer