Ideal Spaced Damping Treatments For Flexural Waves

Layers of viscoelastic material are useful in the damping of flexural waves in plates. Two basic damping treatments using such layers are the homogeneous damping layer analyzed by Oberst [Akust. Beih. 4 (1952)] and the constrained damping layer analyzed by the author [J. Acoust. Soc. Am. 30, 698 (1958)]. In these treatments the viscoelastic layer is distorted in extension and shear, respectively. In the present paper we show that with both basic treatments, the insertion of a “spacer” layer between the plate and the damping treatment serves to magnify the distortion of the damping material, thus increasing the damping factor. The ideal spacer should be incompressible and infinitely stiff in shear. Its extensional stiffness in a plane parallel to the plane of the plate should be negligible. We treat the case of spaced damping treatments with ideal spacers, showing the increase in damping as a function of spacer thickness. Significant increases in damping factor are found, amounting to a factor of 10 or more in some cases. Some measured damping factors are presented for treatments with and without spacers.