Experimental study on damping of flexural waves in rectangular plates by means of one-dimensional acoustic 'Black Holes'

In this paper we present some recent experimental results on new lightweight and broad-band damping treatment for rectangular plates based on the so-called acoustic ‘black hole’ effect [1-5], which represents one of the most efficient ways of creating graded impedance interfaces [6] to reduce edge reflections of flexural waves. These acoustic black holes, or vibration 'traps', use elastic wedges of variable thickness defined by a power-law relationship h(x) = ε·xm (with m ≥ 2) to reduce edge reflections. In the ideal case of no edge truncations, bending wave velocities decrease to zero in such a way that the waves never reach the end and hence do not reflect back. They thus represent one-dimensional acoustic ‘black holes’ for flexural waves. It was predicted [2,3] that very low values of reflection coefficient can be achieved even in the presence of truncations and imperfections when a narrow layer of absorbing material is attached to its surface in order to dissipate the remaining energy (note that direct application of thin layers of absorbing materials to the surfaces of rectangular plates has a negligible influence on damping, which has also been demonstrated during the tests). (Continues...)