Damping of resonant vibrations utilising the acoustic black hole effect

One of the possible ways of damping resonant flexural vibrations of engineering structures or their components, e.g. finite plates or bars, is to reduce reflections of flexural waves from the free edges. A new emerging method of reducing edge reflections, which is being developed by the present author, uses specifically designed attachable plates or bars of variable thickness (non-linear wedges). Such plates or bars utilise gradual change in their thickness from the value corresponding to the thickness of the basic plate or bar, which are to be damped, to almost zero. It is proposed to use specific power-law shapes of these attachable wedges so that they would ideally provide zero reflection of flexural waves even for negligibly small material attenuation, thus materialising the so-called ‘acoustic black hole effect’. To make up for real manufactured wedges, that always have edge truncations, and to increase the efficiency of damping it is suggested also to deposit absorbing thin layers on wedge surfaces. According to the theoretical calculations, the deposition of thin absorbing layers on surfaces of wedges utilising the acoustic black hole effect can dramatically reduce the reflection coefficients, thus resulting in very efficient damping systems for flexural vibrations. The theory of the phenomenon based on geometrical-acoustics approach is discussed for different wedge profiles. Theoretical results are complemented by the preliminary experimental measurements carried out on a steel wedge of quadratic shape.