Acoustic black hole effect

Acoustic black hole effect is a physical phenomenon describing a large increase in damping of flexural vibrations in plates of variable thickness due to the combined action of a thin absorbing layer attached to one or both surfaces of a plate and of the so-called acoustic black hole geometry of a plate. The acoustic black hole geometry can be materialised by creating a gradual reduction in local thickness of a plate with wave propagation distance to almost zero, i.e. by creating sharp elastic wedges. Such a gradual reduction should follow a power-law function of the propagation distance, with the power-law exponent being equal or larger than two. Consequently, the velocity of a propagating flexural wave is reduced with distance according to a power-law function with the exponent equal or larger than one. Under these conditions, the described acoustic black hole effect, often mentioned as simply 'acoustic black hole', provides almost 100% absorption of the incident flexural waves even for nonideally sharp (truncated) elastic wedges and for very small pieces of attached absorbing layers.

The acoustic black hole effect can exist also for air-borne sound propagating in specially modified acoustic waveguides containing some added absorbing materials and providing a required reduction in acoustic wave velocity with propagation distance according to a power-law profile, with the power-law exponent equal or larger than one.