Acoustic black holes and their applications for vibration damping and sound absorption

During the last decade, new interesting physical objects have been invented and investigated - ‘acoustic black holes’, whereby it is possible to achieve almost 100% absorption of the incident wave energy. The main principle of the ‘acoustic black hole effect’ is based on a gradual power-law-type decrease in velocity of the incident wave with propagation distance, linear or faster, to almost zero, which should be accompanied by efficient energy absorption in the area of low velocity via inserted highly absorbing materials. So far, this effect has been investigated mainly for flexural waves in thin plates for which the required gradual reduction in wave velocity with distance can be easily achieved by changing the plate local thickness according to a power law, with the power-law exponent being equal or larger than two. The present paper provides a brief review of the theory of acoustic black holes, including their comparison with ‘optic black holes’ invented about three years ago. It is shown in particular that optic black holes are based on the same principle that governs the behaviour of acoustic black holes. Review is also given of the recent, mainly experimental, work carried out at Loughborough University on damping structural vibrations based on the acoustic black hole effect. This is followed by the discussion on potential applications of the acoustic black hole effect for sound absorption in air.