Experimental Investigation of Sound Absorption Properties of the Damped Sonic Black Holes

The Sonic Black Holes (SBH) absorber consists a cylindrical cavity with a set of rigid thin rings. The inner diameters of the rings are gradually decreasing. The sound speed in SBH can be progressively slowed down, then the effective sound absorption performance can be obtained. In order to further improve the acoustic performance of the SBH, the sponges are used as sound-absorbing material in SBH. The sponges of 2 mm thickness are arranged at the bottom and the different ranges of the inner wall of the SBH. The absorption coefficients of the SBH before and after the sponge arrangement are experimentally investigated. The experimental results reveal that the SBH structure is different from the traditional vibration-based acoustic black hole (ABH) structure. The sound absorption performance cannot be improved effectively when the sponge is placed at the bottom of the SBH. However, when the sponge is placed on the inner wall around the bottom of the SBH, the sound absorption performance can be improved markedly.