Experimental investigation of the acoustic black hole for sound absorption in air

In the present paper, the results of the first experimental investigation of the acoustic black hole for sound absorption in air are described. To achieve the required power-law decrease in sound velocity with propagation distance the inhomogeneous acoustic waveguides earlier proposed by Mironov and Pislyakov (2002) and made of quasi-periodic ribbed structures materialising walls of variable impedance have been used. Two different samples of acoustic black holes formed by these ribbed structures have been manufactured to provide linear and quadratic decreases in acoustic wave velocity with distance. Small pieces of absorbing porous material have been inserted at the end. Measurements of the reflection coefficients for guided acoustic modes incident on the black holes have been carried out in the frequency range of 100-1000 Hz. The results show the possibility of significant reduction in the acoustic reflection without using additional absorbing materials. However, contrary to the expectations, the introduction of absorbing materials did not cause further noticeable reduction in the sound reflection coefficients.