Investigation of the acoustic black hole termination for sound waves propagating in cylindrical waveguides

So far, acoustic black holes have been investigated mainly for flexural waves in thin plates for which the required linear or higher order reduction in wave velocity with distance can be easily achieved by changing the plate’s local thickness. In the present paper, the results of the experimental investigations 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 have been manufactured to provide linear and quadratic decreases in acoustic wave velocity with distance. 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. Initial measurements were conducted without insertion of any absorbing materials. The results show the possibility of significant reduction of the acoustic reflection in this case. Addition of small pieces of absorbing porous materials caused further reduction in the reflection coefficients, albeit not as significant as it could be expected.