Sound absorption with impedance gradient

Duct noise absorption by impedance gradient has attracted significant attention from researchers seeking to achieve broadband performance, and one such configuration is described as a "black-hole" , in theory, waves are slowed down to a complete stop. A typical construction consists of annular rings with the sound passage radius decreasing gradually along the waveguide. The spectrum of the reflection coefficient is smooth and a good performance is achieved when a non-reflecting terminal condition is enforced. However, even under ideal conditions, partial reflection occurs before reaching the terminal, particularly in the low-frequency range. This talk examines the wave behaviour around the terminal, and also compares the sound absorption performance with traditional acoustic wedge as well as flat absorbers of the same profile size. It is found that flat absorber with varying flow resistivity exhibits the highest sound absorption for a pink noise spectrum.