Compact asymmetric treatments for perfect sound absorption in ventilated problems

This presentation introduces compact asymmetric treatments for sound absorption, i.e., simultaneous cancellation of reflection and transmission, in a lined duct of constant cross section. The treatments are composed of side by side and sub-wavelength resonators. They are thus compact, which facilitates their practical use. The simplest asymmetric treatment is formed by a pair of detuned resonators that can lead to a perfect monochromatic sound absorption. Furthermore, the combination of multiple pairs of resonators generates multiple low quality factor absorption peaks leading to an absorption plateau. Two compact asymmetric treatments are presented: one is composed of quarter-wavelength porous resonators to prove their ability to achieve perfect sub-wavelength absorption in a duct problem and the other is composed of Helmholtz resonators flush mounted on the walls of a large cross-section duct. In both cases, the evanescent coupling between the resonators has an important impact on the treatments behavior and is thus accounted for during their optimization. The experimental results are found in good agreement with the theory and a mean absorption coefficient of 99% over a large target and subwavelength frequency range is observed for each treatment.