Optimal damping for a bend containing a microperforated panel

Abstract

Microperforated panels (MPPs) with a backing cavity can be used as liners on duct walls to damp acoustic waves. However, recent studies show that a MPP (without the backing cavity) placed inside a duct also could give rise to an effective damping. But only if there is an acoustic pressure gradient across the MPP, e.g., as in a duct bend. This offers a new way of designing MPP for control of duct acoustics and leads to a variety of potential applications. In particular, the widely used guide vanes in a bend might be replaced by MPPs which serve as an acoustic damping treatment, while at the same time maintain the aerodynamic performance. In this work, the optimal damping by a MPP in a bend will be investigated based on the so called Cremer impedance theory. A derivation of the governing equation associated with this optimal value will be made for this configuration and the procedure for solving the equation presented. The trajectory of the axial wavenumber in the complex domain is investigated as well as the resulting optimal damping and the Cremer impedance. The results will be compared with the corresponding Cremer solution for a straight duct. Finally, possibilities to realize the Cremer impedance required for the optimal damping in a bend is discussed.