Optimum duct liners and modal filters

Abstract

Two strategies for applying micro-perforated plates (MPP:s) to create duct sections with a high or optimum damping of sound are discussed. The first is based on realizing the so called Cremer impedance by creating a locally reacting wall boundary (“liner”) that consists of a MPP backed by an air cavity. This principal can be used to realize the Cremer impedance at a single frequency where typically a damping of several hundred dB/m can be achieved. The proposed solution is robust and gives high damping also in a wide band around the target (optimum) frequency. The second strategy is based on MPP plates arranged along pressure nodes (minima) for a given acoustic cross-mode. This means that the MPP will be located at modal velocity maxima, thereby creating a large modal damping. In principle both strategies are optimum for a particular mode and can be seen as a modal filter, i.e., a device that reduces or “kills” a particular incident mode. The proposed strategies have been investigated theoretically as wel...