An experimental investigation on the acoustic properties of micro-perforated panels in a grazing flow

Micro-perforated panels (MPPs) are widely used for broadband sound absorptions. In the presence of flow, MPPs exhibit complex behavior, which can be characterized by their surface acoustic impedance. Compared with no-flow conditions, studies on MPPs in low-speed grazing flow, especially experimental ones, are scarce. This paper reports an experimental study on the acoustic impedance of MPPs inside a flow duct with grazing incident waves. Through comparisons with experimentally educed impedance data under various flow velocities, the full set of acoustic impedance formulae proposed in our previous work is validated, which is further used to investigate the sound absorptions of a MPP liner with honeycomb backing. Analyses on the sound absorption coefficient curves show that the presence of the grazing flow shifts the absorption peak to a higher frequency, widens the bandwidth and alters the maximum absorption value. Different from the no-flow conditions, however, the peak frequency is found to be less sensitive to the variation of the hole diameter when a flow is present.