Calculation of acoustic absorption in ducts with perforated liners

A method of calculating the absorption of waves emanating from one end of a lined duct is described, together with the theories and correlations used in describing the acoustic impedance of the orifices in the presence of various mean flows. The results for a range of geometries are compared with one of the most comprehensive sets of absorption data from the literature. The approximation of assuming uniform acoustic pressures in the cavity is shown to give acceptable accuracy in all these cases, thus considerably speeding up the calculations. Generally, excellent agreement with the experimental data is obtained when there is no grazing flow, or comparatively weak grazing flow, with or without the bias flow. Peak absorption of more than 50% over a range of frequencies was measured and observed with a small bias flow driven through relatively large orifices in a single liner. The method and orifice data were thus validated for these conditions. However, there were some significant discrepancies for comparatively strong grazing flow with weak or no bias flow.