Abstract Perforated plates are used extensively to absorb sound. A screen perforated by a regular array of parallel slits, with a mean bias flow through the slits, is investigated. The interaction between an incident sound wave and this mean flow converts acoustical energy into unsteady vortical motion. This vorticity radiates sound ineffectively, so that a significant proportion of the incident energy may be absorbed. The presence of a mean bias flow therefore leads to a linear mechanism of sound absorption. The maximum absorption coefficient from an isolated screen is found to be 1 2 , but it can be greatly increased by placing a rigid surface behind the screen. It is predicted theoretically that a backed screen with parallel slits can absorb all the sound incident on it, provided the gap between the screen and the rigid surface is approximately one-quarter of a wavelength and the mean flow velocity through the slits is chosen appropriately. Experimental results are presented which show encouraging agreement with the theory.