A method of attenuating propagating waves in a duct by actively operated sources mounted in the walls of the duct is considered, and a simple control system for the operation of these sources is proposed. The problem considered is as follows. A plane wave disturbance is propagating in the downstream direction through a uniform flow of Mach number M contained in a long duct. It is required to find a localized distribution of point sources mounted in the duct walls which will produce a radiation pattern interfering with the disturbance plane wave and thus attenuating it. It is found that for a circular duct an array consisting of two separated rings of sources, each composed of three point sources, can be made to generate a plane wave output in one direction only. Such an array of sources will not generate any propagating transverse modes for frequencies below ωu(M) = 2·1 ωc(M) where ωc(M) is the fundamental cut-off frequency of the duct. The equivalent result for a square duct involves using two rings of four point sources, and this will generate a unidirectional plane wave output for frequencies up to ωu(M) = 2·8 ωe(M). Such an array of sources will produce an output of useful amplitude over a frequency range of 2⅓ octaves. But it is found that by adding a third ring of sources and by suitably choosing the spacing between the rings, one can generate a plane wave output over a frequency range of 4⅓ octaves. This unidirectional output may then be used to interfere with, the disturbance plane wave, and under these circumstances the sources are effectively absorbing the energy of the disturbance. Before the sources can be operated correctly, it is necessary to detect the incoming plane wave accurately at some point upstream of the sources, and to separate it from any other disturbance which may be present in the duct. It is shown that by using an array of detectors similar to the array of sources it is possible to detect plane waves travelling in the downstream direction only, and to filter out any propagating transverse modes for frequencies up to ωu(M). By ensuring that only waves travelling in the downstream direction are detected, interaction between the source output and the detector input is avoided. Finally a control system is discussed which would operate the sources in the required manner. The principal advantage which such a method of sound attenuation would have over more conventional techniques is the very short length required to achieve absorption: for two sources the separation required is only 0·19(1 — M)λm, where λm is the mid-band wavelength. Moreover the source array should be able to absorb sound of frequency covering a comparatively broad spectrum: the effect is not confined to a narrow “tuned” frequency range.