The acoustic treatment of free-field rooms usually takes the form of projecting wedges of sound absorbing material. Measurements of the reflexion coefficient of such wedges, made at normal incidence in a travelling wave duct, show that it is possible to achieve a reflexion coefficient R less than 0.1 when the ratio of the length L of the wedges to the acoustic wavelength λ is not less than about 0.2. Existing theories of the absorbent properties of wedges are unsatisfactory since they predict that 1/λ has to exceed about 0.4 before R is consistently less than 0.1, and all constants of the material are assumed independent of frequency. A technique is here described whereby theoretical values comparable with the above-mentioned experimental results can be obtained by allowing for the mechanical properties of the material. This technique involves the multiplication of a number of matrices each having two rows and columns, and unit determinant. This can be done by means of an electronic computer programme in which several adjustable parameters can be fed in initially by means of a data-tape. A particular set of values of these parameters (Equation (13)) is such that the calculated R < 0.1 for 1/λ > 0.2. Other sets of values indicate that theoretically R can be less than 0.1 for l/λ above 0.1: this is better than has so far been obtained in practice. Further calculations could be made with little or no modification of the existing computer programme for many other such sets. At present, however, there is insufficient information on the actual variation of compressibility and density within the wedges to determine what values of the parameters are physically acceptable. This information is badly needed before further calculations can be profitable.