A numerical model based on an indirect integral equation technique is presented, with application to horn loudspeakers. The horn is considered as an infinitely thin rigid screen, and is in the model replaced by a distribution of dipole sources. Horn drivers can be described by specifying a velocity distribution on part of the geometry. Numerical results compare favorably with experiments on the directivity of a simple axisymmetric test horn. The model is easily adjustable to different horn shapes, and a comparison has been made of the far field response of three horn geometries.