A waveguide oriented numerical simulation model for horn loudspeakers has been developed. The model is able to predict the output from horns with square openings under general conditions. The primary resonance of the driver is included in the simulations and comparison with measurements on two horn-type loudspeakers is made. The agreement between measured and calculated results is very good provided that a sufficient number of modes is included in the simulation. Simulation models of this kind represent one of the first steps towards a CAD tool for outdoor sound reinforcement systems.