Aims: We suggest an explanation for the high-frequency power excess surrounding active regions known as seismic halos. Methods: We use numerical simulations of magneto-acoustic wave propagation in a magnetostatic sunspot model. Results: We propose that seismic halos can be caused by the additional energy injected by high-frequency fast mode waves refracted in the higher atmosphere due to the rapid increase of the Alfvén speed. Our model qualitatively explains the magnitude of the halo and allows us to make predictions of its behavior that can be checked in future observations.