Geddes tried to define a horn as a device built for the objective of the presenting a target impedance load for the driver, which follows from Webster's horn equation. The waveguide theory would concern a horn's wavefront and expansion, discarding Webster's plane wave assumption, with the new goal of predicting the directivity. In the loosest sense, it could be taken as anything intended to shape the radiated wave, with the acoustic impedance as a secondary concern, as it has become in practice for all high-frequency horns. The cone is normally of a conical [horn] shape, which presumably is used to create a spherical wavefront. There are a variety of practical problems with using the cone driver as a waveguide, but it can provide a means to control the directivity of the high-frequency driver being loaded.