1. A method of modifying a signal representing a sound which is to be applied as a sound to a listener's ear to simulate the origin of that sound at a selected position in space with respect to the listener's ear, comprising the steps of: (a) measuring the filter function for sound originating from a sound source at a plurality of discrete positions in the space surrounding an origin position at which the sound is measured, the measurement position corresponding to the position of a listener's ear; (b) determining a model filter function for each position at which sound originates which approximates in both magnitude and phase the actual measured filter function at each position, the model filter function formed as a sum of a selected number of basic filter functions which are functions only of frequency or time and not of position, with each basic filter function multiplied by a weighting factor for that basic filter function which is a function only of the position at which the sound originated and not of frequency or time; (c) applying the filter function for a selected position as a filter to the signal representing sound to produce a filtered signal; and (d) converting the filtered signal to a sound and applying the sound to the ear of a listener.