This application relates to a system for fitting a hearing aid by testing the hearing aid patient with a three-dimensional sound field having one or more localized sound sources. In one embodiment, a signal processing system employing head-related transfer functions is used to produce audio signals that simulate a three-dimensional sound field when a sound source driven by such audio signals is coupled directly to one or both ears. By transmitting the audio signals produced by the signal processing system to the hearing aid by means of a wired or wireless connection, the hearing aid itself may be used as the sound source.