The use of passive ranging techniques for imaging targets situated in the near field for the audio frequency band is presented in this thesis. A three port interferometer used in millimetre waves is introduced and it is shown that with some approximations, it can be used to passively image targets in the medium-far field. The geometry of this system is then adapted for use in the near field in the audio frequency band, without the need for making any approximations. A model is then constructed to demonstrate the theory. The design, construction and testing of the new system for use in an anechoic chamber is then presented. This thesis also introduces a number of digital signal processing techniques that take full advantage of the recent increase in computing power and decrease in costs. These techniques are then used to perform frequency analysis on real data obtained experimentally from the new three port system. A number of different experiments and results are then presented. A loudspeaker is used as a test source and it is shown that the two separate drive units can be distinguished very accurately with the system. The whole system is then fully characterized and the optimal system setup conditions are found. Finally some ideas for future work and possible applications are presented. It is shown that the system would make an ideal tool for testing the spatial coherence properties of loudspeakers.