Front back confusion in systems for the production of virtual acoustic images

This thesis investigates the causes of front back confusion in two systems that make use of digital signal processing for the production of virtual images. Front back confusion is well in known psychoacoustics and describes the phenomenon occurring when a source placed behind a listener is perceived to be in front, or vice versa. The two virtual imaging systems studied are designed to use respectively two and four channels to give listeners the impression that sources of sound exist at locations that are other than those used for reproduction. The four channel system is shown to be robust with regard to the production of convincing images at any location in the horizontal plane around the listener. The two channel system is able to achieve good images in front of the listener, but not behind. Both reproduction systems are examined with reference to the signals received at the ears of a listener. These signals are compared to those produced by real sources placed at different locations around the listener.This investigation is carried out by using a simple model of the listeners head based on the sound field scattered by a rigid sphere. These models indicate that the difference between the two systems could be explained in terms of the interaural time delay ( ITD ). The ITD was measured by calculating the interaural cross-correlation function ( IACC ). Head movement has been shown by previous work to play a very important part in resolving front back confusion. It is concluded that the rate of change of the IACC with head rotation is an important possible means of resolving these confusions, and that the four channel system is capable of closely replicating this measure. The changes in the IACC resulting from head movements were also modelled and are in accord with the results of the subjective experiments carried out to test the hypothesis from the modelling work. It is believed that this is the method used by the hearing system to interpret the localisation cues contained within the ITD's.