Humans, like most vertebrates, have two ears that are positioned at about equal height at the two sides of the head. Physically, the two ears and the head form an antenna system, mounted on a mobile base. This antenna system receives elastomechanical (acoustic) waves of the medium in which it is immersed, usually air. The two waves received and transmitted by the two ears are the physiologically adequate input to a specific sensory system, the auditory system.
The peripheral parts of the auditory system transform each of the two waves into neural spike trains, after having performed a running spectral decomposition into multiple frequency channels, among other preprocessing. The multi-channel neural spike trains from each of the two ears are then combined in a sophisticated way to generate a running ``binaural-activity pattern'' somewhere in the auditory system. This binaural-activity pattern, most probably in combination with monaural-activity patterns rendered individually by each ear's auditory channels, forms the auditory input to the cortex, which represents a powerful biologic multi-purpose parallel computer with a huge memory and various interfaces and in- and output ports. As an output, the cortex delivers an individual perceptual world and, eventually, neural commands to trigger and control specific motoric expressions.
It goes without saying that a number of constraints must to hold for this story to be true. For example, the acoustic waves must be in the range of audibility with respect to frequency range and intensity, the auditory system must be operative, and the cortex must be in a conscious mode, ready to accept and interpret auditory information. Further, it makes sense to assume that multiple sources of feedback are involved in the processes of reception, processing and interpretation of acoustic signals. Feedback clearly occurs between the modules of the subcortical auditory system, and between this system and the cortex. Obvious feedback from higher centers of the central nervous system to the motoric positioning system of the ears-and-head array can also be observed whenever position-finding movements of the head are induced.
Although humans can hear with one ear only - so called monaural hearing - hearing with two functioning ears is clearly superior. This fact can best be appreciated by considering the biological role of hearing. Specifically, it is the biological role of hearing to gather information about the environment, particularly about the spatial positions and trajectories of sound sources and about their state of activity. Further, it should be recalled in this context that interindividual communication is predominantly performed acoustically, with brains deciphering meanings as encoded into acoustic signals by other brains.
In regard of this generic role of hearing, the advantage of binaural as compared to monaural hearing stands out clearly in terms of performance, particularly in the following areas [27]:
Binaural Technology is a body of methods that involve the acoustic input signals to both ears of the listener for achieving practical purposes, e.g., by recording, analyzing, synthesizing, processing, presenting and evaluating such signals.
Binaural Technology has recently gained in economic momentum, both on its own and as an enabling technology for more complex applications. A specialized industry for Binaural Technology is rapidly developing. It is the purpose of this chapter to take a brief look at this exciting process and to reflect on the bases on which this technology rests, i.e., on its experimental and theoretical foundations. As has been discussed above, there are basically three ``modules'' engaged in the reception, perception and interpretation of acoustical signals: the ears-and-head array, the subcortical auditory system, and the cortex. Binaural Technology makes use of knowledge of the functional principles of each. In the following three sections, particular functions of these three modules are reviewed in the light of their specific application in Binaural Technology.
Esprit Project 8579/MIAMI (Schomaker et al., '95)