The ears-and-head array is an antenna system with complex and specific transmission characteristics. Since it is a physical structure and sound propagation is a linear process, the array can be considered to be a linear system. By taking an incoming sound wave as the input and the sound pressure signals at the two eardrums as the output, it is correct to describe the system as a set of two self-adjusting filters connected to the same input. Self-adjusting, in the sense used here, means that the filters automatically provide transfer functions that are specific with regard to the geometrical orientation of the wavefront relative to the ears-and-head array.
Physically, this behavior is explained by resonances in the open cavity formed from pinna, ear canal and eardrum, and by diffraction and reflection by head and torso. These various phenomena are excited differently when a sound wave impinges from different directions and/or with different curvatures of the wavefront. The resulting transfer functions are generally different for the two filters, thus causing ``interaural'' differences of the sound-pressure signals at the two eardrums. Since the linear distortions superimposed upon the sound wave by the two ``ear filters'' are very specific with respect to the geometric parameters of the sound wave, it is not far from the mark to say that the ears-and-head system encodes information about the position of sound sources in space, relative to this antenna system, into temporal and spectral attributes of the signals at the eardrums and into their interaural differences. All manipulations applied to the sound signals by the ears-and-head array are purely physical and linear. It is obvious, therefore, that they can be simulated. As a matter of fact, there is one important branch of Binaural Technology that attempts to do just this.
It makes sense at this point to begin the technological discussion with the earliest, and still a very important application, of Binaural Technology, namely, authentic auditory reproduction. Authentic auditory reproduction has been achieved when listeners hear exactly the same in a reproduction situation what they would hear in an original sound field, the latter existing at a different time and/or location. As a working hypothesis, Binaural Technology begins with the assumption that listeners hear the same in a reproduction situation as in an original sound field when the signals at the two ear-drums are exactly the same during reproduction as in the original field. Technologically, this goal is achieved by means of so-called artificial heads which are replicas of natural heads in terms of acoustics, i.e. they realize two self-adjusting ear filters like natural heads.
Artificial heads, in combination with adequate playback equipment, are a basic instrumentation for a number of economically appealing applications. The playback equipment, needed for this application, is usually based on headphones. Yet, under specific, restricted conditions, loudspeakers can also be used. A first category of application in this context is subsumed under the following section.
Esprit Project 8579/MIAMI (Schomaker et al., '95)