Fundamentals of directional hearing

Determining the nature of objects in our world is a basic necessity of life. Hear ing allows us to de te rmine objects in our world based on the sounds that they produce . There are several propert ies of a sound that might contr ibute to this process (see Yost, 1992, for a review). For instance, sounds from objects at different locations in space present different acoustic pat terns to the ears of a listener. Listeners can use these differences in sound pat tern to locate a source, and one 's ability to locate the source aids in de te rmining its identity. Tha t is, the auditory system can use the acoustic information that arrives at our ears to de termine the location of that source. This article will review some of the fundamental facts concerning sound localization. There are several recent publications that provide more detailed reviews of directional hear ing than we will be able to provide in this article (Blauert, 1983; Gilkey & Anderson, 1997; Wightman & Kistler, 1993; Yost & Gourevitch, 1987). Objects are located in three spatial dimensions and can be located in these three dimensions based on the sounds they produce. Figure 1 describes the spatial dimensions as they will be used in this chapter. Space is described relative to a listener sitting at the center of a sphere. Depth (range) is defined by the radius of the sphere (the distance from the listener to the sphere's boundary) . Azimuth (horizontal angle) is defined by dividing the circle a round the listener into 360°, such that 0°is straight ahead, +90° is directly out from the right ear of the listener, 180° is directly behind the listener, and —90° (or 270°) is directly out from the left ear. A vertical angle (elevation) is defined by an arc that rises from below the listener to above the listener, such that 0° is again in front, +90° is overhead, and —90° (or 270°) is below. Thus, the position of any object in space can be specified by three numbers: range distance, azimuthal angle, and vertical angle.