In Chapter 1 it was noted that the outer ear (pinna), ear canal, and middle ear modify and distort the acoustic signal before it reaches the inner ear. Although these changes would be considered serious defects if produced by microphones or other physical instruments, they can furnish valuable perceptual information. Thus, the complex acoustic changes produced by the pinnae when a source moves provide information concerning position, and result in perception of an unchanging sound at a changing location. This chapter discusses the effects produced by the pinnae, as well as other examples of how changes in acoustic input are not perceived as differences in the nature of the sound, but rather as differences in the location of the sound source relative to the listener's head.

Obviously, it often is important for us to know where the sound is originating, but there is another advantage associated with the ability to localize sound. As we shall see, mechanisms employed for localization allow us to hear signals which would otherwise be inaudible.

Any position in space can be specified relative to an observer by its azimuth (angle from straight ahead measured in the horizontal plane), elevation (angle from the horizontal measured in a vertical plane), and distance. Unfortunately, it is difficult to mimic the localization of sound sources in space using headphones because the sounds generated by them usually have apparent sources positioned within the listener's head (for reasons that will be discussed subsequently).