The Physical and Psychophysical Basis of Sound Localization
Traditionally, the principal cues to a sound’s location are identified as the differences between the sound field at each ear. The obvious fact that we have two ears sampling the sound field under slightly different conditions makes these binaural cues se
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THE PHYSICAL
AND PsYCHOPHYSICAL BAsis OF SOUND LOCALIZATION Simon Carlile
1. PHYSICAL CUES TO A SOUND'S LOCATION 1.1.
T
THE DUPLEX THEORY OF AUDITORY LOCALIZATION
raditionally, the principal cues to a sound's location are identified as the differences between the sound field at each ear. The obvious fact that we have two ears sampling the sound field under slightly different conditions makes these binaural cues self-evident. A slightly more subtle concept underlying traditional thinking is that the differences between the ears are analyzed on a frequency by frequency basis. This idea has as its basis the notion that the inner ear encodes the sounds in terms of its spectral characteristics as opposed to its time domain characteristics. As a result, complex spectra are thought to be encoded within the nervous system as varying levels of activity across a wide range of auditory channels; each channel corresponding to a different segment of the frequency range. While there is much merit and an enormous amount of data supporting these ideas, they have tended to dominate research efforts to the exclusion of a number of other important features of processing. In contrast to these traditional views, there is a growing body of evidence that: (i) illustrates the important role of information available at each ear alone (monaural cues to sound location);
Virtual Auditory Space: Generation and Applications, edited by Simon Carlile.
© 1996 R.G. Landes Company.
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Virtual Auditory Space: Generation and Applications
(ii) suggests that processing across frequency is an important feature of those mechanisms analyzing cues to sound location (monaural and binaural spectral cues); (iii) suggests that the time (rather than frequency) domain characteristics of the sound may also play an important role in sound localization processing. The principal theoretical statement of the basis of sound localization has become know as the "duplex theory" of sound localization and has its roots in the work of Lord Rayleigh at the turn of the century. It is based on the fact that "the main difference between the two ears is that they are not in the same place." 1 Early formulations were based on a number of fairly rudimentary physical and psychophysical observations. Models of the behavior of sound waves around the head were made with simplifying approximations of the head as a sphere and the ears as two symmetrically placed point receivers (Fig. 2.1). 2 Despite these simplifications the resulting models had great explanatory and predictive power and have tended to dominate the research program for most of this century. The fact that we have two ears separated by a relatively large head means that, for sounds off the mid-line, there are differences in the path lengths from the sound source to each ear. This results in a difference in the time of arrival of the sound at each ear; this is referred to as the interaural time difference (lTD). This lTD manifests as a difference in the onset of sound at each ear and, for more continuous sounds,
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