not be recognized. Whereas the above discussion is rather speculative, it illustrates how acoustic signals could be used, or misused, for monitoring important events in situations like a sonar display in which the operator's visual channel is already overloaded.

In the context of display design, the notion of auditory scene analysis has been most influential in the recent interest in using abstract sounds, environmental sounds, and sonification for information display (Kramer, 1994). The idea is that one can systematically manipulate various features of auditory streams, effectively creating an auditory symbology that operates on a continuum from literal everyday sounds, such as the rattling of bottles being processed in a bottling plant (Gaver et al., 1991), to a completely abstract mapping of statistical data into sound parameters (Smith et al., 1990). Principles for design and synthesis can be gleaned from the fields of music (Blattner et al., 1989), psychoacoustics (Patterson, 1982), and higher-level cognitive studies of the acoustical determinants of perceptual organization (Bregman, 1990; Buxton et al., 1989). Recently, a few studies have also been concerned with methods for directly characterizing and modeling such environmental sounds as propeller cavitation, breaking or bouncing objects, and walking sounds (Howard, 1983; Warren and Verbrugge, 1984; Li et al., 1991). Other relevant research includes physically based acoustic models of sound source characteristics, such as radiation patterns (Morse and Ingard, 1968). Further discussion of some of these issues is presented in the section below on computer generation of nonspeech audio.

Adaptation to Unnatural Perceptual Cues

There are many SE situations in which a sensory display, or the manner in which such a display depends on the behavior of the operator of the SE system, deviates strongly from normal. Such situations can arise because of inadequacies in the design or construction of an SE system or because intentional deviations are introduced to improve performance. Whenever a situation of this type exists, it is important to be able to characterize the operator's ability to adapt.

There have been a number of instances in which acoustic signals have been used as substitutes for signals that would naturally be presented via other modalities (e.g., Massimino and Sheridan, 1993). Unfortunately, however, much of this work has not included careful experimentation on how subjects adapt to such sensory substitutions over time. A major notable exception to this can be found in the work on sonar systems for persons who are blind (Kay, 1974; Warren and Strelow, 1984, 1985; Strelow and Warren, 1985).

Most of the work on adaptation to unnatural perceptual cues has



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