Academic journal article Human Factors

The Impact of Signal Bandwidth on Auditory Localization: Implications for the Design of Three-Dimensional Audio Displays

Academic journal article Human Factors

The Impact of Signal Bandwidth on Auditory Localization: Implications for the Design of Three-Dimensional Audio Displays

Article excerpt

INTRODUCTION

A new type of interface, the three-dimensional (3D) audio display, is being developed to enhance cockpit displays in military aircraft. Recent advances in digital processing technology now allow externalized 3D sound images to be presented in real time over headphones. Real-time presentation coupled with a head-tracking device allows the user to experience virtual sounds interactively. Virtual sounds can be moving or static and can be made to respond appropriately to the listener's head movements.

In order to synthesize a sound's location in space, acoustical transfer functions from the free field to the eardrum are measured for many sound source positions (in space) and incorporated into digital filters through which sounds are passed before presentation. These transfer functions, called head-related transfer functions (HRTFs), preserve the pattern of differences in time (arrival, phase) and intensity cues that occur between the ears. HRTFs also preserve the spectral modifications wrought on the incident waveform by the pinnae, head, and torso before it reaches the basilar membrane. These interaural difference and spectral cues serve to "code" an auditory signal's position in space, and the HRTF captures this code for any given position in auditory space.

It is generally accepted that spectral modifications made to an incoming signal after interacting with the head, torso, and pinnae serve to resolve the "cones of confusion" created by interaural differences cues (Mills, 1972) by supplying a code to that signal's elevation and front/back position (Butler, 1969b; Fisher & Freedman, 1968; Gardner & Gardner, 1973; Musicant & Butler, 1984; Oldfield & Parker, 1984b). These spectral features occur at frequencies from about 4 khz through 16 khz and vary in frequency and amplitude as a function of sound source position and individual pinna, head, and torso characteristics (Kistler & Wightman, 1992; Kuhn, 1987; Mehrgardt & Mellert, 1977; Middlebrooks, Makous, & Green, 1989; Shaw, 1979; Shaw & Teranishi, 1968).

Researchers seeking to implement synthesized 3D audio displays in the military environment are investigating a number of design issues. These issues include (a) whether digital filters based on one generalized set of HRTFs will allow accurate spatial synthesis for all individuals (Wenzel, Arruda, Kistler, & Wightman, 1993; Wenzel, Wightman, Kistler, & Foster, 1988); (b) the intelligibility and localizability of speech signals in 3D auditory displays (Begault & Wenzel, 1993; Ricard & Meirs, 1994); and (c) masking and release from masking effects associated with spatially encoded displays (Doll & Hanna, 1995; Doll, Hanna, & Russotti, 1992).

Another design and implementation issue concerns the effects of limited stimulus bandwidth on spatial synthesis. Most military aircraft, for example, have communication systems that are band-limited in their frequency response, effectively acting as low-pass filters and passing frequencies of up to only 4 to 6 khz. Many auditory displays or warnings currently used or recently designed for military use are also tonal or band-limited in nature (Patterson, 1985, 1989). If a signal's elevation and front/back position are coded by spectral cues that occur at frequencies between 4 and 16 khz, it would be reasonable to predict that band-limiting a signal could affect the accuracy with which listeners can localize the elevation or front/back position of that signal.

Indeed, some evidence suggests that restricting stimulus bandwidth affects the precision with which listeners can localize signals actually presented from elevations in front of the head (Butler, 1969a; Gardner, 1973; Roffler & Butler, 1968) or in the median sagittal plane (Hebrank & Wright, 1974; Blauert, 1969-1970). However, no study has systematically varied signal bandwidth and investigated the effects of this manipulation on the ability to localize signals actually presented from a range of elevation and front/back positions. …

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