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...overall level ambient noise, they may also have the undesirable consequence of attenuating auditory stimuli that are relevant to an operator's task. For example, they may impair an operator's ability to detect and recognize warning signals (Casali Wright, 1995; Robinson & Casali, 1995), understand speech (Gower & Casali, 1994; Robinson & Casali, 2000; Van Wijngaarden & Rots, 2001; Wagstaff & Woxen, 2001), identify operationally relevant environmental sounds, and localize sound (see, e.g., Noble, Murray, & Waugh, 1990). Indeed, the inability to identify or localize sounds has been implicated as a contributing factor to occupational accidents (Laroche, 1994; Laroche, Ross, Lefebvre, & Larocque, 1995).
In an effort to more fully understand the impact of HPDs on operator performance, a number of researchers have investigated the degree to which HPDs influence a listener's ability to localize sound (Abel & Armstrong, 1993; Bolia, D'Angelo, Mishler, & Morris, 2001; Bolia & McKinley, 2000; Noble, 1981; Noble et al., 1990; Noble & Russell, 1972; Vause & Grantham, 1999). Results from these studies indicate that sound localization accuracy in both azimuth and elevation is degraded with HPDs as compared with when the listener's ears are unoccluded.
Azimuthal errors were largely attributable to an increase in front/back confusions, suggesting that spectral features in the 3- to 6-kHz region, which are believed to contribute to front/back discrimination (Shaw, 1997), were disrupted by the HPDs. However, when continuous stimuli were used and unrestricted head movements were allowed, localization in azimuth was largely restored, suggesting that listeners were able to use the corresponding changes in interaural time and level differences, which mediate localization in the left/fight dimension (Mills, 1972), to disambiguate the front/back location of a sound source. The use of exploratory head movements, however, also led to substantial increases in search times. Localization in the vertical dimension, which is mediated by features in the spectral fine structure above approximately 5 kHz (Blauert, 1969/1970; Butler & Belendiuk, 1977; Middlebrooks, 1992), remained relatively poor, however, even when these exploratory head movements were allowed (Noble, 1981; Noble et al., 1990).
Although these effects were found when either earplugs or earmuffs were worn, they were most pronounced with earmuffs, which effectively obscure the pinnae. These findings appear to indicate that high-frequency, pinna-based spectral cues, which mediate localization in the vertical and front/back dimensions, are substantially disrupted by the use of HPDs but that interaural difference cues remain relatively intact.
One issue that these earlier studies failed to consider was the impact of double hearing protection--earplugs and earmuffs worn simultaneously--on sound localization. Double hearing protection is a common practice, and often a requirement, in high-noise environments, including many military, aerospace, and industrial settings (National Institute of Occupational Safety and Health, 1998; U.S. Air Force, 1994). Although it is clear that sound is attenuated to a greater degree with double hearing protection than with either earplugs or earmuffs alone (e.g., see Berger, 1983), the impact of double hearing protection on sound localization per se is unknown. That is, the modifications imposed on the incident sound wave by the two HPDs worn simultaneously may lead to disruptions in localization cues that would not occur when either device alone is worn. Moreover, anecdotal reports from operators in high-noise environments suggest that sound localization is extremely difficult with double hearing protection. These effects need to be quantified.
The effects of HPDs on the orienting behavior of listeners in a sound localization task have not previously been examined in detail. As discussed previously, results in the literature have shown that listeners can improve localization accuracy by employing exploratory head movements to resolve some ambiguous localization cues. However, localization degrades differently in the left/fight, front/back, and vertical dimensions when HPDs are worn, suggesting that the relevant cues for sound localization in each of these dimensions are affected differently. It is reasonable, therefore, to assume these differences would be manifested in specific orienting behaviors and that the types of head motion used when orienting to a sound could provide additional information about the ways that HPDs disrupt localization cues. For example, reliable azimuthal cues and relatively ambiguous elevation cues might lead to a target search consisting of a ballistic azimuthal change in head position followed by a serial search of the vertical plane.
The purpose of this investigation was to extend the findings of previous studies assessing the impact of HPDs on sound localization to the case in which listeners wear double hearing protection. A visual search task was employed in which an auditory stimulus was presented at the same location as the visual target in order to cue the target location. Because this task involves the integration of sensory information across the auditory and visual systems, it represents an ecologically valid scenario that is relevant to occupational...
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