Russell L. Martin

Several studies have described a release from speech-on-speech masking associated with separation of target and masker sources in the median sagittal plane. Some have excluded the possibility that small differences between target and masker interaural time disparities can fully account for this release. This study explored the mechanisms underlying the spatial release from speech-on-speech masking that can be obtained in the absence of such differences. In one condition, interaural time disparities were removed from the nominal median-sagittal-plane, head-related impulse responses used to generate the virtual auditory space within which competing sentences were presented. In other conditions, interaural level and spectral disparities also were manipulated by presenting competing sentences monaurally or diotically after convolution with one ear’s head-related impulse responses. It was found that substantial spatial release from masking can be obtained in the absence of any interaural disparities and that such disparities probably make a relatively minor contribution to spatial release from speech-on-speech masking in the median sagittal plane. It is argued that this release from masking is driven primarily by a reduction in informational masking that occurs when monaural information at one, or both, of the listener’s ears facilitates differentiation of competing sentences that emanate from spatially separated sources.

The contention that normally binaural listeners can localize sound under monaural conditions has been challenged by Wightman and Kistler (J. Acoust. Soc. Am. 101:1050–1063, 1997), who found that listeners are almost completely unable to localize virtual sources of sound when sound is presented to only one ear. Wightman and Kistler’s results raise the question of whether monaural spectral cues are used by listeners to localize sound under binaural conditions. We have examined the possibility that monaural spectral cues provide useful information regarding sound-source elevation and front–back hemifield when interaural time differences are available to specify sound-source lateral angle. The accuracy with which elevation and front–back hemifield could be determined was compared between a monaural condition and a binaural condition in which a wide-band signal was presented to the near ear and a version of the signal that had been lowpass-filtered at 2.5 kHz was presented to the far ear. It was found that accuracy was substantially greater in the latter condition, suggesting that information regarding sound-source lateral angle is required for monaural spectral cues to elevation and front–back hemifield to be correctly interpreted.

Previous studies on the effect of hypobaric hypoxia on auditory sensitivity are not readily interpretable, in most cases because the potential effect of ambient pressure on stimulus level was not considered. In this study, auditory sensitivity to 1, 8, 12, and 16 kHz tones was compared between conditions of hypoxia and normoxia at the same simulated altitude (3700 m). In the hypoxic condition, the partial pressure of oxygen in the inspired air was allowed to decrease with increasing altitude. In the normoxic condition, the partial pressure of oxygen was maintained at a level equivalent to that experienced at mean sea level (MSL). This comparison also controlled for any effect resulting from physiological consequences of hypobaria other than hypoxia (such as a change in middle-ear impedance). A small (2.57 dB) reduction in sensitivity across the frequency range tested was observed. A reduction in sensitivity of this magnitude would not be expected to have a large impact on the effectiveness of information transfer via the auditory modality.

Three-dimensional (3D) audio displays have considerable potential for conveying spatial information in the aviation environment. How reliably these displays will function in that environment will depend in part on the extent to which sound localization is affected by hypoxia. Many aircraft systems operators are routinely exposed to mild hypoxia, and all are at risk of exposure to more severe hypoxia. We have examined hypoxia's effect on localization by measuring the localization performance of four subjects exposed to simulated altitudes of 0, 1200, 2400 and 3700 m above sea level for about 30 min. Sounds to be localized were presented from a free-field source at locations covering 360 degrees of azimuth and extending from -47.6 to + 80 degrees elevation. Localization performance was statistically indistinguishable across the altitudes tested. Average localization errors ranged from 12.6 degrees +/- 0.7 SE at 2400 m to 14 degrees +/- 0.84 SE at 0 m. The finding that hypoxia induced by exposure to simulated altitudes as high as 3700 m has no effect of sound localization is encouraging with respect to the continued development of 3D audio displays for use in the aviation environment.