Sound localization with head movement: implications for 3-d audio displays.

Previous studies have shown that the accuracy of sound localization is improved if listeners are allowed to move their heads during signal presentation. This study describes the function relating localization accuracy to the extent of head movement in azimuth. Sounds that are difficult to localize were presented in the free field from sources at a wide range of azimuths and elevations. Sounds remained active until the participants' heads had rotated through windows ranging in width of 2, 4, 8, 16, 32, or 64° of azimuth. Error in determining sound-source elevation and the rate of front/back confusion were found to decrease with increases in azimuth window width. Error in determining sound-source lateral angle was not found to vary with azimuth window width. Implications for 3-d audio displays: the utility of a 3-d audio display for imparting spatial information is likely to be improved if operators are able to move their heads during signal presentation. Head movement may compensate in part for a paucity of spectral cues to sound-source location resulting from limitations in either the audio signals presented or the directional filters (i.e., head-related transfer functions) used to generate a display. However, head movements of a moderate size (i.e., through around 32° of azimuth) may be required to ensure that spatial information is conveyed with high accuracy.

Spatial release from speech-on-speech masking in the median sagittal plane.

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.

Memory for the locations of environmental sounds.

The accuracy with which a single source of sound can be localized has been examined in many studies, but very few studies have examined the ability of participants to determine the absolute locations of multiple sources of sound. The current study assessed participants' abilities to determine and remember the locations of up to six sources of environmental sound that were positioned at a range of azimuths and elevations in virtual auditory space. In experiment 1, a sequence of one to six sounds was presented one, three, or five times in each trial and the target sound was nominated following presentation of the last sequence. In experiment 2, memory load was held constant by nominating the target sound prior to a single sequence presentation. Localization accuracy was observed to decrease as the number of sounds was increased to three or more under the conditions of experiment 1, but not those of experiment 2. In experiment 1, localization was more accurate when sequences were presented more than once. Pronounced primacy and recency effects were observed for the six sound conditions in experiment 1. An analysis of errors for those conditions indicated that immediate temporal errors, but not immediate spatial errors, were over-represented.

A dual-process account of auditory change detection.

Listeners can be "deaf" to a substantial change in a scene comprising multiple auditory objects unless their attention has been directed to the changed object. It is unclear whether auditory change detection relies on identification of the objects in pre- and post-change scenes. We compared the rates at which listeners correctly identify changed objects with those predicted by change-detection models based on signal detection theory (SDT) and high-threshold theory (HTT). Detected changes were not identified as accurately as predicted by models based on either theory, suggesting that some changes are detected by a process that does not support change identification. Undetected changes were identified as accurately as predicted by the HTT model but much less accurately than predicted by the SDT models. The process underlying change detection was investigated further by determining receiver-operating characteristics (ROCs). ROCs did not conform to those predicted by either a SDT or a HTT model but were well modeled by a dual-process that incorporated HTT and SDT components. The dual-process model also accurately predicted the rates at which detected and undetected changes were correctly identified.

Localization of sound presented via a spatial audio display during visually induced vection in pitch, roll, and yaw.

INTRODUCTION: There is growing interest in the potential application of 3D audio displays to impart spatial information to aircraft pilots. As the aviation environment is dynamic, the effect on sound localization of the visual and vestibular stimulation resulting from motion during flight needs to be assessed. In this study, we investigated whether sound localization is affected by visual cues to self-motion. METHOD: Participants localized bursts of broadband acoustic noise presented from the frontal hemifield under conditions of yaw, pitch, and roll vection (around the z-, y-, and x-axis, respectively) induced by wide-field visual motion of a projected sinusoidal grating. Lateral and elevation components of localization errors were measured. RESULTS: Average signed lateral and elevation errors were less than 1.6 degrees for all vection conditions. The effects of vection condition on lateral and elevation errors were not statistically significant. Also, the interactions between vection condition and sound-source location for these errors were not significant. The magnitudes of the 95% confidence intervals for the differences in lateral and elevation errors between the no-vection condition and each of the other vection conditions did not exceed 3 degrees. CONCLUSION: In the presence of visual cues to yawing, pitching, or rolling self-motion, the perception of auditory space is generally consistent with the available head-centered auditory cues to sound-source location. These results support the application of 3D audio displays in dynamic visual environments such as that experienced during flight.

The role of spatial location in auditory search.

The majority of research findings to date indicate that spatial cues play a minor role in enhancing listeners' ability to parse and detect a sound of interest when it is presented in a complex auditory scene comprising multiple simultaneous sounds. Frequency and temporal differences between sound streams provide more reliable cues for scene analysis as well as for directing attention to relevant auditory 'objects' in complex displays. The present study used naturalistic sounds with varying spectro-temporal profiles to examine whether spatial separation of sound sources can enhance target detection in an auditory search paradigm. The arrays of sounds were presented in virtual auditory space over headphones. The results of Experiment 1 suggest that target detection is enhanced when sound sources are spatially separated relative to when they are presented at the same location. Experiment 2 demonstrated that this effect is most prominent within the first 250 ms of exposure to the array of sounds. These findings suggest that spatial cues may be effective for enhancing early processes such as stream segregation, rather than simply directing attention to objects that have already been segmented.

Sound localisation during illusory self-rotation.

Auditory elevation localisation was investigated under conditions of illusory self-rotation (i.e., vection) induced by movement of wide-field visual stimuli around participants' z-axes. Contrary to previous findings which suggest that auditory cues to sound-source elevation are discounted during vection, we found little evidence that vection affects judgements of source elevation. Our results indicate that the percept of auditory space during vection is generally consistent with the available head-centered auditory cues to source elevation. Auditory information about the head-centered location of a source appears to be integrated, without modification, with visual information about head motion to determine the perceived exocentric location of the source.

Spectral integration time of the auditory localisation system.

For the elevation and front-versus-back hemifield of a sound source to be accurately determined, the sound must contain a broad range of frequencies. Experiment 1 of this study examined the spectral integration time of the auditory localisation system by measuring the accuracy with which frequency-modulated (FM) tones of modulation periods ranging from 0.5 to 200 ms can be localised. For each of the four participants, judgements of sound-source elevation and front-back hemifield were most accurate for a modulation period of 5 ms. Accuracy levels for the 5 ms modulation period approached those for a pink-noise stimulus. This suggests that the spectral integration time of the auditory localisation system is around 5 ms. Supporting evidence for this conclusion was sought in experiment 2, in which two participants localised noise stimuli that had magnitude spectra identical to those of 5 ms equivalent-rectangular-duration samples of the FM tones from experiment 1. For both participants, functions relating localisation error measures (i.e., elevation error and frequency of front-back confusion) to modulation period for spectrally matched noises were similar to those for FM tones.

Spatial audio displays improve the detection of target messages in a continuous monitoring task.

OBJECTIVE: The detection of target messages in a background of competing speech and the identification of the color/number combinations in those messages were examined in a continuous monitoring task. BACKGROUND: Previous research has shown that if listeners know when and where to listen, speech-on-speech intelligibility is improved when signals are presented via a 3-D audio display as compared with a diotic display. However, the effect of display type on detection of infrequent target messages in a continuous monitoring task has not been examined. METHOD: Participants were required to monitor five communications channels conveying messages at random intervals under each of three audio display conditions: diotic, all channels in front, and channels separated in azimuth (3-D). RESULTS: Message detection sensitivity was significantly higher for the 3-D condition than for the in-front condition but did not differ significantly between the in-front and the diotic conditions. There were no differences in response criteria across conditions. Color/number identification sensitivity also was significantly higher for the 3-D condition than for the in-front condition but did not differ significantly between the in-front and the diotic conditions. CONCLUSION: A 3-D audio display enhances both message detection and message identification in a continuous monitoring task. APPLICATION: Three-dimensional audio displays would be particularly beneficial in environments such as aviation, in which the information conveyed to operators via the auditory modality can be crucial to the safe and effective performance of their work.

Learning and retention of associations between auditory icons and denotative referents: implications for the design of auditory warnings.

OBJECTIVE: This study examined the way in which the type and preexisting strength of association between an auditory icon and a warning event affects the ease with which the icon/event pairing can be learned and retained. BACKGROUND: To be effective, an auditory warning must be audible, identifiable, interpretable, and heeded. Warnings consisting of familiar environmental sounds, or auditory icons, have potential to facilitate identification and interpretation. The ease with which pairings between auditory icons and warning events can be learned and retained is likely to depend on the type and strength of the preexisting icon/event association. METHOD: Sixty-three participants each learned eight auditory-icon/denotative-referent pairings and attempted to recall them 4 weeks later. Three icon/denotative-referent association types (direct, related, and unrelated) were employed. Participants rated the strength of the association for each pairing on a 7-point scale. RESULTS: The number of errors made while learning pairings was greater for unrelated than for either related or direct associations, whereas the number of errors made while attempting to recall pairings 4 weeks later was greater for unrelated than for related associations and for related than for direct associations. Irrespective of association type, both learning and retention performance remained at very high levels, provided the strength of the association was rated greater than 5. CONCLUSION: This suggests that strong preexisting associations are used to facilitate learning and retention of icon/denotative-referent pairings. APPLICATION: The practical implication of this study is that auditory icons having either direct or strong, indirect associations with warning events should be preferred.

Directed attention eliminates 'change deafness' in complex auditory scenes.

In natural environments that contain multiple sound sources, acoustic energy arising from the different sources sums to produce a single complex waveform at each of the listener's ears. The auditory system must segregate this waveform into distinct streams to permit identification of the objects from which the signals emanate [1]. Although the processes involved in stream segregation are now reasonably well understood [1, 2 and 3], little is known about the nature of our perception of complex auditory scenes. Here, we examined complex scene perception by having listeners detect a discrete change to an auditory scene comprising multiple concurrent naturalistic sounds. We found that listeners were remarkably poor at detecting the disappearance of an individual auditory object when listening to scenes containing more than four objects, but they performed near perfectly when their attention was directed to the identity of a potential change. In the absence of directed attention, this "change deafness" [4] was greater for objects arising from a common location in space than for objects separated in azimuth. Change deafness was also observed for changes in object location, suggesting that it may reflect a general effect of the dependence of human auditory perception on attention.

Localization of virtual sound at 4 Gz.

INTRODUCTION: Acceleration directed along the body's z-axis (Gz) leads to misperception of the elevation of visual objects (the "elevator illusion"), most probably as a result of errors in the transformation from eye-centered to head-centered coordinates. We have investigated whether the location of sound sources is misperceived under increased Gz. METHOD: Visually guided localization responses were made, using a remotely controlled laser pointer, to virtual auditory targets under conditions of 1 and 4 Gz induced in a human centrifuge. As these responses would be expected to be affected by the elevator illusion, we also measured the effect of Gz on the accuracy with which subjects could point to the horizon. RESULTS: Horizon judgments were lower at 4 Gz than at 1 Gz, so sound localization responses at 4 Gz were corrected for this error in the transformation from eye-centered to head-centered coordinates. We found that the accuracy and bias of sound localization are not significantly affected by increased Gz. CONCLUSION: The auditory modality is likely to provide a reliable means of conveying spatial information to operators in dynamic environments in which Gz can vary.

Utility of monaural spectral cues is enhanced in the presence of cues to sound-source lateral angle.

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.

Effect of hypobaric hypoxia on auditory sensitivity.

INTRODUCTION: 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). METHOD: 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). RESULTS: A small (2.57 dB) reduction in sensitivity across the frequency range tested was observed. CONCLUSION: 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.