The detection of 'virtual' objects using echoes by humans: Spectral cues.

Some blind people use echoes to detect discrete, silent objects to support their spatial orientation/navigation, independence, safety and wellbeing. The acoustical features that people use for this are not well understood. Listening to changes in spectral shape due to the presence of an object could be important for object detection and avoidance, especially at short range, although it is currently not known whether it is possible with echolocation-related sounds. Bands of noise were convolved with recordings of binaural impulse responses of objects in an anechoic chamber to create 'virtual objects', which were analysed and played to sighted and blind listeners inexperienced in echolocation. The sounds were also manipulated to remove cues unrelated to spectral shape. Most listeners could accurately detect hard flat objects using changes in spectral shape. The useful spectral changes for object detection occurred above approximately 3 kHz, as with object localisation. However, energy in the sounds below 3 kHz was required to exploit changes in spectral shape for object detection, whereas energy below 3 kHz impaired object localisation. Further recordings showed that the spectral changes were diminished by room reverberation. While good high-frequency hearing is generally important for echolocation, the optimal echo-generating stimulus will probably depend on the task.

Use of binaural and monaural cues to identify the lateral position of a virtual object using echoes.

Under certain conditions, sighted and blind humans can use echoes to discern characteristics of otherwise silent objects. Previous research concluded that robust horizontal-plane object localisation ability, without using head movement, depends on information above 2 kHz. While a strong interaural level difference (ILD) cue is available, it was not clear if listeners were using that or the monaural level cue that necessarily accompanies ILD. In this experiment, 13 sighted and normal-hearing listeners were asked to identify the right-vs.-left position of an object in virtual auditory space. Sounds were manipulated to remove binaural cues (binaural vs. diotic presentation) and prevent the use of monaural level cues (using level roving). With low- (<2 kHz) and high- (>2 kHz) frequency bands of noise, performance with binaural presentation and level rove exceeded that expected from use of monaural level cues and that with diotic presentation. It is argued that a high-frequency binaural cue (most likely ILD), and not a monaural level cue, is crucial for robust object localisation without head movement.

Identification of the lateral position of a virtual object based on echoes by humans.

Echolocation offers a promising approach to improve the quality of life of people with blindness although little is known about the factors influencing object localisation using a 'searching' strategy. In this paper, we describe a series of experiments using sighted and blind human listeners and a 'virtual auditory space' technique to investigate the effects of the distance and orientation of a reflective object and the effect of stimulus bandwidth on ability to identify the right-versus-left position of the object, with bands of noise and durations from 10-400 ms. We found that performance reduced with increasing object distance. This was more rapid for object orientations where mirror-like reflection paths do not exist to both ears (i.e., most possible orientations); performance with these orientations was indistinguishable from chance at 1.8 m for even the best performing listeners in other conditions. Above-chance performance extended to larger distances when the echo was artificially presented in isolation, as might be achieved in practice by an assistive device. We also found that performance was primarily based on information above 2 kHz. Further research should extend these investigations to include other factors that are relevant to real-life echolocation.