Sonification of 3D Scenes in an Electronic Travel Aid for the Blind

Sight, hearing and touch are the sensory modalities that play a dominating role in spatial perception in humans, i.e. the ability to recognize the geometrical structure of the surrounding environment, awareness of self-location in surrounding space and determining in terms of depth and directions the location of nearby objects. Information streams from these senses are continuously integrated and processed in the brain, so that a cognitive representation of the 3D environment can be accurately built whether stationary or in movement. Each of the three senses uses different cues for exploring the environment and features a different perception range (Hall, 1966). Touch provides information on the so called near space (termed also haptic space), whereas vision and hearing are capable of yielding percepts representing objects or events in the so called far space. Spatial orientation in terms of locating scene elements is the key capability allowing humans to interact with the surrounding environment, e.g. reaching objects, avoiding obstacles, wayfinding (Gollage, 1999) and determining own location with respect to the environment. An important aspect of locating objects in 3D space is the integration of percepts coming from different senses. Understanding distance to objects (depth perception) has been possible by concurrent binocular seeing and touching experience of near space objects (Millar, 1994). For locating and recognition of far space objects, vision and hearing cooperate in order to determine distance, bearings and the type of objects. The field of view of vision is limited to the space in front of the observer whereas hearing is ominidirectional and sound sources can be located even if occluded by other objects. Correct reproduction of sensory stimuli is important in virtual reality systems in which 3D vision based technologies are predominantly employed for creating immersive artificial environments. Many applications can greatly benefit from building acoustic 3D spaces (e.g. operators of complex control panels, in-field communication of combating soldiers or firemen). If such spaces are appropriately synthesized, perception capacity and immersion in the environment can be considerably enhanced (Castro, 2006). It has been also evidenced that if spatial instead of monophonic sounds are applied, the reaction time to acoustic stimuli becomes shorter and the listener is less prone to fatigue (Moore, 2004). Because of the enriched acoustic experience such devices offer (e.g. spaciousness and interactivity) they are frequently termed auditory display systems. Recently, such systems gain also in importance