Auditory perception of walls via spectral variations in the ambient sound field

Individuals with visual disabilities often use their hearing in order to ma intain a line of travel parallel to walls, such as when walking down a hall way or along the side of a building. Previous studies established that this ability depends on the sense of hearing, but the specific acoustic informa tion has not been investigated. The present paper describes a model of how sound pressure builds up within a meter or so in front of a wall, particula rly in the low frequency end of the sound spectrum. This buildup of sound p ressure is based on ambient or "background" sound, not self-produced sound such as footsteps. The model leads to a prediction that walls are detected by means of a spectral shift toward low frequencies. This prediction was te sted in three experiments, in which sighted adults listened for such spectr al shifts. In each experiment, a threshold value was obtained corresponding to the farthest simulated distance from a wall that could be detected. Tre shold values were in good agreement with previous observations of the dista nce at which pedestrians can utilize acoustic information from walls. There was no evidence that simulated listener motion enhanced perception of wall s. The model underlying these experiments implies that the term echolocatio n carries inappropriate connotations about the auditory processes that are involved in walking along walls. It is suggested that a more apt descriptio n is that pedestrians listen for spatial variations in the structure of the ambient sound field.