The Localisation of Spectrally Restricted Sounds by Human Listeners

The two principal binaural cues to sound location are interaural time differences (ITDs), which are thought to be dominant at low frequencies, and interaural level differences (ILDs), which are thought to dominate at mid to high frequencies. The outer ear also filters the sound in a location dependent manner and provides spectral cues to location. In these experiments we have examined the relative contribution of these cues to the auditory localisation performance by humans. Six subjects localised sounds by pointing their face toward the perceived location of stimuli presented in complete darkness in an anechoic chamber. Control stimuli were spectrally flat (400 Hz to 16 kHz), while the relative contribution of location cues in the low frequency channels was determined using noise high passed at 2 kHz and in the high frequency channels using stimuli low passed at 2 kHz. The removal of frequencies below 2 kHz had little effect on either the pattern of systematic errors or the distribution of localisation estimates with the exception of an increase in the size of the standard deviations associated with a few rear locations. This suggests considerable redundancy in the auditory localisation information contained within a broadband sound. In contrast, restricting the target spectrum to frequencies below 2 kHz resulted in a large increase in the cone-of-confusion errors as well as a subject dependent biasing of the front-to-back or back-to-front confusions. These biases and the reduction in localisation accuracy for high pass stimuli at some posterior locations are consistent with a contribution of spectral information at low frequencies.