Auditory localisation of low-frequency sound sources

This thesis investigates the extent of the human ability to perceive changes in the direction of low-frequency sound sources, within the range of 31.5 and 100 Hz, in a large, highly damped environment. As it stands, this investigation is the only attempt made at determining the MAA for the low-frequency spectrum. In addition, the binaural cues utilised by the auditory system that enable this localisation ability have been studied. These objectives have been reached through two experiments, a relative localisation and a lateralisation task, conducted using a 2AFC method. The first psychoacoustic experiment, with the subwoofers positioned at 0°, 10°, 20° and 45° in the left azimuth plane, disprove the general held belief that there is little directional information or that localisation is limited to a left-right discrimination in the low-frequency spectrum. Results indicate that humans can reliably detect a change as small as 10° in the direction of pure tones with a frequency of over 63.5 Hz, and octave bands of pink noise with a centre frequency as low as 31.5 Hz. For sinusoids under 63.5 Hz, a separation angle higher than 45° is needed for reaching the MAA threshold. Furthermore, using an auditory model to extract binaural cues from the experimental setup, the concept of MAITD, described as the minimum ITD difference between two subwoofer positions that results in 75\% of correct responses, has been introduced. The second experiment utilised variations in these extracted binaural cues in a psychoacoustic test conducted over headphones. Results show that ITD is the primary cue utilised by the auditory system to determine changes in the location of a low-frequency sound source. Moreover, manipulation of the ITD cues in the lateralisation task according to the computed MAITD of the previous experiment result in similar percentages of correct responses across both experiments and a displacement of the auditory events. It has also been demonstrated that the presence of modal behaviour in the listening environment hinders the localisation ability, as it leads to erroneous ITD cues. This fact could have contributed to the belief that, as a rule, low-frequency sound sources are not localisable, a view which this thesis aims to refute