Stimulus factors which influence the perceived externalization of sound presented through headphones

Several experimental conditions were arranged to study the influence of binaural difference cues on the externalization of sound. Broadband, high- and low-pass noise bursts were generated in a sound-treated but not echo-free room and tape recorded from electret microphones placed at the entrance to the ear canals in a live model. The tapes were processed in special ways in an attempt to dissociate the binaural differences in time, intensity, and spectrum which normally appear together and congruently when listening to a sound 90 degrees off midline. These variously processed tapes of trains of noise bursts were played back via headphones to 12 normal-hearing Ss; they were asked to estimate the apparent distance of the acoustic image. No significant differences in distance judgments were found among listening conditions which consisted of: (1) interaural differences in time, intensity, and spectrum (i.e., simulating free-field listening); (2) interaural differences in time and spectrum only; and (3) interaural differences in time only. With respect to interaural time differences, it was found in a second experiment with 16 normal-hearing Ss that differences in time of stimulus arrival promoted externalization of sound, since when these were eliminated (leaving only interaural differences in the ongoing aspects of the fine structure of the stimuli) all sounds appeared significantly nearer the head. A consistent finding was that high-pass noise bursts appeared nearer the head than low-pass noise bursts irrespective of the conditions of stimulus presentation.