Auditory fitness for duty: localising small arms gunfire

Abstract

Locating the source of small arms fire is deemed a mission-critical auditory task by infantry personnel (Bevis et al. 2014; Semeraro et al. 2015). Little is known about the acoustic localisation cues within a gunshot and human ability to localise gunshots. Binaural recordings of ‘live’ gunshots from an SA80 rifle were obtained using a KEMAR dummy head placed 100 m from the firer, within 30 cm of the bullet trajectory and with 13 azimuth angles from 90° left to 90º right. The ‘crack’, created by the supersonic bullet passing the target, produced smaller interaural time and level differences than the ‘thump’, created by the muzzle blast, for the rifle at the same angle. Forty normal-hearing listeners (20 civilian, 20 military personnel) and 12 hearing impaired listeners (all military personnel) completed a virtual azimuthal localisation task using three stimuli created from the recordings (whole gunshot, ‘crack’ only and ‘thump’ only) plus a 50 ms broadband noise burst convolved with KEMAR impulse responses. All listeners localised all stimuli types above chance level. Average localisation error increased in the order of: noise burst < thump < gunshot < crack, for all cohorts. Military personnel (regardless of their hearing level) performed significantly worse than civilians for all stimuli; they had a higher tendency to select the extreme left and right sources, resulting in an increased lateral bias. The difference between military and civilian participants may be due to their understanding of the task or military training/experience. Mild to moderate bilateral symmetrical sensorineural hearing loss did not have a significant impact on localisation accuracy. This suggests that, providing the gunshot is clearly audible and audiometric thresholds are equal between the ears, binaural cues will still be accessible and localisation accuracy will be preserved. Further work is recommended to investigate the relationship between other hearing loss configurations and small arms gunshot localisation accuracy before considering gunshot localisation as a measure of auditory fitness for infantry personnel