Novel audio analysis helps identify multiple sounds in forensic gunshot recordings

Steven Beck – stevendbeck@alumni.rice.edu

Beck Audio Forensics, 7618 Rockpoint Dr, Austin, Texas, 78731, United States

Popular version of 2pEA8 – Dissecting Recorded Gunshot Sounds
Presented at the 186th ASA Meeting
Read the abstract at https://doi.org/10.1121/10.0027107

–The research described in this Acoustics Lay Language Paper may not have yet been peer reviewed–

Forensic audio can provide important evidence, especially when the scene information is not captured on video.  Audio recordings of gunshots often supply additional information, or the only information, from a shooting incident.  Analysis of the recorded audio can help determine who fired first, how many guns were fired, and sometimes identify each gunshot in a sequence.  The muzzle blast, or “bang”, and the ballistic shockwave, or “crack” from a supersonic bullet are the most common sounds analyzed.  These very loud sounds often obscure other gunshot sounds that can provide important forensic evidence.  New recording technology, like floating point recorders and high sensitivity police body cameras, may capture multiple acoustic sound sources from a gunshot, depending on the shooting, propagation, and recording conditions.

In order to investigate the multiple sounds from a gunshot, a body camera and Zoom F6 multi-channel floating point recorder were used to record gunshot sounds using microphones placed at 90o, 135o, and 180o relative to the line of fire.  Revolvers, semiautomatic pistols, and long barrel firearms (rifles and shotguns) are shown to have different sequences of acoustic source events:

• Revolver have the primer blast, chamber gas jet, muzzle blast, mechanical sounds
• Pistols have the primer blast, muzzle blast, slide/bolt sound, mechanical sounds
• Semiautomatic rifles have primer blast, slide/bolt sound, muzzle blast, mechanical sounds
• Bold action rifles have primer blast, muzzle blast, later mechanical sounds

Figure 1 shows an example of multiple acoustic sources in a gunshot.  The left side plots show a muzzle blast followed by a mechanical sound.  Zooming in on the amplitude shows a much quieter primer blast that occurs before the muzzle blast.  The bottom right plot is the same gunshot recorded on a police-style body camera.  The primer blast is very clear, but the other gunshot sounds are clipped.  Since the primer blast can only be recorded behind or to the side of a close-by shooter, it’s presence in a recording can help determine “who fired first” or help identify individual gunshots.

Figure 1 – A Primer Blast Prior to the Muzzle Blast Indicates the Presence of a Near-by Shooter

In addition to blast-related sounds, there are sounds related to ballistics.  Recording these sounds are very position dependent, and require the recording system to be close to the source.  These sounds include the ballistic shockwave and ballistic flow sound (recorded close to a passing bullet), tumbling bullet, reverberation and reflections, and the ballistic impact.  Ballistic sounds can help identify a gunshot or a possible shooting location.