Imagine you’re a crime scene investigator and you’ve been called to the aftermath of a shooting. There’s been a bloody gun battle between two rival gangs, and you arrive to find a bullet-ridden corpse lying in the middle of a busy street. What would be your top priority?
If, like me, your gut instinct would be to get out of there as soon as possible, forensics probably isn’t for you. If, on the other hand, you’re a calm-headed pro, you’d first need to secure and isolate the crime scene. Then you’d photograph the area and maybe start making a note of incriminating evidence such as abandoned weapons or scraps of clothing.
In any case, trying to locate an audio recording of the gunfight probably wouldn’t be at the forefront of your mind.
However, there’s a growing number of forensic scientists who study gunshot sounds, because, surprisingly, a bang isn’t just a bang. Instead, gunshots are like fingerprints: master their subtle differences and you may be one step closer to solving the crime.
This field of study – ‘forensic gunshot acoustics’ – is a burgeoning research topic because amateur recordings of gun crimes are becoming more and more common, thanks to the proliferation of mobile phones and other digital recorders.
To make sense of these recordings, forensic scientists need to be able to identify gunshots belonging to different guns and ammunition types, as well as understanding how the recording location could have affected the gunshot.
It’s a tough task, but successfully identifying the gunshots could help to answer some crucial questions in a trial situation. How many different guns were fired? Who fired first? Is the loud bang actually a gunshot, or just a car backfiring?
Earlier this year, researchers at BAE Systems and the FBI published a comprehensive paper on gunshot forensics. In their study, Steven Beck and colleagues explain that a gunshot is made up of two primary sounds – there’s a crack and there’s a BANG!
The bang is the ‘muzzle blast’ – the sound of pressurised gases escaping as the bullet leaves the barrel of the gun. The initial sound only lasts a few milliseconds, but it’s louder than a jet engine and can reverberate for over a second.
The crack is the shock wave created as the bullet breaks the sound barrier, like a miniature version of the sonic boom created by a supersonic jet. The shock wave forms a cone which trails behind the bullet, and you hear the crack when the shock wave passes by.
So, a typical sound wave for a single gunshot looks like this:
In practice, of course, a real-life gunshot recording will never be so perfectly formed. Amateur audio recordings are usually poor quality, with interference from voices, screams and echoes. Furthermore, the sound of the gunshot will depend on the position of the recorder relative to the shooter.
To begin investigating some of these effects, the scientists carried out a series of controlled experiments, using different gun types and a range of different microphone positions relative to the gun:
Their results highlight a number of factors that can affect the sound of a gunshot. The sound of the muzzle blast depends on the gun make, model, and barrel length, as well as its distance from the microphone. The sound of the shock wave, on the other hand, is set by the bullet speed and size, its miss distance from the microphone, and the angle of the microphone relative to the gun.
Each of these factors can come into play in a real-world situation, and that’s without even considering the effects of interference and echoes, or the variations between the recorders themselves.
But although this may all seem terribly complicated, breaking down the sound of a gunshot into its two main components is a promising step forward.
“I think both [the muzzle blast and the shock wave] are very important,” says Steven Beck. “The direction of the shooter can be determined by the muzzle blast, but not by the shock wave. You can determine the range to the shooter if you use both the muzzle blast and shock wave.”
There’s still some way to go before these techniques can be routinely used to help solve crimes, but, who knows, maybe forensic scientists will one day be able to piece together a gunfight with their eyes closed. CSI, eat your heart out!
Steven Beck and colleagues present their study at this week’s Acoustical Society of America meeting in San Diego, California.
Beck SD, Nakasone H, & Marr KW (2011). Variations in recorded acoustic gunshot waveforms generated by small firearms. The Journal of the Acoustical Society of America, 129 (4), 1748-59 PMID: 21476632