Last August, the U.S. National Institute of Justice (NIJ) published the findings of an eight-year investigation by Dr. Robert C. Maher on the use of new forensic audio techniques to document and interpret firearm gunshot recordings.
The publication of his research came from the Office of Justice Programs’ National Criminal Justice Reference Service, and the author recalls the beginnings of his research through a phone call asking if a gun allegedly used in the commission of a crime could be matched with an audio recording from a crime scene of a fired gun.
Dr. Maher’s initial work in this field began with understanding the acoustic characteristics of gunshots by obtaining repeated, high-quality recordings that were carried out under controlled conditions. This effort matched a strategic objective of NIJ’s Office of Investigative and Forensic Sciences to support foundational research in forensic sciences.
To do so, he created a device and methodology that collected recordings of gunshots. In order to measure consistency, reliability and shot-to-shot variability, he collected data from a variety of firearms (five handguns, one revolver, one shotgun and two rifles).
Maher found that, although there are similarities when one fires the same gun 10 times, there are also notable differences from shot to shot. The duration of the blast varies from one firearm to another, but a given firearm also varies from one shot to another. Although the reason for the variability in duration is not yet known, the doctor suggests that this variation will have an effect on the forensic analysis of recordings that include gunshots of unknown origin.
As soon as he found a repeatable method for accurately recording gunshot acoustics under ideal conditions, Dr. Maher was ready to study the limitations of forensic interpretation of standard recording devices. This could cover mobile phones, land-mobile radios, personal audio recorders, audio data collected by emergency call centres and dispatch centre recording systems.
He compared signals at 11 different locations from microphones and personal recording devices, plus a body camera worn by the shooter and an internal recording system in a police vehicle. This allowed him to verify geometric predictions of arrival time and level at each recording site. For verification purposes, he also compared the times with a recording made by a mobile phone call to a corporate voice mail system.
He then examined several gunshot recordings simultaneously to see if relevant forensic information could be obtained, despite reflections, distortion, coding artefacts and other non-ideal characteristics. From the analyses, the doctor created a processing method to locate the source of the gunshots and reduce incoherent background noise, as well as a method to identify the most likely synchronisation point for multiple audio recordings.
For audio forensic analysis, it is more and more likely that several user-generated recordings may be presented as evidence in a criminal investigation. Audio evidence can come from portable smartphones, private surveillance systems, body-worn cameras and other unsynchronized recording devices. When numerous user-generated recordings are available, analysing the audio can yield spatial and temporal data about sound source location and orientation, including gunshots and other sounds.
Dr. Maher’s gunshot audio analysis was already used in the Cleveland Police Bureau’s trial of Michael Brelo, where Dr. Maher concluded that 15 of the 18 shots were fired from Brelo’s gun. Independent FBI investigations corroborated their findings.
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