BOZEMAN -- Hollywood movies tend to dramatize gunshots as loud bangs, but in reality they are more often quick pops that witnesses can easily mistake for firecrackers.
In fact, audio recordings of gunshots consist of pulses of sound as brief as two-thousandths of a second, according to Rob Maher, professor of electrical and computer engineering at Montana State University. For that reason, gunshots "are very difficult to play back realistically using conventional speakers," which aren't designed to respond to such short bursts, he said.
That's one of the surprising things Maher has learned during more than a decade of research about audio forensics -- the use of sound recordings as evidence in court proceedings or official investigations. In his new book, titled "Principles of Forensic Audio Analysis," Maher summarizes some of his key findings to provide a practical overview intended for law enforcement officials and forensic investigators.
The 147-page book discusses the fundamentals of acoustics, the history of audio forensics and methods for authenticating, enhancing and interpreting audio recordings. The book provides detailed examples of gunshot scenarios and cockpit sound recordings.
"To my knowledge," Maher said, "this is essentially the first book focused on this topic."
Having published more than 20 papers on the subject, Maher is a sought-after expert. The majority of Maher's research has been funded by the National Institute of Justice, the research and development wing of the U.S. Justice Department. He regularly receives calls from attorneys and investigators and has testified about audio evidence in courtrooms around the country.
The cases often involve dialogue, background sounds and sometimes gunshots recorded by a witness's phone, an emergency dispatcher or a police car's dashboard camera. The question in the case of gunshots is usually: Who shot, and when?
"In a court case, that's important to both the prosecution and the defense, because it's about who might go to jail," Maher said.
"If you watch a TV show like 'CSI,' sometimes they'll take a crummy recording and magically make it perfect," Maher said. Those fictions have a real consequence that justice officials refer to as the "CSI effect" -- jury members too easily trusting an attorney's assertions that a piece of evidence supports a definite conclusion.
"The reality is that you're seldom able to do that," said Maher, whose role in a courtroom is often being an educator for the judge and jury, testifying about the nuances and limitations of interpreting audio evidence.
On the other hand, much can be deduced from an audio recording. By visually charting the recording's sound intensity over time, careful analysis of the recording can reveal distinctive signatures. When interpreting recordings involving firearms, for example, a rifle's blast looks different than a shotgun's or handgun's.
Over the years, Maher has recorded a variety of guns fired in controlled scientific experiments at MSU's Red Bluff Research Ranch near Norris. The recordings are made with a dozen microphones arrayed in a semicircle around the shooter, measuring the major differences in sound in front of the firearm versus beside it.
"That's the kind of thing that our scientific understanding can add to forensics," Maher said.
A musician whose early-career research focused on acoustics in the entertainment industry, Maher traces his forensics interest to a call he received from an attorney about 10 years ago. The attorney wanted to know whether it was possible to match a gunshot recorded on a phone answering machine to a specific gun. Maher initially dismissed the question and said he thought it was impossible, but the question triggered his curiosity. When he began researching the topic, he found more questions than answers and saw that he could contribute to the field.
At a time when cellphones and police cameras have made recordings more available than ever, and advances in electronics have reduced the time and effort required to interpret audio recordings, forensic practices are still catching up, Maher said. A 2009 report by the National Academy of Sciences identified a need for greater scientific rigor in many areas of forensics, including audio analysis.
"Hopefully this book is a step in that direction," Maher said.
"Principles of Forensic Audio Analysis" is available from Springer Publishing Company as a downloadable e-book and a hardcover book.