Owen, a court-qualified expert witness and former chief engineer for the New York Public Library's Rodgers and Hammerstein Archives of Recorded Sound, is an authority on biometric voice analysis — a computerized process comparing attributes of voices to determine whether they match.

After the Sentinel contacted Owen, he used software called Easy Voice Biometrics to compare Zimmerman's voice to the 911 call screams.

"I took all of the screams and put those together, and cut out everything else," Owen says.

The software compared that audio to Zimmerman's voice. It returned a 48 percent match. Owen said to reach a positive match with audio of this quality, he'd expect higher than 90 percent.

"As a result of that, you can say with reasonable scientific certainty that it's not Zimmerman," Owen says, stressing that he cannot confirm the voice as Trayvon's, because he didn't have a sample of the teen's voice to compare.

Forensic voice identification is not a new or novel concept; in fact, a recent U.S. Department of Justicecommittee report notes that federal interest in the technology "has a history of nearly 70 years."

"These people don't leave fingerprints, but they do still need to talk to one another," he says.

'The home run'

Though the term "biometric analysis" may sound futuristic, it basically just means using personal characteristics for identification. A fingerprint scanner is an example of a biometric device.

Much as the ridges of a human hand produce a fingerprint, each human voice has unique, distinguishable traits, Owen says. "They're all particular to the individual."

Another benefit of modern biometric analysis, Owen said, is it doesn't require an "in context" comparison. In other words, Owen didn't need a sample of Zimmerman screaming in order to compare his voice to the call. 

The technology Owen used to analyze the Zimmerman tape has a wide range of applications, including national security and international policing, he said. A recently as January, Owen used the same technology to identify accused murderer Sheila Davalloo in a 911 call made almost a decade ago.

Owen testified that it was Davalloo, accused of stabbing another woman nine times in a condo in Shippan, Conn., who reported the killing to police from a pay phone in November 2002.

Davalloo was convicted, according to news reports.

Owen says the audio from Zimmerman's call is much better quality than the 911 call in the Davalloo case. Voice identification experts judge the quality based on a signal-to-noise ratio; in other words, comparing the usable audio in a clip to the environmental noises that make a match difficult.

And the call on which the screams are heard is better quality than is necessary, Owen says.

"In our world, that's the home run," he says.

Not all experts rely on biometrics. Ed Primeau, a Michigan-based audio engineer and forensics expert, is not a believer in the technology's use in courtroom settings.

He relies instead on audio enhancement and human analysis based on forensic experience. After listening closely to the 911 tape on which the screams are heard, Primeau also has a strong opinion.

"I believe that's Trayvon Martin in the background, without a doubt," Primeau says, stressing that the tone of the voice is a giveaway. "That's a young man screaming."

Zimmerman's call to authorities minutes before the shooting provides a good standard for comparison, Primeau says, because it captures his voice both at rest and in an agitated state.

'CSI' effect

Only one person alive knows exactly what transpired in the moments immediately before Trayvon was fatally shot: Zimmerman, who has claimed he fired in self-defense.

Zimmerman told police he was walking back to his SUV after a brief foot pursuit of Martin, and the teen confronted and attacked him, punching him and slamming his head into the pavement.

Arriving police said Zimmerman was bloodied. One officer wrote in a police report that he overheard Zimmerman telling a paramedic, "I was yelling for someone to help me, but no one would help me."

Angela Corey, the special prosecutor assigned to the case, has yet to decide whether to charge Zimmerman, send the case to a grand jury or decide against charging.

If Zimmerman's self-defense claim is tested at trial, legal experts say a forensic identification of the voice in the 911 audio could be key evidence, either in Zimmerman's favor or to his detriment.

Still, Maine-based audio enhancement expert Arlo West says that today's juries sometimes seem reluctant to accept evidence that's any less conclusive than what they're used to seeing on television.

"I call it the 'CSI' effect," he says, referring to "CSI: Crime Scene Investigation," the popular — if not always realistic — forensics-based TV drama. "You get in front of a jury, and they expect a miracle."

jeweiner@tribune.com or 407-420-5171