Monday, October 29, 2012

About Those Two Forensic Audio Experts

    By now we are all familiar with the uninformed and typically uneducated Trayvon Mafia mindlessly parroting that "two audio experts proved that the screams for help did not come from Mr Zimmeran." In a largely undereported article The Daily Caller ran an article which refuted the nonsense spewed by the scheme team affialiated experts.

(click the link to see the original article)

Comparing apples to oranges

Dr. James Wayman, a San Jose State University expert in the field of speech science, told The Daily Caller that he questions the grounds on which Owen based his analysis.

Wayman also said he would be willing to testify against the admissibility of Owen’s findings on the grounds that they don’t meet the criteria required for evidence in federal courts.

“There is no history of, or data on, the comparison of a questioned scream to a known speech sample,” Wayman said.

The problem, he said, is that the two voice samples were recorded in difficult acoustic conditions over different cell phones.

“Even if we were to have Mr. Zimmerman recreate the scream under identical conditions with the same cell phone,” Wayman explained, “it would be difficult to attribute the scream to him without a sample of a similar scream from Mr. Martin under the same conditions. This is clearly not possible.”

Reached for comment, Owen told TheDC that he has conducted his own study — “The Owen Study” — of more than 400 different pitches, screams, and voice disguises. The study is unpublished.

He explained that he has attempted, without success, to obtain a “voice exemplar” from Zimmerman, consisting of recordings of both his speaking voice and a scream.

And Wayman, he said, “assumes that the voice software is not able to make a determination on each voice independently.”

Wayman fired back in a later email exchange. “There is no accepted standard regarding metrics for voice comparisons,” he insisted, “either if done forensically or using automated comparison software.”

‘Naïve’ voice recognition

The Sentinel also contracted with Ed Primeau, a trained audio engineer and registered investigator whose expert testimony has been used in dozens of criminal court proceedings. Primeau used a more intuitive approach to determine that Zimmerman was not the person heard screaming on the 911 call.

“That’s a young man screaming,” Primeau told the Sentinel.

Comparing the human voice to a symphony full of varying timbres, Primeau wrote on his blog that the “male voice yelling for help … cracks like teen male’s does when going through puberty.”

Dr. Philip Rose of the Australian National University told TheDC that scientific experts refer to Primeau’s method as “naïve voice recognition.” His influential 2002 book Forensic Speaker Identification draws a major distinction between naïve and “technical forensics” voice recognition.

“Naïve voice recognition is so prone to error that it is acknowledged that it is worthless as evidence,” Rose said via email.

A forensic expert’s job, he said, is to assess the strength of evidence, not to estimate the probability of a hypothesis. And “the value of the evidence depends … on the similarity of the samples.”

In a properly conducted analysis, he told TheDC, “you would still have to do the comparison using screamed and phone samples, with many speakers.”

One voice authentication expert whose work is commercial in nature told TheDC that screaming, stress, and a recording’s audio quality can “wreak havoc” on voice biometric software and its ability to interpret data.

And speaking of Owen’s findings, another industry insider said that “a legitimate biometrics expert would likely refute the contentions” and suggests that these were “incendiary publicity plays.”

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