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Newsletters > July

AUDIO FORENSICS
Or Digging through the Trash for the Treasure

 

 
(discontinuities), spectral analysis, metadata analysis (an increasingly rich area for data gathering), hashing to compare digital identity, and Electrical Network Frequency Analysis (using background electrical grid noise and frequency variation to narrow down locations).  Dr. Begault noted that the Audio Engineering Society was a pioneer of forensic audiology with its 1972 report on the Nixon tapes and established many o the practices still followed today.
 
 
Dr. Begault described how vocal analysis can be quite challenging, and despite tools such as voiceprints and voice biometrics, the intra-speaker variability of the human voice is quite large and often confounds inter-speaker variability.  Dialects, emotion, different languages and recording quality all have a deleterious effect on establishing whether a particular person is the source of a recorded voice.  A notorious recent case involved the Trayvon Martin/George Zimmerman case, in which the opinions of two forensic audiologists were excluded from evidence - one asserted that a scream heard on the 911 recording was “not George Zimmerman” while the other offered that it was “probably Mr. Martin.”  Beyond the variability and capability of the human voice, the circumstances under which recordings are made create further uncertainty - non-optimal microphone placement, multiple microphones in a cell phone and noise cancellation, room acoustics, and audio codecs all limit the “Speech Transmission Index,” a “single-value quantifier of the effects of signal-noise ratio and reverberation on modulation depth of the speech envelope.”  For instance, a narrow-band cutoff of 3400 Hz can make it impossible to distinguish seed from feed or thin from fin.  As a forensic audiologist, Dr. Begault has visited locations and recreated the conditions under which the original recordings were obtained, in order to determine whether the verbal testimony of victim and accused are consistent with what is available on cell phone and 911 recordings.
 
 
Moving on to the topic of gunshot analysis, Dr. Begault described gunfire as an “extremely brief and powerful impulse event, loud enough to cause hearing damage and capable of overloading a recorder."




......Recap by John Svetlik

 

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