I just came across this Bruce Schneier talk about the big-picture issues of increasingly powerful technology for communication / surveillance and found it to be relevant to the discussion here:

On Thu, Oct 17, 2013 at 10:27 AM, Patrik D'haeseleer <patrikd@gmail.com> wrote:
On Thu, Oct 17, 2013 at 6:41 AM, GtwoG PublicOhOne <g2g-public01@att.net> wrote:
And voiceprint attribution was 99.96% accurate as of 1962
(published); fast-forward at the speed of Moore's law, draw your own
conclusions, and more details in person.

Note that the accuracy of voiceprint identification has been hugely exaggerated. That number has been touted primarily by prosecutors, and by voiceprint identification "experts" who make a nice buck from offering their services. But just as has been the case with lie detectors, these claims have not stood up to rigorous scientific validation.

Of course, that doesn't mean that they won't be used against you in a court of law, regardless of your guilt or innocence! And some people have been falsely convicted to decades in jail or worse on the basis of this kind of "evidence".

Forensic speaker recognition has many limitations and is currently inadmissible in federal court as expert testimony. Bonastre et al (2003) summarize these limitations quite well:  
"The term voiceprint gives the false impression that voice has characteristics that are as unique and reliable as fingerprints... this is absolutely not the case."
The thing about voices is that they are susceptible to a myriad of external factors such as psychological/emotional state, age, health, weather... the list goes on. From an application standpoint, the most prominent of these factors is intentional vocal disguise. There are a number of things people can intentionally do to their voices to drastically reduce the ability of machine or human expert to identify their voice correctly (you would be amazed at how difficult it is - nearly impossible - to identify a whispered voice). Under these conditions, identification accuracy falls to 40 - 52 percent (Thompson 1987), 36 percent (Andruski 2007), 26 percent (Clifford 1980).


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