Speak, Voiceprint

It was just after midnight on May 22, 1970, when St. Paul police headquarters got an anonymous call asking help for a woman about to give birth. Two policemen sped to the scene but found only a darkened house. While one policeman went round to a back door, a sniper suddenly opened fire from across the street. The second policeman fell mortally wounded.

It seemed a random attack on "police in general," and the only clue was the telephone call, which had been routinely taped. To find a matching voice, police interrogated 13 women in the neighborhood. At each interview they made "voiceprints"--electronic "pictures" of the individual's voice. Because her voiceprint matched the taped call, Caroline Trimble, 18, was arrested and later indicted for first-degree murder.

Conceived in 1941 as a way for the deaf to "read" speech, the voice-print machine analyzes patterns of frequency and amplitude, transcribing each variation into a spectrogram. One of the chief developers, Physicist Lawrence Kersta, claims that everyone's voiceprint is as unique as his fingerprints, and that any skilled technician can identify a voiceprint with more than 99% accuracy. Other scientists have disputed his claims.

Two earlier cases involving voiceprints came before courts in New Jersey (1967) and California (1968), and both times the appeals courts rejected the device as unreliable. Among the experts who opposed voiceprints at the time was Oscar Tosi, a professor of audiology at Michigan State; since then, however, Tosi has compared some 34,000 voiceprints under a $300,000 grant from the Justice Department, and the evidence has convinced him. In the case of Caroline Trimble, he testified in favor of voiceprint evidence, and Minnesota Supreme Court Chief Justice Oscar Knutson agreed with him.

Knutson observed that courts permit witnesses to identify voices on the basis of what they heard with their own ears, even via telephone. "We are convinced," the court ruled, "that spectrograms ought to be admissible at least for the purpose of corroborating opinions as to identification by means of ear alone."

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