Anyone who talks on a phone or tape recorder is fair game for voice analysis, especially if they have criminal intent.  Increasingly, more law enforcement officers are getting trained in voiceprint analysis, and with the development of computer and digital spectrogram technology, the procedure is becoming widely used. 

Lawrence Kersta noted that each person's voice has a unique quality that can be mapped on a graph.  The individuality derives primarily from differences in physical vocal mechanisms.  One person's vocal chords, no matter how similar they might look, process sounds differently than someone else's.  The size and shape of someone's vocal cavity, tongue, and nasal cavities contribute to this, as well as how that person coordinates lips, jaw, tongue, and soft palate to make speech.  No combination of these things is like any other.  That means that our voices are sufficiently unique to make personal identification based on voice sounds possible.

Although Kersta also believed that an individual's voice does not change over his or her lifetime, other experts have disputed him on this point.  If the body changes, so does the voice.  Even where a person lives can effect voice changes, as well as illness, stress, aging, and other factors.  Nevertheless, Kersta maintained that the essential qualities of the voice remain constant.  He felt that he finally proved this in one of the most famous cases involving the spectrograph: that of the reclusive Howard Hughes.

In 1971, a man named Clifford Irving came to New York to cut a deal for what he claimed was Hughes' autobiography, ghosted by him.  He had letters that he insisted were written by Hughes and experts soon authenticated them.  The publisher McGraw-Hill bought into his claim, advancing him $765,000 and announcing their intent to publish the book.  Eventually Irving turned in a 1200 page manuscript.

It was difficult to ascertain whether Hughes had actually authorized this transaction since for the past fifteen years he had been exceedingly elusive.  That Irving had letters from him seemed a good indicator that they knew each other.  Several people who had known Hughes read the manuscript and felt convinced that it was genuinely his story.  However, he finally surfaced from his retreat on Paradise Island in the Bahamas to renounce the book. 

Hughes claimed that he had never met Clifford Irving and that the whole thing was a fake.  He added that he did not know where Irving had gotten his information.  However, he was not willing to make his renunciation in person.  He agreed only to do this by phone.  That meant that he could be identified only by his voice—how it sounded and what he said.

A group of reporters familiar with him from his early days was assembled by NBC in Los Angeles to ask him questions for two hours.  Their purpose was to authenticate the voice on the phone as that of the famous, eccentric billionaire, and they were to ask some key questions that would trip up an imposter.  The man on the phone responded in convincing detail.  He talked about such things as the make of his plane and trips that he had made, but he stumbled when asked about the good luck charm that a woman had presented to him before his 1938 trip around the world.  He said that he could not recall the incident, but moments later he did: She had placed chewing gum on the tail of his plane.

This entire phone conversation was recorded and as they listened again, the reporters all believed that Hughes had been the man on the phone.  That meant that Irving was a fraud.       

Irving defended himself by insisting that the person who had called was the imposter, but NBC had hired Lawrence Kersta to make a voiceprint analysis.  He measured pitch, tone, and volume to compare the voice pictures on a line-by-line basis, comparing a recording of a speech that Hughes had made in 1947 with the recordings from the interview.  Finally he announced that the man who had spoken to reporters was Howard Hughes. 

Even one of Kersta's most vocal critics, phonetics professor Peter Ladefoged, admitted that the recordings were remarkably identical. Irving was arrested and convicted of forgery.  He repaid the publisher and was sentenced to thirty months in prison.

Since the recordings had been made nearly a quarter of a century apart and Hughes' voice had deepened, there had been concern that changes would make the reading impossible.  However, the spectrographic patterns proved to be impressively similar.  This result further convinced Kersta that the inherent uniqueness of an individual's voice remains constant.

Spectrographic analysis of the human voice has made a similar impact in other criminal cases, so let's see more specifically how an interpretation is made.