It might be fair to say that the scientific community, on whole, hasn't exactly warmed up to the idea of spectrographic voice recognition. The human voice is incapable of producing one pitch at a time. Instead, it produces a simultaneous series of fundamentals and overtones. Some overtones are random and others are multiples of the fundamental, called harmonics. Of all the characteristics of voice, two of the most important ones are frequency and intensity .
Frequency is the speed at which air particles vibrate, measured in centimeters per second.
Humans can only produce and hear frequencies in the 60-16,000 cps range. Uniqueness in voice is a product of both physiology and learning. With learning, the process is mostly imitation and trial and error, but the brain's speech center is actually sending signals to various organs. A person may try to disguise their voice, or even learn a foreign language, but the way their brain controls speech habit and especially the way their resonators and articulators are shaped and used cannot be changed. This makes each and every individual voice unique.