All instruments, including our singing voices, have a sound source, a resonator that reinforces (amplifies) the basic sound and a radiator that transmits the sound to listeners. In people, the source is vibrating vocal folds (vocal cords) of the larynx or voice box; the resonator is the sound-boosting airway above the larynx; and the radiator is the opening of the mouth.
The human voice can create an impressive array of sounds because it relies on non-linear feedback by which a small input can result in a disproportionately large output. One of the voice’s more effective nonlinear mechanisms is inertive reactance, whereby singers create special conditions in their vocal tract to amplify sounds generated by the vocal folds.
To better understand the complex phenomena that produce the incredible sounds acclaimed vocalists demonstrate in the following sound clips and elsewhere, take a look at my article—The Human Instrument—in the January issue of Scientific American.
SOUND CLIPSSteven Tyler
Steven Tyler, lead singer of the rock band Aerosmith, is celebrated for his ability to scream tunefully. Here, he produces several interesting vocal effects. Tyler first uses some inharmonic (noise-like) sounds to match the timbre of his voice to percussive instruments. He also demonstrates a “flip” into falsetto register, but later employs a bright vowel on the word “same” to continue his belt-like voice (as in “belt” it out) into a high pitch.
Georgia Brown is a Brazilian pop singer who is noted for her wide vocal range (eight octaves) and is thus classified as a full dramatic coloratura soprano. In this example, she is likely using inertive reactance in her vocal tract to reinforce a very high-pitched whistle voice that she creates with her vocal folds. No vowels are heard because the pitch sits above the first two vocal-tract resonances that define (perceptually) what a vowel is.
Rollin Rachele is one of the world’s foremost overtone singers, a technique in which a person vocalizes two notes simultaneously. Overtone singing and related techniques are most widely recognized in the Tuvan, Mongolian and Tibetan cultures. Rachele never uses the fundamental frequency to change pitch. Rather, he maintains the fundamental frequency as a constant drone, then applies varying vocal tract shapes to resonate a single harmonic of this drone at any one time. By skipping from harmonic to harmonic he can play a tune with these high frequencies, also known as overtones.