03/09/2026
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"Let us examine some of the sources of randomness in the acoustic signal. They originate at several levels in the speech production chain.
It is known that a muscle contraction is a composite of a large number of individual twitches of groups of muscle fibers called motor units. A single twitch associated with a given motor unit lasts only a fraction of a second. It can be repeated, but not immediately, and usually not at predictable intervals. The “firing” of a motor unit depends on the arrival of neural signals at the myo-neural junctions, i.e., the junctions between nerve endings and muscle fibers. Since the neural signals have a large random component, the size and time interval between twitches will also vary randomly. Although a muscle contraction is smoothed out by the summation process of many of these twitches, a small random component remains. This contributes to randomness in the fundamental frequency, randomness in vocal fold adduction, randomness in vocal intensity, since these are all regulated by coordinated muscle activity.
If the random variations are rapid, they will be perceived as roughness in the voice, but only if excessive. If they are slow, they will be perceived as tremor, an instability in pitch, loudness and quality. A certain amount of this perturbation in the acoustic signal is not only tolerated, but preferred by the listener. One normally does not attend to it, but misses it when it is entirely absent."
from "Random Acoustic Factors in Voice Production" by Dr. Ingo Titze. First published in the NATS Bulletin, a predecessor to the Journal of Singing, Sept/Oct 1982