Speech consists of sound waves that propagate in the air from talker to hearer. Sound waves consist of rapidly changing pressure within the medium, normally air in the instance of speech. If the variations of pressure are regular, then the sound has a tonal quality. Most, but not all, speech sounds have this tonal quality. Sound amplitude relates to the amount of energy and is perceived as loudness. The speed of pressure variation is called frequency and is perceived as pitch. Individual sound waves combine additively, creating complex sound waves. Acoustic analysis of speech creates spectrograms, giving a visual representation of the original sound. In certain conditions, such as the interior of the vocal tract, sound reverberates in a self-additive way, giving rise to cavity resonance. The frequencies of this resonance can be modified by adjustments in the position of the speech articulators, creating different speech sounds, which occurs through the source--filter theory.
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