The Source-Filter Theory is an acoustic model that explains how the human voice produces sound and how this sound is shaped into speech. This theory is fundamental to the fields of phonetics and speech science. It divides the process of vocalization into two parts: the creation of a sound source, and the modification of this sound by the vocal tract.
Source:
The “source” in Source-Filter Theory refers to the sound generated by the vibration of the vocal folds within the larynx. When air from the lungs is expelled through the larynx, it causes the vocal folds to come together and then rapidly part—a process known as phonation. This action produces a series of pulses that create a buzz-like sound. The rate of these pulses determines the fundamental frequency of the sound, which we perceive as pitch. For speech, this fundamental frequency is the basis for the vocal tone or the voice’s pitch contour.
Filter:
The “filter” describes the role of the vocal tract—a tube-shaped structure comprised of the throat (hypo-, oro-, and nasopharynx), mouth, and nasal passages—in shaping the sound produced by the vocal folds. As the sound travels through the vocal tract, its characteristics are altered by the tract’s resonant properties, which are determined by its shape at any given moment.
The vocal tract acts as a series of linked tubes that can change shape to modulate the sound. These changes in shape are achieved by movements of the articulators, including the tongue, jaw, soft palate (velum), and lips. The vocal tract’s resonant frequencies, known as formants, are crucial for the differentiation of vowels and certain consonants in speech.
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