= Solution
A <free reed> is a thin elastic tongue fixed at one end over a closely fitting opening in a rigid frame. A pressure difference drives an airflow; displacement of the tongue alters the available opening, and the changing aerodynamic force can sustain its vibration against elastic restoring force and damping. Unlike a beating reed, the tongue passes through the opening without regularly striking its rim. The air supply provides the <energy>; the reed's dimensions and stiffness largely set the pitch, while its enclosure and any attached pipe modify the response and radiated <sound waves>.
The useful analogy with <phonation> is an airflow-driven valve coupled to an elastic oscillator. Air from the <lungs> passes between the <vocal folds> in the <larynx>. Their motion periodically modulates the flow, generating a rich <sound spectrum>; the <pharynx>, mouth and nasal passages filter it, while the <tongue> and lips help form speech sounds. The <vocal folds> are paired living tissues with adjustable tension and geometry, rather than a single metal tongue. Their oscillation should not be explained as a passive string struck repeatedly by air, nor does the free-reed analogy establish every detail of tissue motion. <Taking Care of Your Voice by the National Institute on Deafness and Other Communication Disorders>[“What is voice?” and the labelled structures of voice production]{ref}
Changing vocal-fold tension and effective vibrating mass changes the source pitch. Changing the resonating passages changes its spectral envelope and the vowel quality. Those are different controls: a singer can change pitch while retaining a recognisable vowel, or change vowel while sustaining approximately the same pitch.
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