Past exam of the medicine course of the University of Oxford / 1866 / First examination / Physics / 5

Describe the construction and action of a free reed organ pipe.

Explain upon the principle of the free reed the action of the human organs of speech.

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. ^[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.