Helmholtz resonance

Helmholtz resonance is the amplification of sound near a cavity’s resonant frequency, produced by air oscillating through an opening against the compressibility of air inside.

Helmholtz resonator: A cavity with a neck or opening whose enclosed air resonates at a characteristic frequency. This physical system produces Helmholtz resonance through oscillation of air in its opening.
Acoustic pressure: The fluctuating pressure component of a sound wave. Pressure differences drive air through the cavity’s neck.
Helmholtz resonator (musical instrument): A musical resonator in which air in a cavity and its opening oscillates at a characteristic pitch. This is the instrument form of the cavity resonance, used to reinforce selected tones.
Standing wave: A wave pattern with stationary nodes and antinodes, formed by interference of waves traveling in opposite directions. Unlike a cavity’s lumped air oscillation, this resonance depends on spatial wave patterns.
Acoustic compliance: A measure of how much a volume of air changes in response to a change in acoustic pressure. Compressible air inside the cavity supplies the spring-like restoring force.
Speed of sound: The speed at which small pressure disturbances propagate through a medium. It affects the resonant frequency through the air’s compressibility and density.
Guitar body: The hollow body of a guitar that couples string vibration to the surrounding air. Its air cavity has a Helmholtz resonance that shapes the instrument’s low-frequency response.
Organ pipe: A wind instrument in which an air column inside a pipe resonates to produce musical tones. Its resonances are chiefly set by standing waves along the pipe, not a compact cavity neck.
Acoustic inertance: A measure of the resistance an air volume offers to acceleration in oscillatory flow. Air moving in the neck behaves like the resonator’s oscillating mass.
Air density: The mass of air per unit volume. Air density contributes to the inertial load of oscillating air in the opening.

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