Helmholtz resonator: principle of operation

Glossary of physics

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Acoustic resonator, Helmholtz resonator

Acoustic resonator, Helmholtz resonator
—
a vessel that communicates with the environment through a small opening
or pipe (throat). A characteristic feature of an acoustic resonator is that the length
waves of his own
LF oscillations are much larger than the dimensions of the acoustic resonator.

For an acoustic resonator with a throat, natural frequency ,
where c is the speed of sound in air, S is the area
cross section, l is the length of the tube, V is the volume of the vessel. If
place the acoustic resonator in a harmonic sound field with a frequency f, in
it generates oscillations with an amplitude, in many
times greater than the field amplitude (resonance ).
In a non-harmonic sound field, the acoustic resonator responds only to
oscillations with frequency f. Therefore, a set of acoustic
resonators with different natural frequencies can be used to analyze
sound. In the presence of friction in the throat of an acoustic resonator,
strong sound absorption at frequency f, which is used
to create a so-called. resonant sound absorbers in architectural acoustics.
Acoustic resonators placed on the walls of sound ducts serve as
elements of resonant reflectors to reduce the transmission of low-frequency noise along
sound ducts. Bubbles in liquid and air cavity in some other media
(for example, rubber) are also acoustic resonators, so the presence
a large number of bubbles in the water causes a strong absorption of sound, which prevents the propagation
sound waves.

The theory of acoustic resonators was developed by G. Helmholtz
(1860).

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