Helmholtz resonators

Principle of Helmholtz resonator

Principle of the Helmholtz resonator: The air plug in the opening oscillates on the elastic cushion formed by the air in the corpus.

Helmholtz resonators are acoustic systems consisting of an oscillating air plug connected to an air volume. There are many different forms of Helmholtz resonators: an empty wine bottle, the corpus of a string instrument, bass reflex enclosures of loudspeakers and wall linings made from perforated wood or gypsum boards. The empty wine bottle is best suited to explain the geometry and operation of the Helmholtz resonator in a simple way: The air in the bottleneck forms the said air plug, and the air contained in the remainder of the bottle is the connected air volume. The air plug has an acoustic mass which results from its geometry and the specific density of the air. It rests on the elastic cushion formed by the air contained in the remainder of the bottle. Together they form an oscillating system with a specific resonance frequency that can be easily excited, as is well known, by blowing across the opening of the bottle.

Helmholtz resonators are often used to amplify sound. With string instruments, for example, the sound energy emitted by the vibration of the strings alone would by no means be sufficient. Only when the strings are connected to the corpus with its openings sufficient loudness can be achieved.

In order to prevent a Helmholtz resonator from amplifying sound and make it absorb sound, the air oscillating in the opening must be slowed down by friction. This is usually achieved by means of a thin fleece material glued to the rear of the opening, sometimes with an additional layer of mineral wool or foam.

Perforated linings of walls and ceilings:
Probably the most common application of the Helmholtz resonator are suspended ceilings or wall linings made from perforated metal, wood, or gypsum board. The air contained in the numerous holes of the boards is oscillating in front of the air volume enclosed between the boards and the ceiling or wall. These systems are tuned in such a way that several resonance frequencies lie next to each other and absorption over a broad frequency band can be achieved.

Special constructions to be used as bass absorbers:
Helmholtz resonators can also be used to damp individual eigenmodes of a room which develop at low frequencies. To attain a low resonance frequency, a relatively large volume with only one or a few openings is needed. The Taschenbuch der Technischen Akustik (Pocket Book of Technical Acoustics) includes a comprehensive collection of formulas for the calculation of the resonance frequency.