What is a Helmholtz resonator?


The Helmholtz resonance is named after Hermann von Helmholtz, who created a resonator device in the 1850s. He used to identify the various frequencies and musical pitches present in music and other complex sounds. Helmholtz described in his 1862 book, “On the Sensations of Tone”, an apparatus able to pick out specific frequencies from a complex sound. So, the Helmholtz resonator consists of a rigid container of a known volume, nearly spherical in shape, with a small neck. It has a hole in one end and a larger hole in the other end to emit the sound.


Specific frequency


When the resonator ‘nipple’ resides inside your ear, you can hear a specific frequency of the complex sound clearly. In Helmholtz’s book, we read: When we “apply a resonator to the ear, most of the tones that the surrounding air produces get considerably damped; but if the proper tone of the resonator sounds, it brays into the ear most powerfully…. You can sometimes hear the proper tone of the resonator cropping up in the whistling of the wind. Also, the rattling of carriage wheels or the splashing of water. Helmholtz resonance finds application in internal combustion engines subwoofers and acoustics.


The construction


An acoustic pipeline houses the Helmholtz resonator. The pipeline comprises a cavity and a neck part, wherein one side of the cavity arranges the neck part and communicates with the cavity and the acoustic pipeline. The Helmholtz resonator further comprises a sound absorbing structure, wherein the sound absorbing structure fills the neck part. The sound absorbing structure includes a plurality of through holes which communicate the cavity with the acoustic pipeline, the hydraulic diameter d and the porosity phi of the through holes in the sound absorbing structure meet the following relational expressions.

Specifications describe the expression, wherein Re(Zb)apt represents the optimal acoustic resistance value of the Helmholtz resonator. The R represents the reflection coefficient of the downstream acoustic pipeline of the Helmholtz resonator and is a real number. Furthermore, Re(Zb) represents the inlet acoustic resistance of the Helmholtz resonator. The Re(Zb) is the relational expression which is related to the hydraulic diameter d, the porosity phi, the flow resistance and the length of the through holes in the sound absorbing structure. The invention further relates to a design method of the Helmholtz resonator.


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