The Helmholtz-Resonator Thing ...

Every hollow body with an opening to the outside is a Helmholtz-Resonator

So is a guitar, a violin even a drum or any other musical instrument that fullfills the marks:

  • hollow body
  • opening to the outside

... regardless of the shape of the instrument and the shape of the so-called 'soundhole'.

 

The background and basics


The effect of 'Helmholtz resonance' was revealed by Hermann von Helmholtz in the late 1850s.

A indispensable mark of such a Helmholtz resonator is its 'Helmholtz resonator frequency'. This is the frequency where one gets an tremendous over-shot in sound strength at the opening (e.g. a 'soundhole')  compared to the emited strength of sound of the body, if the body is stimulated by a mechanical frequency. (See Wikipedia for "Helmholtz Resonance" and/or "Helmholtz Resonator")

An excellent article about Helmholtz resonance [↑] can be found at the University of New South-Wales (UNSW) in Sydney/Australia and the 'Basics' section [↑] provides essential knowledge about the physics of musical instruments and the human voice as well.

 

Bringing this into practice ...


Below are two pictures from a 3D-spectrum analyser.

These pictures  show the impact of the Helmholtz-Resonator-Frequency to an instrument if the soundhole is not covered and if the soundhole is covered: i.e. the opening is closed and therefore the Helmholtz-Resonator is 'turned off. Mounted into the instrument was an AnyMic/i from the normal production.

The chosen instrument was a Western-/Steel-string guitar that has a typical Helmholtz-Resonator-Frequency of 55 Hertz. This is half of the frequency of the the base-frequency that is for a steel-string guitar the open A string (A5; 110 Hz). The bad impacts of an uncovered soundhole can be seen clearly ...

A5 String uncovered SS
A5 String covered SS

UN-covered soundhole

  • Tremendous over-shot @ H.R frequency
  • Drastic cutdown of over-tones
  • fast degrading sustain

Covered soundhole

  • Clear 'spike' from the string pluck
  • Clear built-out of over-tones
  • better sustain

Using an internal microphone in the instrument demands, that the soundhole is covered by a 'soundhole cover'.
Only this turns the Helmholtz-Resonator 'off'' with all the bad impact. Some call that a 'feedback killer' - which is wrong, because Helmholtz-Resonance has nothing to do with 'feedback'.

 

Specific instruments have typical H.R. frequencies.
I can speak here only for instruments, for which I have investigated it or have tested it with instrument that I have at hand.

The following table shows results from my invstigations and measures:

   H.R. frequency [Hz]  Base frequency [Hz] Note/Tone
Steel-string guitar 55  110 A5 (open A string)
Classical guitar 48 96 G5
Flamenco guitar 43 86 F#5
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