Helmholtz Resonance

thomasross20 · May 2023

Do not read this unless you want to go insane...

I spotted that one particular note on one of my guitars decay's a lot more quickly than the others. I got reading... It's late but the acoustic air chamber vibrates at this hemholtz frequency and if the guitar body/chamber is not "tuned" then it can interfere with notes. I read this is why some notes are accentuated when played or the decay more quickly than others (depending on whether the frequencies add or subtract).

Apparently this Helmholtz frequency is around the F-F#-G notes and can often be heard most clearly on the bass strings (I may be wrong there...). It's why often you get that "thunk" when hitting the F#/G when playing those chords. Not every guitar has it TO THE SAME DEGREE.. it can be tuned out but I believe it is always there (soundhole size, shaving braces etc... There are some inventive fixes players can try).

This frequency can only be moved, never killed... So it could be moved to sit exactly between F# and G... But then both those notes might have half the effect instead of one having the full effect.

I had a panic as noticed a bit of it on just one note on one guitar tonight. It decays quickly. So I tried my trusty old OM-02. Dear God, I never noticed before but it's there around the G on the A string. Try playing the fifth as well as root and you can hear it even more.

In fact, the more resonant (better) a guitar is, the more noticeable this is.

So on my parlour it's not overly noticeable but the low E definitely rings more than others so I think this is a sympathetic resonance that boosts the output (preferable imo).

I checked the frets re the ones doing this.. and they're fine. Action is also fine and strings are only a week old. You can check for it by detuning one tone. The same fret should now be fine as is lower pitch but the one two frets above is the original pitch and will have the resonance effect.

Thinking back, I have heard this on plenty of guitars but never cottoned on. Some say it bugs them and they sell to get one "without the issue" but I'm not sure the issue ever goes away, it's just "moved" a bit. I don't know how this changes over time as a guitar ages - probably not much as my OM-02 has a little of it after 20 years. I might contact the manufacturer but I think this is just a phenomenon that is there, unless it is particularly bad.

I've been reading up on this on the AGF forum and other. Is this just news to me? Feel free to correct any mistakes or misunderstandings above.

Acoustic guitar.. so much more "difficult" than electric and in more ways than one! How often do you really go up and down every single note before purchase, also trying fifths to make sure both notes truly ring and sustain?

Only try this with your own guitar if you are ok with going a little bit mad lol

thomasross20 · May 2023

Think this is also called "wolf note" (the boosted note... Not decaying one)

Link to just one AGF forum post on this:

https://www.acousticguitarforum.com/forums/showthread.php?t=616914

thomasross20 · May 2023

Btw, as an aside....... If this issue was particularly frustrating and you wanted it covered under warranty and it was a USA manufacturer, that is a real pain and I think you could be liable for shipping etc. There is definitely value in buying "local" (I am specifically thinking of Brook here tbh!) as that would obviously be a lot easier

Tannin · May 2023

There are many resonances in an acoustic guitar. The body resonance is (from memory) the second or third-most important of them. The main top resonance is #1. Yes, it is important to locate each of the main resonances off a scale note, but also to make certain that none of them are close together.

I could add a heap of fascinating tedious detail to that summary, but not without refreshing my memory from the textbooks. I might do that sometime over the next few days if the weather behaves as forecast (very cold!) and I'm indoors twiddling my thumbs for any length of time. Or I might see something shiny and forget, of course.

thomasross20 · May 2023

Yes, feel free!

On my OM-02 it's the G note which is very slightly muted. That pitch occurs at different frequencies. 3rd fret low E there's a tiny bit of it. It's more noticeable 10th fret A string.

The new OM03BH it is happening around F but again, more noticeable on the A string. It's annoying to have realised this - means can't expect to hit and sustain that note during a chord / passage . I was just playing again this morning and it's not a killer, just a bit of a peculiarity. Amazing lol

Fyi

DavidR · May 2023

Yeh. I've always called these wolf notes too. Interesting it has an official name though @thomasross20 . Thanks.

When I'm buying I will usually play every note (to about 7th fret anyway) to try and identify them if I can concentrate in the shop enough. It's a sign of a nice guitar if there aren't many, but as you've said, there's always 'gonna be one or two.

I've always assumed it's a function of the natural resonance of the soundbox and the varying strength of the harmonics which form the overall tone of any note played at any fret. Some are complex. Some aren't. The ones that aren't? 'Wolf Notes' Due to the Helmholtz resonance obviously! :-)

thomasross20 · May 2023

I'm going to be trying a lot of very expensive guitars at the weekend for some fun. I'm going to check for this on those...

WezV · May 2023

the ideal is to have the assembly tuned to a less commonly used note.

It still needs to be a note in A440, and that less commonly used note will pop when played. No point tuning it off frequency, as you will likely hear beats instead of wolf notes and that will drive you crazy instead

stufisher · May 2023

I recall studying the phenomenon in O grade physics ... I'd completely forgotten about it TBH but if anyone has a hankering to understand the mathematics then ...

https://en.m.wikipedia.org/wiki/Helmholtz_resonance

sev112 · May 2023

thomasross20 said:

I'm going to be trying a lot of very expensive guitars at the weekend for some fun. I'm going to check for this on those...

Ask for discounts on all the sub-standard guitars you find with one

thomasross20 · May 2023

Haha!

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Helmholtz Resonance

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