Question about main air resonance frequency

Construction and repair of Classical Guitar and related instruments
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Question about main air resonance frequency

Post by LBrandt » Tue Jun 13, 2017 11:54 pm

Every guitar has a main air resonance pitch (or whatever it is otherwise called). If I know the main air resonance pitch of a guitar, should plucking a note of that pitch create a sound with a different characteristic than it would if the air resonance were of a different pitch than the plucked note?

In other words, if the air resonance were F#, and I plucked an F#, should I expect to hear anything different than if the air resonance were A, and I plucked an F#?

If the question sounds stupid, please excuse.

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Re: Question about main air resonance frequency

Post by Paul Micheletti » Wed Jun 14, 2017 12:49 am

If the main air resonance lines up perfectly with a note, then the air resonance can suck all of the power out of the string when that note is played. It can cause a note to boom loudly (which is difficult for the human ear to pick out many times), and will also cause the note to have a short duration (which is a much more noticable effect).

A friend of mine made his first guitar with a tiny soundhole for some unknown reason. He played an open E on the low string and it had a duration of only about 1 second. Blumpffff Blumpfff. What an anemic E. I picked up the guitar and hummed into the soundhole, varying the pitch of my voice until I heard the feedback resonance. It was exactly aligned with the open E string pitch. Raising or lowering the E string fixed the problem. He wound up expanding the soundhole size which raised the air resonance pitch and fixed his thuddy E. It actually was not a bad sounding guitar for a first guitar after that surgery.

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Re: Question about main air resonance frequency

Post by bftobin » Wed Jun 14, 2017 12:56 am

Reuben Diaz(sp?) has some youtube videos about how this works. That's why some performers have multiple guitars on stage.
They will change guitars depending on what key they're playing in.


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James Lister
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Re: Question about main air resonance frequency

Post by James Lister » Wed Jun 14, 2017 6:44 am

Much as Paul has said - if the body resonance (or any other resonances) coincide fairly closely with a scale note, then that note will tend to have a short sustain and a "thuddy" attack. The resonances all have a certain bandwidth, or frequency range, so in fact, even if the resonance lies exactly between two notes, both will still be affected, although to a much lesser extent.
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Trevor Gore
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Re: Question about main air resonance frequency

Post by Trevor Gore » Wed Jun 14, 2017 10:03 am

As well as the "wolf note" effect (short duration, "thuddy" notes that others have mentioned) there is also a frequency shift effect. When two resonators of similar resonant frequency couple strongly, the resonances tend to "repel" each other. So a string trying to play G at 98Hz and a guitar box with a main air resonance at, say, 99Hz will result in the string playing at a lower frequency on that fret compared to the 98Hz it should be sounding at. This type of over-coupling problem can result in local intonation errors on the fretboad of + or - 30 cents, depending on whether the played note is higher or lower in frequency than a nearby body resonance. This level or error is more than audible and is why some guitars seem impossible to tune. The more responsive the guitar, the bigger the problem. The fix is the accurate placement of body resonances between scale tones and making the "geometric" intonation as accurate as possible so that errors don't aggregate, but there is nearly always some degree of residual intonation error due to over-coupling. It is up to the builder to place these and minimise them so they cause least offense and reduce the player's effort required to push or pull a note into tune.

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Re: Question about main air resonance frequency

Post by Rasputin » Wed Jun 14, 2017 12:44 pm

My low G has a boomy quality to it but the sustain is more or less the same as the C on the next string. Some sort of feedback thing happens when I hum into the sound hole at a pitch between F# and G, so presumably the boominess is down to the air resonance - but I am now wondering why the sustain is not affected.

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Re: Question about main air resonance frequency

Post by eno » Wed Jun 14, 2017 1:24 pm

I'm wondering how the body resonance, specifically the soundboard resonances, change with humidity. As the wood absorbs water its mass increases and that should affect the resonance frequencies, so if a luthier carefullty tunes the resonance to be between notes at one level humidity, at different level the resonances may shift and coinside with notes. Can that happen?
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Re: Question about main air resonance frequency

Post by Pat Foster » Wed Jun 14, 2017 1:51 pm

There are degrees of coupling between the plucked note and the main air resonant frequency. If the coupling is strong, the sustain is more likely to be affected, especially if there are other components at or near the main air resonant frequency, such as a back and or top whose resonant frequencies happen to be an octave above the main air. That's my understanding.

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Re: Question about main air resonance frequency

Post by Alan Carruth » Wed Jun 14, 2017 5:46 pm

Yes, the resonant pitches of the guitar can change with changes in humidity. One problem is that the 'air' and 'wood' resonances move in opposite directions; as the humidity rises air resonances rise in pitch with wood resonances fall. The damping of the modes is also affected. This accounts for some of the 'moodiness' you get with certain guitars.

There are actually two types of sustain. What I think of as 'Les Paul' sustain involves having a heavy or stiff guitar that basically traps all the energy in the string, only allowing it to leak out very slowly. You get a low level of sound for along time. 'Banjo' sustain happens when you get a very responsive top that is quite efficient at turning string energy into sound. The sound level builds up quickly and dies off rapidly, but it can reach such a high level that it can actually remain audible for a long time. Since your ears are not very good at detecting rather large changes in sound level you might not notice that the boomy note actually builds to a much higher level than the ones around it.

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