A question about a " -resonance note" -

Construction and repair of Classical Guitar and related instruments
yooper

A question about a " -resonance note" -

Post by yooper » Sat Jan 16, 2010 2:10 pm

Hi all - just a quick question about a resonance note on a guitar that I'm trying out.

It's got a fairly pronounced resonance note that produces a "thuddy" tone on F#, 4th string. One fret lower, the F is a bit thuddy too. Otherwise, the guitar sounds fine.

Given that nearly all guitars I've owned and played have a resonance note of some kind, for classical guitar, is there such a thing as a preferred note or a preferred place that the resonance note shows itself? I've got another classical guitar that has its resonance on A, but it's quite subtle everywhere on the guitar.

Thanks very much for your thoughts...

Alan Carruth
Luthier
Posts: 3134
Joined: Mon Dec 24, 2007 6:56 pm

Re: A question about a " -resonance note" -

Post by Alan Carruth » Sun Jan 17, 2010 1:09 am

It sounds to me as though you've got a guitar with a very active top: if the top were less active it would effect the sound less.

All guitars have a 'main top' resonance, and the issue is not so much what pitch it is at, as it is whether it's a problem for some reason. Unfortunately, it's difficult to alter the pitch of the main top resonance much without major surgery in most cases. The folks who have done controlled studies of this (see, in particular, Howard Wright's PhD thesis: "The Acoustics and Pschycoacoustics of the Guitar", given at the U/Wales in Cardiff in 1996. You can download it from their web site) seem to feel that the pitch itself doesn't matter a lot in the overall perception of the guitar's sound, within reason. Usually it's only the timbre of the notes close to the resonance that are effected, as you've noticed.

You might be able to spread out the top resonance, and weaken it a bit, by getting the back working with it. Usually getting the 'main back' tap tone fairly close to the 'main top' pitch will do the trick: I shoot for having the back a semitone higher (and sometimes even get it!). But if it's more than a third higher now any drop might help. Try adding some weight, such as a lump of poster adhesive to the back, which will lower it's pitch, and see if it has any beneficial effect. If it doesn't work, all you're out is the cost of the lump of goo.

If altering the pitch of the back doesn't work, parhaps reducing the activity of the top will. Again, you could add some weight, this time to the bridge. It will cut down the volume some, and possibly increase the sustain. See how little you can get away with to bring the problem under control.

These are only a couple of ideas, and the actual problem could be far different and require entirely different aproaches. In the end, internet diagnoses via ASCII text are not nearly as good as having the thing on the bench.

One of my 'test mule' guitars has a very strong 'main top' resonance just below the pitch of the open G string. When you play that string the top moves enough so that the fundamental of the note is actually split, showing up as two peaks in the spectrum about 6 Hz apart. If you listen you can hear the 6Hz beat frequency. Most people don't seem to notice unless I point it out, and, in some ways, it adds a little 'shimmer' to the sound. It's only a problem when it's a problem.

yooper

Re: A question about a " -resonance note" -

Post by yooper » Sun Jan 17, 2010 1:32 am

Very interesting - thank you. I have noticed that my recently-damaged thumbnail might have something to do with how ugly the F and F# are sounding on the 4th string. It split off a few days ago and I'm only getting flesh on those notes - when I hit the string with another finger with some nail, or as an experiment, a flatpick, it rings a fair bit better. At times, it seems like it's more about what I feel coming back at me from the body of the guitar as it is about tone - the guitar thuds, and it feels like the entire guitar is thumping my chest a bit. Anyway - I appreciate the response.

riffmeister
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Location: Philadelphia, PA, USA

Re: A question about a " -resonance note" -

Post by riffmeister » Sun Jan 17, 2010 2:19 am

I often get a little fret rattling on that F# (and the F# at the 2nd fret on the 6th string) which I believe is due to a 'sympathetic vibration' of the string and top at that frequency. But instead of of a cancellation leading to a 'thud' as in your case, it's an addition leading to more string vibration and hence the rattling on the fret.

Alan Carruth
Luthier
Posts: 3134
Joined: Mon Dec 24, 2007 6:56 pm

Re: A question about a " -resonance note" -

Post by Alan Carruth » Sun Jan 17, 2010 7:58 pm

riffmeister:
It's amazing sometimes what can cause a buzz. I remember one repair that came in with a number of buzzes. Most were the 'usual suspects': a low nut slot, high frets and so on. But after we went through all of those there was still a buzz. There were no loose parts either. Finally I found that the 'main back' resonance was too close in pitch to the 'main top', and the two were interacting in such a way as to produce a difference frequency that sounded for all the world like a fret buzz at G on the low E string. A few grams of mass added to the bridge dropped the top pitch enough to eliminate the problem without harming the sound noticably. That's why I can only make suggestions on line: the real world can be far more interesting and hard to pin down.

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