MIMF

This is a repair question. A guitar I made 2 years ago came to me for repair of a buzz. The client thought it was a lose bracing buzz. This guitar is a Dreadnaught and very deep. It is very resonant and loud. It has a mechanical vibration when open G is played loud. When other strings are tuned to G, the vibration is activated. My initial impression is that this is NOT a body buzz. I believe all braces are intact. The truss rod was lightly tightened. I loosened the truss rod and tightened in both direction. I checked the neck and put back the tension plus a 1/16 turn. Strings re-tensioned to pitch. Vibration still present but not as strong. A note about clients setup. He just bought the guitar from me, 2 months ago. He uses medium strings and a heavy G string (unwound) I want to say it is 0.026". It sounds a little dead to me. This set up changes the guitar to less responsive, not as lush but possibly a little louder. I would not set it up this way. The guitar was intended to use light strings.
I think the G is activating a resonant frequency in the neck and that this represents a wolf tone.
Any suggestions?
I tuned it to pitch this morning and the vibration is much diminished.
I don't feel it is wise to instruct the client to change to light strings and lose the plain G string. What do you think

A plain G string? On an acoustic? Why? And moving to light strings wouldn't have to change that. No sets that I know of come with a plain G anyway, light or medium.
The only thing that could be rattling in that neck is the trussrod. Moving the rod one way or another should change that, I'd try adjusting the rod with the strings on and the guitar tuned up to see what happens with the rattle. Don't forget that back braces can cause rattles just like top braces can. If there is a rattle beyond a doubt- a buzz or rattle is not a wolf tone- then it has to be coming from somewhere.

Time to start eliminating possibilities.

The lowest order whole-body resonance, in which the entire guitar vibrates like xylophone bar, is usually lower in pitch than the low E note, and the next one up is generally higher in pitch than the open G string, so I'm doubtful it could be that. The lowest mode is often called a 'neck' resonance, because the neck does bend a lot. To check for the lowest one hold the guitar up by pinching the neck near the nut between your thumb and finer, damping the strings with your finger. Hold your ear close to the surface of the headstock and tap lightly on the back of the head with a finger tip. you should hear that lowest mode pretty clearly, somewhere between B below the low E and (seldom) as high as G on the low E. In theory you might be able to hear the next one up as well, but it could be hard to pick out. Recording the sound on a computer and using an FFT program to find the peak frequencies might get it.

Often the 'main top' resonant mode is close to the open G pitch, and that is a more likely candidate than a 'neck' mode. Hold the guitar by putting a few fingers in between the strings and pinching the upper transverse bar between them and your thumb. Your hand in the hole blocks the 'air' resonance that masks the 'wood' sound, and your fingers deaden the strings. Tap in the center of the bridge and try to hear the pitch. Some experiments I did a while ago indicate that changing the stress on the top can alter the pitches of some of the resonances, so it's possible that switching string gauges could have changed thins so they line up differently.

If you can't hear the resonant pitches you can try altering the mode frequencies directly by adding mass in selected spots. Poster putty is good for this: don't use modeling clay, which has oils that can leach into the wood. Slapping a wad of the stuff on the top of the headstock will change any 'neck' resonant modes, and the result will tell you whether that's the issue. Similarly, adding a bit of mass on the bridge or the top in the middle will drop the 'main top mode pitch, and alert you if that's the issue.

It can take a fair amount of patience to track down a wolf, although it's usually not too hard to fix once you've got it cornered.

Alan Carruth / Luthier

A "mechanical vibration"? Fixed on of those on a friend's tenor uke, and it turned out to be a brace which had come unglued for just a couple of inches. It took hours to track down, isolating each possible cause. I found this one by taking a dowel and pressing it against each brace (in several spots) while playing the offending note. When it suddenly stopped was the eureka moment.

I'm not saying it is a brace, but unless you've checked them all you can't be certain.

A plain 3rd string is common with the much lighter electric guitar sets but as Mark said, not found on light or medium acoustic sets. In some experimentation with Hawaiian guitars I have found that heavy plain strings are terrible sounding. You might as well string it with fence wire. The wound g strings have a smaller core wire which is more flexible than the plain string, which allows more complex tone compared to the plain string.

The 3rd string is probably not the cause of the buzzing but you should try a regular g string for a test. If you sold the guitar to be used with light strings I would think you should reemphasize that to your customer. Light structures may sound great with heavier strings but at the cost of early deformation or other issues.

Have you checked the tuners for rattling washers, bushes, etc.?

I've had the ball ends of strings cause rattles, too. It can happen if the end doesn't pull up tight against the bridge plate. This could be a possibility if your customer is using larger diameter strings than you cut the string ramps for.

Thanks all. I have rechecked for the vibration and it seems gone. I have noticed on this guitar that there is doming at the bridge area that is more that is usual for my guitars. The action is a little higher and the G string is dead. I advised the client of this and recommended that he use light strings only. I need to verify my thinking on string gauge. So far I have only made steel string guitars and they have all been made with the intent of using light gauge strings. What happens when I have a client who wants medium or (ick) heavy gauge strings. What sort of changes do I make. I have made a few 12 string guitars that have turned out well. Do I apply the same reasoning top thickness and brace stoutness? How do the mass produced guitars seem to handle any gauge strings?
Is it reasonable for me, as the maker of the guitar to go any further than recommend that he only use light gauge strings?
I am thinking about suggesting that a return and refund are his best course. I am not sure this is the best business course.

I make my warrenty contingent on the use of light gauge strings only.