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I have a few acoustics I'm working on. So far in this batch, I have 3 boxes without tops. One more to go.

I attempted to post a few .mp3 sound clips of this, but I guess OLF doesn't allow them.

Anyway, I was tapping the 3 boxes (2 Dreadnoughts, 1 OM) on the back just to listen to the difference so far. On the EI Rosewood one, I noticed a "buzzing" sound on brace #3. My knee jerk reaction was to check and see if a brace had come loose somewhere. I couldn't find anything. Couldn't find any separation between the rims and back or around the perimeter of the reverse kerf linings. Everything looks solid but that buzz started to concern me. Especially since I couldn't explain it. I started listening to the others with a critical ear and found that they all do it to some degree. One of them was made from Goncalo Alves and the other, an OM, is Sapele. I can't find any reason on any of them for the buzzing/rattling sound. The only thing I can think is that the backs have not been flush trimmed to the rims yet and they hang over varying amounts around the perimeter. Still just rough cut. Maybe lack of symmetry?

Has anyone here ever experienced this?

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

Can't say that I ever experienced that but in the repair world it's standard fare when evaluating for loose braces to do some tapping and listen for a buzz. A buzz often, not always, indicates a loose brace.

Now since you can make all three rims do this if you want to I would suspect that it's not a loose brace unless all three have loose braces. Maybe give us some more information on exactly how you tap, where, if the rims are out of the mold, spreaders gone, anything that helps paint a bigger picture?

The overhang of the back not being trimmed back yet should not be an issue IMO unless the back is not glued down well in some spots, a loose back can buzz too.

How did you check for separation? I had a guitar come in recently from a conservatory student from Nashville, where the guitar had gone to a Nashville repair shop for a buzz. They found nothing and told her that maybe it was humidity (you're getting it 3rd hand so who knows what was really said). Anyway it was not buzzing in my shop. But given she was on travel, I took it in nonetheless. Upon close inspection I discovered a separating back brace. It was very tight, but separated. Certainly humidity moved the wood around enough to cause buzzing in certain instances.

But my point is separation can be elusive. It could be a dry glue-up. Also depending on humidity when gluing up, height of braces, newly glued up back can get some movement and cause some aches and pains. All you need for a buzz is two spans of wood somewhere that are separated and vibrating.

Andy

When installing the reverse liners did any of the crack, even a little bit at all? That crack can cause buzzing.

Also, are the rims held in a mould or are they out of the mould? If in a mould there can be buzzing between the side and the mould.


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Check around top rim. It vibrates at high amplitude when there's no plate glued to it, so any butt joints between linings and end blocks can break and rattle. And even more likely, if your reverse kerf linings aren't in single continuous strips, then check all the joining spots on those. Generally those kind of fractured butt joints will quit rattling after the top (or in my case, back) is glued on so they can't flex open and closed anymore, but it's still good to locate them and reglue first. With hide glue, you can just put a wet paper towel on it for an hour or so to rehydrate the glue, and then heat it to reactivate it. But of course if you tap it hard again after that, it might just break again in the same place.

Another place I have this happen is on the butt joint between side brace and upper transverse brace, as can be seen here:

The rest of the side brace to soundboard joints don't seem to be an issue since they're supported on either side by the dentellones. And the dentellones themselves are all butted and glued together, so the fact that I've never had rattles between them before is more evidence that it's not really a concern for closed boxes.

Thanks everyone for the info. I do have a couple places where the kerfing is a little cracked. I always make sure to move those pieces to the top where it won't be seen.

It's not in a mould. No spreaders. Just rims glued to a back. Everything was glued up in 40-50% RH or less. I always make sure there's plenty of squeeze-out and of course it gets cleaned up immediately.

These boxes all vibrate like crazy too. Just by brushing your finger on the overhanging part of the backs will make these things start to sing. They just want to resonate. I've never noticed this kind of thing before but I get maybe I wasn't this critical with my first few acoustics. I thinking since all 3 do it, it's gotta be those little pieces of separated kerfing.


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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

The fix is to find the areas that are cracked along the liners and cut through them. I know this defeats the purpose of the reverse liners but gluing them might not fix them.

Better to fix this now versus having to find those cracked liner locations once the box is closed.

Next time around, make sure you prebend the liners to avoid this problem. Spritz the solid side of the liner with water and clamp to the outside of the body. The next day you can install on the inside. Works like a charm.

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