Acoustic Guitar

[willsie01]

I’m making an acoustic guitar. The sound board is designed to have a very slight curvature outward. Between sessions at the college it has flattened out. I was pondering on the possibility of restoring it with an inflatable diaphragm or suchlike. Is there anything like this I could try?

 

[CliveB]

Most likely due to changes in relative humidity (RH), the spruce will shrink as the humidity decreases and later expand if the humidity rises again and so on. I am no expert but I have built one acoustic guitar. You don't say how much the movement has been? If you're currently in a "dryish" environment and it's flat I'd be inclined to be happy with it. From observing this sort of movement in my own prototype soundboard I realised that it's best to glue the struts to the soundboard in a place of lowish (for the UK) RH, 45% in my case.

Clive

 

[willsie01]

It’s more about learning a way of restoring the curvature rather than if it is ok flat. Because it’s there!

 

[sploo]

There are inflatable bags used for lifting; e.g. https://www.screwfix.com/p/hedgehog...le-lifting-levelling-tool-170mm-x-165mm/6548k

Whether it would be suitable (or successful) I don't know.

 

[willsie01]

I’ll check it out.

 

[profchris]

Don't waste your time trying to restore the doming to the top. You locked in the geometry when you glued on the bracing, and more so when you closed the box of the body. The top will expand widthways in higher humidity and some dome will come back. It will shrink with lower humidity - worst case is that this produces cracks in the soundboard or even a dip.

I'd start by finding out the % relative humidity (RH) the guitar is currently living in. If, as a previous poster wrote, that is low (40% or less) then you're OK. The dome wil come back a little in normal humidity (in a UK house that's maybe 55%) and the strings will pull the top upwards at the bridge another 1-2mm.

If the guitar's top is flat at, say, 60% RH then you could have a problem. It will still work, but you'll have to make sure it's kept at that RH or higher (keep it in a case with a humidifier, topped up regularly).

At this stage your only alternative is as follows:

1. Rout off the binding
2. Detach the top
3. Rebrace it (you can use the same braces if they come off cleanly) in low (around 45%) RH
4. Reattach the top, again in low RH - could be a problem here, as the top may be a different shape, but you can compensate by adding extra purfling to cover the gaps.
5. Re-bind

If your teacher knows about guitars, she or he can explain to you the importance of RH and when you need to pay attention to it - if not, there's lots of information online, or ask here.

For context I've made around 60 instruments in the guitar family (ukuleles, tenor guitars, 6 string guitars) so I learnt this early on (after my first two did exactly what yours has done, except over time they also cracked their soundboards mightily!).

 

[willsie01]

You locked in the geometry when you glued on the bracing, and more so when you closed the box of the body.

The braces were clamped and glued to the soundboard in a dished jig. The curvature remained for quite a few months after the body was boxed up, as you describe it, before the soundboard flattened. The teacher is a professional luthier and university teacher.

 

[profchris]

Ah, that helps. And I'm assuming the curvature was still there when you glued up the box? If so, unlikely to be a humidity shrinkage issue unless it's in the UK equivalent of a desert (eg central heating set really high).

In that case, get inside with a light and inspection mirror (or a phone might get some pics if it'll go through the soundhole). If a brace end or two have sprung free, that might be the cause. Your teacher can help you work out how to reglue them if that's the case. If you can't see inside, probing with a feeler gauge or thin palette knife might find the issue.

I've just fixed the brace ends on a friend's guitar - it was in her music shed, and the really wet weather expanded the top enough to pop them free, at which point it went flat.