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String pressure crushing boxes.

So, I have found that on some builds, after all the careful selection of boxes and doing what I can to insure good acoustic tone and volume that the string pressure on the bridge tends to deform the soundboard and the bridge ends up sitting on the through neck. All of the work in cutting the relief on the through neck to keep space between it and the soundboard has caused it's own set of "issues". If this happens on one with already low string action, fret buzz can become a problem. This seems to happen even on builds that do not have a very severe break angle at the bridge. Anyone else? Is this where internal bracing comes into my building picture? I use the thinnest surface of the box for the soundboard, often the "bottom" of the box. There is never a free lunch is there?

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Permalink Reply by Bluesheart on December 16, 2014 at 4:35am

Now that I see this picture I notice it is causing the bridge to tip forward as well and this is a very shallow break angle.

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Permalink Reply by ChickenboneJohn on December 16, 2014 at 4:49am

Simple solution - don't bother leaving a gap, and build the guitar with the underside of the top bearing directly on the neckstick. With a thin top, this can yield surprising good acoustic results - counter intuitive and simple. And by the way, that IS a very shallow break angle over the bridge -try setting the fingerboard higher above the top of the guitar. Also, try making your bridge triangular in profile..wide at the base and knife edged at the top..less tendency to keel over like that.

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Permalink Reply by Richey Kay on December 16, 2014 at 5:30am

I think John has a point here. I would also add that as I mostly make CBGs with the intent to amplify them I actually use the thickest side, which seems to dramatically improve sustain. I am often surprised by the acoustic volume, especially as I tend not to make soundholes.

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Permalink Reply by Bad Finger (Eric) on December 16, 2014 at 6:59am

The "relief"ers fail to consider how much the neck is actually vibrating in combination with the strings. Sometimes (often) there is significant transfer of energy into the neck and when the neck is not in contact with the soundboard that energy is wasted. I am really surprised at how much even a well-built neck built with 4 pieces of wood will still vibrate.

Even this one which has a 1/4" thick purpleheart fretboard on the other side. 1x2 maple with 1/4" oak stripe and purpleheart heel. The neck is in direct contact with the masonite lid. Surprisingly loud acoustic even though it was intended to be electric-only.

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Permalink Reply by David McQueen on December 16, 2014 at 12:53pm

I love the grain on that neck. Just lovely.

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Permalink Reply by Jim Morris on December 16, 2014 at 8:18am

Alright here's my 2 pence worth. I'm with ChickenboneJohn provided the instrument is meant to be amplified. Most of mine are acoustic instruments and I'm trying to get the top, sides and back vibrating as much as possible. Spruce or pine bracing does dampen vibrations a bit but you certainly don't want the top to bow under bridge pressure so there is a little trade off. I've started using a sound post, like on my CB Fiddles, on other instruments as well. This supports the top and transfers vibrations to the back. You can get almost a wah-wah effect on these instruments by touching the back to your belly and moving it away. And I agree with Eric that the neck itself vibrates quite a bit hence the need for a good tight fit where it attaches or goes through the body especially on acoustics but also the tight fit adds to the overall sustain of an amplified instrument. My limited knowledge prevents me from getting too technical but with the soundpost support design you can also change the tone of an instrument by adjusting the post's position. Good luck and all of this has come to me by experimenting and having numerous failures which I choose to look at as learning experiences!

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Permalink Reply by Jim Morris on December 16, 2014 at 8:20am

Oh, and I like your idea of making 3 identical instruments using different neck connections. I'd be interested to hear your conclusions. Also I forgot to mention the fact that dulcimers have full, above body neck contact and they have a wonderful tone! Lot's of right ways!

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Permalink Reply by Bluesheart on December 16, 2014 at 11:21am

Thanks for all the feedback! Just love that about the Nation! Taking from the suggestions so far, I'm wondering if a sound post, block or something fitted inside the box between the top and the neck relief cut ( not like a violin post that connects top and back) would both support the bridge/top and provide a mechanical connection to the neck to get those vibrations AND keep as much of the top free to vibrate as possible! Make sense?

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Permalink Reply by Ron "Oily" Sprague on December 16, 2014 at 11:12pm

Just to throw in from the commercial Git side of things:

This happens on my Ovation Elite. Due to the high gloss finish on the walnut veneered composite top, you can see the dimple in the top from string pressure. Yes, I put mediums on what is supposed to use lights or extra lights. It's a super shallow bowl, and I needed to drive it for rhythm playing.

This will also happen on cardboard CBGs, as well as wooden ones. I find bracing the top along the sides and corners helps with this. Chickenbone John's suggestion to make the bridge triangular is a good one. You can also try the footed violin or banjo bridge. And notice that in most commercial acoustics, the bridge is usually about1.5" wide, and 4-5" long, to spread out contact with the top. The saddle is the thin bit that rides in a slot carved in the bridge. If that slot is a tad too big, even the saddle will bend over a bit like your bridge is doing in your pick. Wider base, narrower top. I also notice that your bridge has not string slots and does not appear to be angled for compensation. You could do all of those on a triangular, higher bridge. But then you might have to raise the fretboard to keep the action low for fret fingering. Luthiery is all trade-offs and compromises; one fix leads to another.

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Permalink Reply by Michael Fred Johnson on December 17, 2014 at 1:25am

Fanstrut the box,This one is a year old and still fine,it has a raked neck to give a low action with a high bridge

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Permalink Reply by Bluesheart on December 17, 2014 at 3:04am

Yep, this bridge is not typical of what I do, it was just a piece of Corian I had laying around that I tried for size, it worked well so I got lazy and never changed it out. It is a floater so it gets knocked around. I do angle it and set the intonation before doing any playing on it.This particular guitar is one I knocked together using the first fretted neck I made (that was on another box) and it does not have a separate fret board so there is very little height available for the bridge.