I am planning my next build.
3-string, red oak neck with scarf joint, 24.5 scale, bone nut and saddle,
rod piezo mounted under the saddle.
What are the effects of bracing the underside of the box lid.
Is it worth the effort ?
Also is adding internal shaping to the inside of the box, like rounding all f the corners,
something to seriously consider for the good sound.
Almost certainly no need for and no point in bracing the box. The only reason for doing so would be if you were putting a lot of extra load on it somehow - like if the strings were to be anchored at the bridge as on a traditional acoustic guitar. With typical CBG designs, where the strings are anchored to the protruding end of the thru neck or where there is some sort of tailpiece, then there's generally no need for internal bracing.
Bracing would stiffen the lid, which would tend to raise the pitch of its natural resonances and affect the amplitude of the instrument's sound. That's the simple version - the full picture is much more complicated and the fact is that there are still aspects which are unknown.
As for shaping the inside of the box, I'd say that's unlikely to make much difference and it's just not worth the additional complication.
Despite lots of time and effort by professional luthiers and acoustic scientists there is still a lot that remains unknown about the best way to design guitar bodies and soundboards. Many people have their own theories - some of which are more convincing than others. The bottom line is that guitar building, even at the highest professional level, still comes down to trying things out and seeing if people like the sound.
My next build is with a sheet of very thin plywood for the top of the CGB, the plywood was for another project (on biodiversity - building next boxes for a schools project) the timber was rubbish, the sheets started de-laminating.
So I got a thin board off it for the top of a cigar box. Good thing is the thin board had great acoustics but the down side is it doesn't appear too strong, So I've glued up a cross brace, slightly forward of the bridge, so the edges of the bridge will be over the arm of each cross and I'll use a sound post to connect the back and the top. It will have a simple piezo pick up, I'm about a weeka way from fixing it up....
I'll take some pix and report back later.
I use 2mm thick plywood for almost all my builds (3-stringers, mostly). I have to build some kind of sound box as cigar boxes are rare in here in China. And for most of my builds I've found no need to add additional bracing for the plywood veneer sound- and back-boards, especially on small to medium boxes with 3 or 4 wire strings. On the couple larger boxes I have done, I did add a bit of bracing in the form of a 3cm by 0.5cm length of trim wood a bit longer than the bridge and located under where the bridge will be located. By large box, I mean one with a width > 8". So far, no problems, and these plywood topped sound boxes sound a lot better than the instruments built around cigar boxes. Most cigar boxes have fairly thick tops and so don't really need bracing unless you're building (maybe) a six stringer or 8-string mandolin, or a 12 stringer guitar. In these cases, with the additional string tension on the bridge and sound boxes, I'd consult with builders of these "bigger" instruments to see how they brace them.
Love this question sense Im starting on my second CBG build........I plan on using some bracing tho I agree it isn't necessary . I want to see what effect it may have...even tho I won't have a head to head comparison. I have done some WWW in depth research on bracing and regard to the effects on tone....but these were with regards to standard sized guitars. My box (a boat shaped shelving unit 14 x 7.5 (at wides point) inches) and 3,5' in depth) I plan on using the bracing mainly for distribution of vibrations.......and the bracing will be thinner but protrude deeper in the box than normal bracing .. it's all for experimentation .... I've read that when bracing is applied the cross brace may help amplify the vibration due to the grain running in a perpendicular directing on the soundboard grain ......it also helps vibration travel further down the soundboard to the outer edges of the box / guitar sides. I can't get to in depth here but I gust wanted to add that bracing has more that one purpose..well at least on a standard acoustic guitar.......
IMHO lids are too thick. I take a dremel tool with a large sanding bit and make passes that radiate out from where the bridge would sit.Sometimes I carefully remove to top and take my belt sander to it. I leave a few thick radiating areas which become the bracing or stronger areas. It kind of resembles a sun pattern when I'm finished. I also make the edges of the soundboard a bit thinner. This is a lot of work and does improve tone, but I usually only do it for my acoustic keepers - too much Time, work, dirt and dust to sell. Here's a real quick one I did - this only took a few minutes....
Thats pretty cool Wade, I was thinking of rigging up a router jig to do some thinning experiments my self. Neat idea!
Thanks. Thinning makes a huge difference acoustically - at least to my ears. Thinning plywood maybe tricky with a router - not sure with the alternating grain.
Re bracing; I built a box mando last year and used a thin piece of solid koa for the top. When all was complete, it played dull.klunky. I was so dissappointed. It looked great but I wanted it to sound good accoustically. So on a whim I cut a brace and by reaching thru the hole , glued it crosswise as far in as I could reach. It made all the difference. Now it sounds better than any I have built. so maybe if the top wood is very thin it needs bracing. I will post a sound bite on my page when I can find it. Dan
I always like to put a little brace near where my bridge sits, even though boxes dont have any stress on em in the neck through designs...your strings push down where the bridge is considerably.