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Old 12-26-2017, 12:04 PM
LouieAtienza LouieAtienza is offline
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Quote:
Originally Posted by Carl1Mayer View Post
Ive decided to start building a new guitar with a cylindrical bend in the soundboard. I had previously built an upright bass using a similar approach to get out of having to carve the top (and to get out of paying ~$400 for a blank of spruce that big).

Its documented here if anyones interested https://www.talkbass.com/threads/alm...-bass.1194755/

I was happy with the result and pretty impressed that a minimally braced 1/8" plywood top could hold ~400lbs of string tension (the steep break angle on a bass bridge directs ~100lb of that normal to the soundboard). Since I typically play guitar more regularly than bass I started digging to see if anyone uses a similar approach with guitars and found out the Howe-Orme company had stolen my good idea about a century before I thought of it (pictured below from google). In addition to the Howe-Orme guitars I would be remiss if I didn't mention Nigel Forster (http://www.nkforsterguitars.com) who is a modern builder using this approach as well, and from the sound clips Ive been able to find he's using it to great effect.




Since the bracing used isn't well documented for the original Howe-Ormes and would depend on how tight of a radius I decide to put in the top I did a bit of finite element modeling to come up with a bracing pattern where the stresses don't exceed what a traditional flattop sees while also cutting weight as much as possible





(Max Deviatoric Stress image is the mostly blue one out of plane displacement is the mostly green one)

Comparing the unbraced soundboards you can see that adding the radius to the top only gives you a little improvement when you put a sound hole in the middle of it because its almost flat on either side of the hole making it much weaker than the rest of the top. Moving the sound hole off of the radiuses part makes the radius much more effective. Another option would be to use a smaller oval sound hole like the Selmer jazz guitars.

Anyway after experimenting with a few design iterations I got the stress pretty comparable to a representative flat top (roughly based on a gibson L00) with only 63% of the weight using a pretty simple X brace pattern (Im using the max deviatoric stress as the metric since wood has pretty significant tension-compression asymmetry). At the same stress the Z displacement for this design is also more meaning it should in theory move more air and be louder.

Next step is to start on all the molds and other such prepwork.
Cool...

I believe Nigel Forster worked with Stefan Sobell, of who he learned the approach from...
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