Official Luthiers Forum!

I know there are a lot of varibles, but is there a weight range a braced top should come at based on type of soundboard used???
I just finished bracing a Sitka top for an OM , I used forward bracing with no popsickle stick, I preshaped the braces but haven't finished shaving and finishing them yet and the top came in at 278 g. The soundboard is around .100 thick and the braces are 1/4 inch thick.
Since I am in no hurry I thought I would collect some data on this one and try doing chladni patterns , which I don't have a clue what I will be looking at, then comparing weight to pattens etc.. ??...
any insight would be greatly appreciated.

Thanks

_________________
MK5acoustics.com

That sounds quite heavy to me. And it sounds heavy for what you're describing. Did you weigh the un-braced top? I'm wondering if you have a very dense top or very large braces. How about a photo?

There's no right answer and no magic number but I would guess that most hand builders would be between 180 - 240 grams for an OM.

_________________
http://www.chassonguitars.com

every thing is rough shaped at this point...I didn't weigh the top before bracing, but It seemed like a very stiff top board.

_________________
MK5acoustics.com

Might be helpful if you can measure the density of the soundboard wood from an offcut if you still have them.

_________________
Evan McCartney

Hmm, don't know what to tell you. You may be able to lose 20 grams by profiling the braces but that is probably optimistic. If that color is correct, that could mean you have a heavy piece of sitka. The darker stuff tends to be heavier in my experience.

Are you sure your scale is correct?

_________________
http://www.chassonguitars.com

ITs my trusty harbor freight 500 g scale...it seems accurate enough. The sitka is a little dark, and it did feel stiff I thicknessed ot to .100 and a little under that .095 or so depending where it is measured.

_________________
MK5acoustics.com

First attempt at chladni
using computer speaker
freq 221

_________________
MK5acoustics.com

Most of my 000/OM size tops seem to run around 220-260 gm, with one closer to 270. It's hard to remove as much as ten grams of bracing without risk, I think.

That mode at 221 could be a ring+, but it's a little hard to say. Try moving your supports around so that they're right under the node lines. I don'tusually see that mode much below 230 on a steel string.

I'll look at the weight of my most recent top when I get home. I don't remember specifically, but I believe it was 248 gm.

after shaping braces weight is at 269.5 g
chladni freq 217

_________________
MK5acoustics.com

Chladni as I thined braces... weight is 263.6g tapping is definately louder...observations greatly appreciated!!!

_________________
MK5acoustics.com

the photos in the last post are reversed the first one is when I finished shaving braces.

_________________
MK5acoustics.com

It looks like you're on the right track. FWIW, my most recent Sitka top was a Dreadnought that weighed 270g after final voicing and it taps beautifully - strong and clear. Thckness was about .112".

Good Luck!

Chuck

_________________
"To live a creative life, we must lose our fear of being wrong"
- Joseph Chilton Pearce

now if I could just figure out what those chladni patterns mean!!!!!

_________________
MK5acoustics.com

Weight is another variable that is controlled to give the customer what they're looking for. Some players want a faster response, while others want more sustain. The weight of the top including the bridge is a big factor in this, with lighter tops being faster of course. Most of the weight is in the top, not the braces, so choosing a top of appropriate density and then determining the proper thickness offers the most control. The bridge is then the second largest factor as far as weight is concerned.

_________________
Randy Muth
RS Muth Guitars Website
RS Muth Guitars Blog
Facebook Fan Page

I've been having some problems posting to this thread: happens once in a while.

welewis wrote:
"now if I could just figure out what those chladni patterns mean!!!!!"

The Eternal Question.

Basically, the patterns are readings of the distribution of mass and stiffness in the plate. The shapes tell you a lot about how smooth and 'proper' that distribution is, and the frequencies give an indication of the ratio of stiffness to mass. Things like narrow node lines and well defined frequencies give a reading on the losses in the plate. There's a lot of interpretation involved, so a bit of practice helps. OTOH, this is stuff that can be posted as pictures and recorded for future reference, so it's easier to learn over the internet than, say 'tap tones'.

From what I can see you've got good patterns there. The 'ring+' is smooth, and at a decent frequency. It's often hard to get it to 'close' wth asymetric bracing, and that doen'st seem to be as important for the tone of the guitar as how smooth the pattern is. I'd say ship it.