Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

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OiAcoustics
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Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Wed Nov 17, 2021 2:24 am

My bodies and necks are built and finished entirely separate from each other than fitting the neck to match the fixed bridge location (length and sideways) before finishing.

It would far more expeditious to also check the targeted values for an 0 guitar of 95 Air/180 Top/226 Back hz, coupled and uncoupled, BEFORE mounting the bridge so I can muck around with anything that looks like a wolf note before finish.

Have any of you had success anticipating the final loaded results (strung) from an untensioned state? I could mount the bridge with double side tape for the weight, but I wouldn't think that an untensioned top would have nearly the same response as under tension, but might be predictably lower as it's loser, if everything else were constant (which of course they aren't). The air and back resonances might be reasonably close...

I thought I read something about this in The Book on an early read, but haven't seen that again. If anyone has thoughts or a page reference, do tell... if not I'll work up my own before/after tension chart to see if I can come up with anything consistent, as the first X to falcate rebuild is being boxed up tomorrow. Exciting! As all I'll have to do is rebind the back, and refinish, I'm going to go ahead a string it up for an initial ear & FRC test (compare to the X state) before those final steps... just in case...

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by johnparchem » Wed Nov 17, 2021 2:45 am

On a closed and bound guitar I have tried taping on the bridge and just adding mass with Blue tac neither really told me much. The bridge is also a brace stiffening the top; that is lost with taping and just adding mass. On repeat guitars I am consistent enough that I have a pretty good knowledge about the effect of the bridge and the strings at tension. So I do check the resonances before I finish and glue on the bridge. This lets me know that I am in the range. At that point the only remediation I would do is thin the edges of the top if I think the resonances are too high.

OiAcoustics
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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Wed Nov 17, 2021 7:46 am

johnparchem wrote:
Wed Nov 17, 2021 2:45 am
... I do check the resonances before I finish and glue on the bridge. This lets me know that I am in the range. At that point the only remediation I would do is thin the edges of the top if I think the resonances are too high.
Hi John

Thanks for your thoughts. Do you find the initial untensioned top f to be lower than the final after tensioning - that it raises with tension and therefore you don't want to start out too high?

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kiwigeo
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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by kiwigeo » Wed Nov 17, 2021 9:05 am

Recreating conditions for a fully strung up instrument without gluing the bridge on is difficult...that's the beauty of the allowance for trimming resonances by adding mass to the sides.

Ive tried double sided taping the bridge on but as already noted on a Gore build the bridge has significant stiffness. Ive found with most of the builds the shift in main top frequency with bridge added is close to what Trevor predicts in his books.
Martin

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by johnparchem » Wed Nov 17, 2021 11:57 am

OiAcoustics wrote:
Wed Nov 17, 2021 7:46 am
johnparchem wrote:
Wed Nov 17, 2021 2:45 am
... I do check the resonances before I finish and glue on the bridge. This lets me know that I am in the range. At that point the only remediation I would do is thin the edges of the top if I think the resonances are too high.
Hi John

Thanks for your thoughts. Do you find the initial untensioned top f to be lower than the final after tensioning - that it raises with tension and therefore you don't want to start out too high?
I am not sure. if I have strings on I tension them and test. From no strings to strings tensioned my top resonance goes down about 4 hz.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Wed Nov 17, 2021 2:15 pm

kiwigeo wrote:
Wed Nov 17, 2021 9:05 am
Recreating conditions for a fully strung up instrument without gluing the bridge on is difficult...that's the beauty of the allowance for trimming ...as already noted on a Gore build the bridge has significant stiffness. Ive found with most of the builds the shift in main top frequency with bridge added is close to what Trevor predicts in his books.
Hi again

thanks for that comment, which stirred up my memory cells to look under Bridge mass, and there is was!

It appears that for the bridge assembly, mass controls over stiffness, and per Fig.4.6-3, to expect a drop in top f of .4Hz per gram of bridge/saddle (no pins for me), so for an average of 25 grams that's a 10hz drop after loading the bridge/saddle. Of course then it gets more interesting as under load, with presumably more "stiffness" as the top tenses, it will raise back up some I expect. So John's 4hz # looks interesting... Will track as I prepare this one to hear it...

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by Jim watts » Wed Nov 17, 2021 3:27 pm

I have a build going right now that I temporarily glued the bridge on using hide glue and masking tape. I put tape down on the top and glued the bridge to that. I'm not sure it really mimic's the stiffness due to the tape but the weights right and certainly has some stiffness and it was easy to do.
I find out out in the near future.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by seeaxe » Wed Nov 17, 2021 5:49 pm

"of course then it gets more interesting as under load, with presumably more "stiffness" as the top tenses, it will raise back up some I expect."

Nope.

Stiffness does not change with load. Stiffness is a constant. Deflection changes with load.
Richard

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Thu Nov 18, 2021 2:06 am

Jim watts wrote:
Wed Nov 17, 2021 3:27 pm
I have a build going right now that I temporarily glued the bridge on using hide glue and masking tape. I put tape down on the top and glued the bridge to that. I'm not sure it really mimic's the stiffness due to the tape but the weights right and certainly has some stiffness and it was easy to do.
I find out out in the near future.
Interested to hear your final take as to whether it get you into the right territory... if so I'll keep a "temporary bridge" around just for this purpose.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Thu Nov 18, 2021 2:26 am

seeaxe wrote:
Wed Nov 17, 2021 5:49 pm

Stiffness does not change with load. Stiffness is a constant. Deflection changes with load.
Are we sure on that? Yes, load first causes deflection, which changes the location of the neutral axis in a beam (towards the fibers under compression), which I think we can presume happens with the soundboard assembly. Various complex things happen, but in general when a beam goes from unloaded or unstressed to under load, it gets stiffer (before the point where failure begins). In deflection testing a back, for instance, it will deflect quickly from neutral but not as much per gram once it's loaded.

Or for an easy practical example, how about playing a saw? As you bend it further, it stiffens and the pitch raises.

Or maybe I'm getting different physical terms intermixed...

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by Jim watts » Thu Nov 18, 2021 2:54 am

Oi has it right. The modulus of elasticity of the material is fixed, but the section modulus changes with shape, your saw example is great.
Total stiffness is the material modulus x section modulus.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by johnparchem » Thu Nov 18, 2021 4:22 am

OiAcoustics wrote:
Thu Nov 18, 2021 2:26 am
seeaxe wrote:
Wed Nov 17, 2021 5:49 pm

Stiffness does not change with load. Stiffness is a constant. Deflection changes with load.
Are we sure on that? Yes, load first causes deflection, which changes the location of the neutral axis in a beam (towards the fibers under compression), which I think we can presume happens with the soundboard assembly. Various complex things happen, but in general when a beam goes from unloaded or unstressed to under load, it gets stiffer (before the point where failure begins). In deflection testing a back, for instance, it will deflect quickly from neutral but not as much per gram once it's loaded.

Or for an easy practical example, how about playing a saw? As you bend it further, it stiffens and the pitch raises.

Or maybe I'm getting different physical terms intermixed...
I also agree that stiffness changes, that is one of the reasons Trevor suggests a 3 meter dome for an active back. Able to get extra stiffness while keeping the back thin enough to have low mass.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by seeaxe » Thu Nov 18, 2021 6:14 am

I disagree.

Stiffness = EI/L all of which are constants for a given guitar.

If you deform a flat plate into a curved shape that certainly increases its stiffness but in the range under which it is loaded by guitar strings, all response is in the elastic range of the material.

If you look at the response of any structural system it will respond elastically until its plastic deformation limits are exceeded.
Richard

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Thu Nov 18, 2021 11:50 am

I also agree that stiffness changes, that is one of the reasons Trevor suggests a 3 meter dome for an active back. Able to get extra stiffness while keeping the back thin enough to have low mass.
[/quote]

I just glued up the back of my myrtle 0 rebuild last night, thinned down and radius decreased (tighter) to 3m (10') from 30' (for the top, but was too lazy to CNC a different form for the back until now). Poingggggg! What a difference even in its unglued to sides state, with a substantially higher pitch, resonant fullness and sustain. The box is complete (with the bridge on from before and neck completely reusable to just bolt on and restring) - tomorrow will tell the tale of how the FRC compares to prior to the rebuild, both in loaded and untensioned states...and how the 3 main resonances match up...

Thanks for all your thoughts - I really like these discussions instead of just turning these considerations around in my head...

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by OiAcoustics » Thu Nov 18, 2021 12:05 pm

seeaxe wrote:
Thu Nov 18, 2021 6:14 am

If you look at the response of any structural system it will respond elastically until its plastic deformation limits are exceeded.
My background was in residential architecture, engineering and construction for way too long... so I'm influenced by that. An underbuilt floor system of plywood and joists (not tall enough for the span) has an initial very elastic response and you can stomp around on it like a drum - boom boom boom. However, load it up, let's say with drywall, so that it's under more compression (top fibers)/tension (bottom fibers), and that disappears - the flex is almost gone... it has reached a fairly static state well before failure. Conversely, laminated or I shaped "Silent Floor" joists are the opposite - super rigid, but they can attenuate high frequencies (depending on shape) more than standard, more flexible, wood joists, which transmit low frequencies more readily.

So I see the falcate top plate system similarly, which is why I'm drawn to it and willing to rebuild a slew of X braced guitars - we're trying to achieve the exact right amount of "underbuilt" to achieve a high response ... before failure (the dreaded wave).

Or another example might be to take a banjo or drum skin - tune them by stretching them to a higher pitch.

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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by seeaxe » Fri Nov 19, 2021 11:03 am

I still disagree but I'm not going to rain on your parade mate.
At the end of the day if they sound good, you have succeeded.

All the best!!
Richard


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Re: Air, Top and Back Resonances, Coupled and Uncoupled - Untensioned?

Post by Jim watts » Thu Nov 25, 2021 9:48 am

That is interesting, thanks for pointing us to it.

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