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  #31  
Old 03-21-2017, 05:38 PM
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Bruce Sexauer Bruce Sexauer is offline
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You can get a sense of my actual thinking here:
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  #32  
Old 03-21-2017, 06:06 PM
redir redir is offline
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Originally Posted by Bruce Sexauer View Post
You can get a sense of my actual thinking here:
When you say node you mean the big bulbous part of the brace right? I think I do see what you are saying, the node in the above pic is offset to the left of the center seam?
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Old 03-21-2017, 06:33 PM
printer2 printer2 is offline
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Too annoying to flip the pages back and forth to look at both images. Hope you don't mind.



After looking back and forth I am wondering if both of you are causing the lower half of the bout to behave a little asymmetrical along with limiting how much the string tension is pulling up the bridge. I am guessing that when the bridge is rocking back and forth the sound waves that would cancel out if the bottom were symmetrical allows one to be the stronger with more surface area radiating. If you look at the two sides getting the same amount of energy driving them the one side with the higher mass would have a lower amplitude then the side with less mass. Mind you, the different stiffness of both the sides would also change how much amplitude each side has. I'll have to think about this some more. But while I am gluing my tail block, I have some work I wanted to get done tonight.
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Last edited by printer2; 03-21-2017 at 06:45 PM.
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  #34  
Old 03-21-2017, 08:44 PM
dekutree64 dekutree64 is offline
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Originally Posted by Bruce Sexauer View Post
I view the top as a membrane. It is all about integrity of the membrane. The membrane has greater integrity fore and aft than it does side to side. The bridge footprint already has 99+% membrane integrity, and this integrity projects further fore and aft than in other directions. The area of the top with the least membrane integrity is halfway between the high integrity bridge and the high integrity rim in the cross grain direction. Assuming mission critical structure is handled, the big issue getting tone and balance in a guitar is the even distribution of structural integrity within the membrane. IMO, the vast majority of guitars I have ever seen are vastly overbuilt where mission critical structure is concerned.
Thank you! I will ponder more on this long grain to cross grain difference, and whether scalloping actually distributes the stress more evenly than tapering as a result.

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Originally Posted by printer2 View Post
After looking back and forth I am wondering if both of you are causing the lower half of the bout to behave a little asymmetrical along with limiting how much the string tension is pulling up the bridge. I am guessing that when the bridge is rocking back and forth the sound waves that would cancel out if the bottom were symmetrical allows one to be the stronger with more surface area radiating. If you look at the two sides getting the same amount of energy driving them the one side with the higher mass would have a lower amplitude then the side with less mass. Mind you, the different stiffness of both the sides would also change how much amplitude each side has. I'll have to think about this some more. But while I am gluing my tail block, I have some work I wanted to get done tonight.
According to Gore/Gilet, it takes quite a lot of asymmetry in the bracing to make a difference in the mode shapes. Most likely both of those guitars will have a largely cancelled cross dipole. Violins are the extreme case, where the soundpost completely stops the treble foot of the bridge moving up and down, so what would be the cross dipole actually functions as a monopole. In that case, the downward pressure of the bow suppresses the true monopole while inducing high amplitude side-to-side motion, so it works out nicely. But for guitars, it seems to be worth sacrificing the cross dipole to get more out of the monopole.

Some classical bracing designs do have enough asymmetry that you probably get some usable cross dipole radiation (the ones with a diagonal "harmonic bar" reducing the size of the treble side's active area). It would be interesting to try something similar on a steel string, probably in the form of a very stiff diagonal brace toward the tail (notched into the linings with plenty of height, creating a "false perimeter"). Best to try on something like a dreadnought so the remaining active area is still plenty large. Other braces and soundboard thickness could then be reduced to optimize for the new smaller effective area. Armrest bevels already do this to some extent, cutting into the active area. But I'm thinking a bit more than that.

And it would also be possible to create asymmetry by making one side light and stiff while the other is loose and heavy, while the actual shape is symmetrical. But total mass and total stiffness would both be higher than you can get with symmetrical bracing, so it probably wouldn't be worth it.
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  #35  
Old 03-22-2017, 02:10 PM
Alan Carruth Alan Carruth is offline
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Bruce's bracing is probably more symmetric left and right than the usual tone bar setup. I tend to think that more symmetry is better, which is why I've taken to using double-X bracing. To each his own.

In my case, I made an 'almost matched' pair of guitars, one with the usual tone bar setup, and another with double-X braces. Essentially, the double X takes the lower tone bar, moves it up a little, and rotates it 90 degrees, so you end up with the same amount of brace material, more or less, but just symmetrically disposed. I matched everything on those two as closely as I could, and took them to an ASIA Symposium to have other luthiers try them out, without telling them what the difference was until they'd tried them. Overall they slightly preferred the sound of the double-X braced top by a wide margin: that is, twice as many thought it was a little bit better. Some commented that the double-X had a 'modern' sound, while the other sounded more 'traditional'.

I used to use the diagonal waist bar in Classicals, with the intention of improving the output from the cross dipole. The problem is, as printer2 says, that there's no guarantee that the larger vibrating area will still have the same amplitude as the smaller, and momentum considerations would suggest that it's unlikely to. In the end it seemed to me that asymmetric bracing would tend to alter the pattern of radiation at high frequencies without necessarily enhancing the output. Also, I became convinced that asymmetric patterns are inherently less efficient vibrators, unless something is done to make them more symmetric. I can't cite much in the way of hard evidence for this, but since I've gone to more symmetric bracing I've gotten results I like better.

Duketree64 wrote:
"Thank you! I will ponder more on this long grain to cross grain difference, and whether scalloping actually distributes the stress more evenly than tapering as a result."

It's not that scalloped bracing distributes the stress any better; the difference is in how the top works at low frequencies. A few years ago Evan Davis gave a paper at an ASA meeting that dealt with this. He modeled guitar tops with various bracing profiles, and looked at the way they moved air. I'll note that Evan's PhD thesis was on guitar acoustics, and he made a career with Boeing working out ways to control noise, and he makes nice instruments, so he knows a few things. What he found was that, assuming the tops were all structurally sound, a scalloped top tended to move more air in the 'bass reflex' range of the guitar than tops with either straight or tapered bracing. Also, thinning the top around the edge was less helpful in increasing sound output than thinning in the middle, for the same reason. According to Richardson, who has done a lot of work on guitar acoustics, the low frequency 'main top' resonant mode may actually produce more sound than any other, all the way up to 1000 Hz. This is due to the fact that it's simply more effective at producing a net volume change in the box, and pumping more air, than any other mode. All the mulitpole modes cancel out in one way or another, unless they happen to couple strongly with something like an 'air' resonance in the box.

The result of scalloping braces is the famous 'Martin thump'" a strong attack and quick decay, particularly on the lower notes. This is a great tone for certain kinds of music, but its not everybody's cup 'o tea. That's why guitars with other styles of brace profile are also made and sell well enough. 'Better' is a value judgment, and pretty much subjective. We can figure out what happens objectively when you make something a certain way, and report on it, but whether it's any good has to be determined by listeners and players.

Last edited by Alan Carruth; 03-22-2017 at 02:42 PM.
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  #36  
Old 03-22-2017, 07:05 PM
printer2 printer2 is offline
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Quote:
Originally Posted by Alan Carruth View Post
The result of scalloping braces is the famous 'Martin thump'" a strong attack and quick decay, particularly on the lower notes. This is a great tone for certain kinds of music, but its not everybody's cup 'o tea. That's why guitars with other styles of brace profile are also made and sell well enough. 'Better' is a value judgment, and pretty much subjective. We can figure out what happens objectively when you make something a certain way, and report on it, but whether it's any good has to be determined by listeners and players.
This is the first time I read mention of scalloping braces and the attack and decay of the bass notes. It makes sense, the more you use the energy the quicker you use it up, just never thought of it in terms of the thump. I love when the light comes on. That is why I like these discussions.
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