Bracing stiffness and top mode

Construction and repair of Classical Guitar and related instruments
sphinx
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Bracing stiffness and top mode

Post by sphinx » Sat Nov 02, 2013 6:03 pm

Is there a direct connection between the stiffness of the top bracing and the pitch of the main top resonance? If such connection exists, what is the part of the top bracing that affects the resonance pitch the most (or the least)? In particular, does the stiffness of the lower harmonic bar and shoundhole bracing have a significant effect?

ChristianSchwengeler
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Re: Bracing stiffness and top mode

Post by ChristianSchwengeler » Sat Nov 02, 2013 6:35 pm

I think it is the overall stiffness of the top, in combination with the mode of the body and the back, which will affect the mode. The bracing is one factor of a complex system. The thickness and the doming of the top also add a lot of stiffness. The angle of the bracing bars has influence on the directional stiffness. The braces themselves may be more or less stiff if they are shaped to the vault or not, if they are higher in the center and decreasing to the ends or of all equal height on the whole length and depending on the particular stiffness of the wood used. I use western red cedar for some bars as it is lighter and less stiff. The lower harmony bar has in my point of view more structural functions and a lot of builders make it pretty heavy and continuous, without any notable effect on the mode. There must be many other opinions out there...

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Alexandru Marian
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Re: Bracing stiffness and top mode

Post by Alexandru Marian » Sat Nov 02, 2013 6:37 pm

Of course. The stiffer they are (both plate and bracing) the higher the pitch. Cross grain stiffness helps too (ie having better quartering for the plate) as it comes for free (no weight penalty). On the other hand, weight (most of it in the plate, then harmonic bars) lowers the pitch, so you are constantly juggling with these 2 factors. Height is important for getting stiffness, that is why we use braces - more efficient than a bare plate. When tuning you'll notice the most impact comes from lowering the height, and less so from making the profile slimmer or cutting longer bevels at the ends. Soundhole reinforcements (and braces near it) do have an effect on the pitch, in a way it's all part of the lower bout fan. Crossgrain braces (including harmonic bars) on the other hand have a less strong impact on the pitch, as they usually add about equal amount of stiffness and weight. You must keep them a reasonable height though, as they are important in preventing the top collapsing under the string tension. They also have a role in the air resonance, by using less height you can gain bass, but then again don't forget that string tension...

sphinx
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Re: Bracing stiffness and top mode

Post by sphinx » Mon Nov 04, 2013 11:40 am

Thanks Christian and Alexandru for sharing your experience, I learned a lot.

One more question. Apart from the height and material of the braces, does the length of the braces affect the resonance pitch? I feel that shorter braces (fans and bridge bars) might lower the pitch because they reduce the stiffness of the perimeter of the main vibratory membrane, but I'd like to know the opinion of experienced luthiers.

Alan Carruth
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Re: Bracing stiffness and top mode

Post by Alan Carruth » Mon Nov 04, 2013 7:58 pm

Stiffness in the middle makes a lot more difference than stiffness around the edges. Short braces that taper out at the ends can be very effective at raising the pitch of a plate.

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Steven D'Antonio
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Re: Bracing stiffness and top mode

Post by Steven D'Antonio » Mon Nov 04, 2013 8:04 pm

Also keep in mind the single largest cross grain brace on the guitar top... the bridge. There was a study done, unfortunately I don;t have a copy with me, where they found the most effective bridge for producing a good sound was one with just the center portion and no wings. It wasn't in this particular set of papers, but it was done by one of the members of this university group http://www.speech.kth.se/music/acviguit4/

Steven
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ChristianSchwengeler
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Re: Bracing stiffness and top mode

Post by ChristianSchwengeler » Mon Nov 04, 2013 9:03 pm

Steven D'Antonio wrote:Also keep in mind the single largest cross grain brace on the guitar top... the bridge. There was a study done, unfortunately I don;t have a copy with me, where they found the most effective bridge for producing a good sound was one with just the center portion and no wings. It wasn't in this particular set of papers, but it was done by one of the members of this university group http://www.speech.kth.se/music/acviguit4/

Steven
This study was made in Sweden and I can send a PDF copy of the study to the members who are interested in(just PM me with your email address included): Function, Construction and Quality of the Guitar by Erik V, Jansson. The question is if the sound is only scientifically better, or if it is also more beautiful with this kind of bridge. We are used to a certain timber and even if I for example personally recognize that a lattice braced guitar has it's very strong points generally I am not so amazed of the sound, at least in the players position

Adam S. Vernon

Re: Bracing stiffness and top mode

Post by Adam S. Vernon » Mon Nov 04, 2013 9:14 pm

I always assumed that the long wings had more to do with gluing surface than adding stiffness. Stiffness can gained through other (lighter) means.

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Steven D'Antonio
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Re: Bracing stiffness and top mode

Post by Steven D'Antonio » Tue Nov 05, 2013 2:28 pm

Hi Adam,

This study seems to indicate they add a lot to the stiffness, but, like you, I had assumed their main function was gluing surface. One reason I hesitate to abandon them. Though in the past, on a pyramid bridge I filed the point where the wings meet the center section down to almost nothing and the top modes opened up nicely. Makes you think though.

Steven
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The Luthiers Bench

If you cut it to short you can always nail another piece on the end. But if you cut it to long; then what the heck are you going to do?

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Steven D'Antonio
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Re: Bracing stiffness and top mode

Post by Steven D'Antonio » Tue Nov 05, 2013 2:38 pm

Found it

http://www.speech.kth.se/music/publicat ... 38-ocr.pdf

Look for one of the Jugen Meyer articles about half way through the collection, he also talks about fan spacing relative to the bridge wings.

Steven
Steven D'Antonio
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If you cut it to short you can always nail another piece on the end. But if you cut it to long; then what the heck are you going to do?

Alan Carruth
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Re: Bracing stiffness and top mode

Post by Alan Carruth » Tue Nov 05, 2013 6:00 pm

Reducing the height of the bridge wings at the tie block end is a pretty common practice. This lowers stiffness, and thus the frequency, for the 'main top' mode, and also for the 'cross tripole', both of which are strong radiating modes. This illustrates one of the rules of the game: you can't change just one thing...

bullpuppy
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Re: Bracing stiffness and top mode

Post by bullpuppy » Fri Apr 14, 2017 4:19 pm

Just out of curiosity has anyone experimented with a floating cross brace near the widest part of the lower portion of the guitar. I have several kohnos and one has a 7 in cross brace bellow the bridge and another about 3 inches below that the runs all the way just short of the edge that are solid. Probably to increase the lateral stiffness. What if you cross braced a floating brace just short of the edge and have it varied width where it contacted the center of the soundboard. I'm just curious about such opinions of such arrangement.

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Re: Bracing stiffness and top mode

Post by Alan Carruth » Fri Apr 14, 2017 5:28 pm

Without spending too much time thinking about that (for one thing, the image is not quite clear to me), I'll say that almost anything can be made to work. The questions to ask are:
1) how much work will it take to do it,
2) how good is it likely to be, and
3) will the resulting instrument be useful for the repertoire you want to play on it?

Almost twenty years ago I sat on an acoustics panel discussion with Greg Byers, among others. He put forth an argument that is well known in some circles, although I can't remember who originated the idea. Basically it says that you can reduce the set of design features to a point on a plane; a seven-fan strutted top represents a certain point, while a nine-fan strut top will be in a slightly different location, Bouchet braces are further over and lattice braces someplace else. Given this distribution you can represent the fitness of the design for it's purpose as a height above the plane. This yields a topographical map, with hills and valleys of different designs that work more or less well.

If you start out at random with some design you'll probably be somewhere on the slope of a hill; the guitar you make will probably not be the worst one ever down in a deep valley, but you're unlikely to be at the top of a hill either. Starting from there you can make small changes in the design or construction, which will represent slightly different points on the plane, and that will move you up hill (you hope). With luck and perseverance you can work your way to the top of the hill; the best guitar that can be made using that basic design and material.

The problem is that if you start out at some random point it's hard to know know whether the hill you're on is the tallest one. You might spend decades making improvements and reach the top, only to find that some other hill is a lot taller. It's possible that the bracing scheme you are thinking of would be better than anything else out there, but you can't know that for sure until you've put in a lot of time and effort.

In his case, Greg chose to stay on the hill we know; the more or less 'standard' Torres style guitar. As with climbing real mountains every foot you go higher gets a bit harder to climb, but at least you know where the top is. I've been a bit more adventurous, with some experiments working out better than others. Keep in mind that 'fitness' includes a lot of things, such as basic timbre, power, ease of playing, breadth of tone, and so on. It's not too hard to think of things that improve one or another aspect of the guitar: the current effort seems to be all about getting more power, and people have had some success at that. Whether the resulting instruments are better fit for the job of playing the music we love is debated. Musical taste changes, of course, as do the needs of players, and composers work with the sounds that are available, so that new instruments with different timbres can open up new possibilities for the music. It's a moving target.

In the end it's very hard to come up with something that will be generally accepted as 'better'. Literally thousands of man-years of effort have gone into refining the 'standard' designs. As David Pye said, trying to come up with a big improvement starting from scratch is in effect saying that all of the designers that came before were idiots. Not likely.

Dan Seufert
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Re: Bracing stiffness and top mode

Post by Dan Seufert » Fri Apr 14, 2017 5:51 pm

Almost twenty years ago I sat on an acoustics panel discussion with Greg Byers, among others. He put forth an argument that is well known in some circles, although I can't remember who originated the idea. Basically it says that you can reduce the set of design features to a point on a plane; a seven-fan strutted top represents a certain point, while a nine-fan strut top will be in a slightly different location, Bouchet braces are further over and lattice braces someplace else. Given this distribution you can represent the fitness of the design for it's purpose as a height above the plane. This yields a topographical map, with hills and valleys of different designs that work more or less well.
Alan,

This sounds like Sewall Wright's "Shifting Balance" evolutionary theory from the 1930's. I don't know whether he played guitar!

bullpuppy
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Re: Bracing stiffness and top mode

Post by bullpuppy » Fri Apr 14, 2017 7:24 pm

Thanks so much for the detailed response. Yes it's not very wise to recreate the wheel. I had a weird theory about a floating brace acting like a spring i.e absorbing energy and springing it back up. It may apply somewhat to a guitar with a tail piece and not so much to classical. I may just try comparing a solid to floating brace of equal weight and measure the lateral stiffness and then put it on my test bed and to hear the sound results -- however inclusive the results may be.

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