Official Luthiers Forum!

I'm at the point of gluing in struts or side braces on the rims. I'm building a dreadnought and am gluing in five spruce struts per side. They are quarter sawn with grain running perpendicular to the rim and straight up and down. Rims are East Indian Rosewood.
After gluing on one side yesterday using titebond, this morning I got to tapping on the side and it just doesn't ring like the side without them does, sounds dampened
Are there specific deminsions that the struts need to be? These are 10 mm wide and 6 mm high. Does the bracing affect the tone of the guitar by taking up air space inside the box?

Thanks for your input!

_________________
"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

The answer to your question the way you phrased it is....So little you could not perceive it

Now the mass could make may have an effect but I don't think you affected the air chambers area enough to come close to noticing.

You can shave them to a max height of 3mm, and win a few grams and cc's. Really negligible, but this type of tiny differences, when gathered from all the various parts of the guitar make the difference between light and heavy.

That being said, the first rims I made sounded quite nice after I bent them. Rung like a bell. In the completed guitar they sound like cardboard and i didn't put a single brace. I imagine that bracing the sides with the top and back has more effect than the transverses.

_________________
Build log

I use side tapes and not wooden struts/braces and here is my thinking as to why:

1) I don't believe that the sides have much impact on the tone of the guitar beyond the very important part that they play in providing a rigid frame for the top and back. I know others will disagree and I don't care......

2) The primary purpose of any side support is to stop or prevent a side crack. If what ever you use, tapes, struts, pieces of an old Kerry/Edwards bumper sticker do not extend under the "kerfed linings" you will have the possibility of the crack taking the path of least resistance and passing between the struts, tapes, political memorabilia and the Kerfed linings..... There is also the issue with struts that end at the linings and are not inlet into the linings of stress risers.

3) The double sides guys will also tell you that they see the sides as being important in terms of providing an uber-stiff rim for the back and top. I agree with this and hope to join the schizophrenic sides club one day too.

So no - I see no tonal benefits to side struts (wooden or tapes) and only see them as structural supports/stiffeners, and future insurance against run away side cracks.

"Are there specific deminsions that the struts need to be? These are 10 mm wide and 6 mm high. Does the bracing affect the tone of the guitar by taking up air space inside the box?"

No and no. It's a better practice to inlet them into the liners to avoid a stress riser along the top of the liners. Tapering down as you did will avoid creating the stress riser, but by inletting them you can help with the existing riser created by the liners.

Technically, these are not struts.

"I see no tonal benefits to side struts (wooden or tapes) and only see them as structural supports/stiffeners, and future insurance against run away side cracks."

Stiffening the sides affects the tone. IMO this is usually beneficial.

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Let's see if we can break this down.

A top that is not mounted to the sides can flex along the edges in ways that a mounted top can't. As far as I can tell, you could consider the edges of a mounted top to be 'perfectly rigid' within pretty good limits: the top edges certainly don't show any motion perpendicular to the plane of the top and any frequency I can look at with 'glitter patterns'. Adding stiffness or mass to the sides won't change that to speak of.

Since momentum has to be conserved, if most of the top is moving 'up', something else has to be moving 'down' to cancel that out. The strings take care of a lot of that, but I guess the rest of the guitar can get into the act as well. Making the B&S heavier might give you more top motion. I guess somebody's going to have to insttrument one and find out.

The top is not 'hinged' at the edge. If the side can flex a bit it's possible that the whole top-liner-upper edge system could rotate a little. Given that the sides are curved that also seems like a dubious proposition _except_ for the 'flat' spot below the waist.

Similarly, that's the only part of the side that can vibrate much to speak of at 'normal' frequencies. In fact, you can get a little 'ring mode' on that part of the sides on most guitars, at a pretty high frequency, and it's likely to be putting out sound. On the 'sorta-Baroque' guitar that I made a few years aog, the total area of the sides was probably greater than that of the top, and I suspect they made a major contribution to the tone. It's also possible that this 'side mode' can couple with one or more of the air resonant modes in the box, and that could effect the tone.

From what I can tell from measurements, almost all of the energy that vibrates the back comes from pressure chages in the air in the box, not through direct transmission from the sides. Because the speed of sound for compression waves in the sides is high, 'side' sound and 'air' sound to the back would tend to be out of phase, and cancel. Fred Dickens, among others, used to put weights in the neck- and tail blocks to try to imobilize the sides, but he found it didn't make any difference. Another piece of evidence.

In sum, I don't think side fillets will make a huge difference in the sound _except_ for whatever effect they might have in stiffening the 'flat' part below the waist. They are effective crack stoppers, but you _must_ inlet them into the liners to avoid the stress riser problem that has been mentioned. Side tapes are nearly as effective in stopping cracks, and don't cause stress risers.

Hmmm.....thats very interesting. I was inclined to believe the opposite....here is my reasoning:

More often than not, when I strum a string I can feel vibration along the edge of the back. Depending on the note being strummed, the vibration may be located at one place or another. You can almost always feel more vibration on the edge of the back than you can in the middle. If it is the air that is exciting the back, wouldn't you expect to sense the vibration in the middle? Alternatively, if you can detect more vibration along the edges, it seems that this would indicate that the sides are primary in driving the back...and not the air.

_________________
http://www.PeakeGuitars.com

Good observation and I concur with what you are feeling vibration wise. But I don't think that the vibrations that you are feeling along the edges of the sides and back are intense enough to produce audible sound.

Wow, I just realized that the top nor the back is in direct contact with the sides. They are glued to the lining and the lining is glued to the sides. Whereas a violin's top and back are glued directly to the rims. Not that this makes a difference in the thread discussion, but a light bulb just come on in my head.

_________________
"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

I think it depends on the guitar. I think that most acoustic guitars do not have very responsive backs. Benedetto mentions in his book that it is harder to tune the back than it is the top (he was referring to archtops of course, but I think the same principle applies).

_________________
http://www.PeakeGuitars.com

I've come to believe that stiffer sides give a guitar more power. Can't prove it but I've perceived it.

I've also come to believe that what happens at the edges (linings and binding) has very audible consequences. I am in the middle of an an experiment with my test body where I strung it up with no binding and played it. I then routed for and glued in binding that was only the thickness of the top (didn't extend into the sides) and played it. In my opinion (and the opinion of another builder who listened to it as well), the sound changed noticeably. It had more clarity and focus and sounded, for lack of an objective word, tighter.

Next up when I get the time is to rout for and install full height binding. I've done this part of the experiment before and the effect was similar to adding the short binding, but more so. The goal of this was to see whether capping the grain of the top was responsible for the change in sound or whether it had to do with better coupling between the top, lining, and sides.

As others have said, if you feel around on a lively guitar, there is a lot going on at the edges. I am of the belief that the details there have more of an effect on sound than most folks believe.

_________________
http://www.chassonguitars.com

Are you assuming that vibration which is not revealed in glitter patterns does not make an audible contribution? That's assuming a lot.

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

thats cool that you can perceive a difference with binding as well..

Maybe binding a guitar gives you a more flawless glue joint...and this is why it would act differently?

_________________
http://www.PeakeGuitars.com

The chladni patterns are only going to indicate the location of soundboard nodes. It would be hard to interpret what role the sides have in the entire system by only looking at the modes of the soundboard. also, even though the amplitude of the vibrations at the edge may be small (and this is why the chladni patterns don't tell us much here), the energy involved in moving the sides is probably not insignificant as that is a fairly rigid portion of the guitar. As Al mentioned, there must be "an equal and opposite" reaction occurring along the periphery of the guitar...so really...a good portion of the string energy is being funnelled into the sides..what happens to it afterwards would depend on how the rest of the box is built.

_________________
http://www.PeakeGuitars.com

Howard Klepper asked:
"Are you assuming that vibration which is not revealed in glitter patterns does not make an audible contribution? That's assuming a lot."

I'm not assuming that; far from it, actually. But, as usual, it's pretty complicated.

There are two problems with using Chladni patterns on assembled guitars. One is that it takes as lot of power to get them to form, particularly at high frequencies, and eventually your signal generator poops out or you smoke the speaker. The other, and more important, is that as you go up in pitch you get less and less useful information out of them. That's because there are so many resonant modes as you go up that they overlap, and what you see, whether it's with Chladni patterns or holography, is less and less solely about the thing you're looking at and more and more about everything else too. You might be looking at the top, but the influence of the air and the back are so strong (and the neck, and the sides) that you can't say that what you're seeing is any longer a 'top' resonance.

Now, most of the actual _power_ output of the guitar seems to be in the low range, and to come from the 'bass reflex couple'. Most of the 'tone' and 'color', and maybe the 'projection', are up in that 'resonance continuum' where things run together and you can't seperate them out. Just to show you how this can work: if you start out with a 'pure' tone, you have to almost double the power if you want it to sound just audibly louder. However, if you start with the same pure tone, and add in a higher harmonic, that harmonic only needs to have 1% of the power of the original tone to make an audible difference in the sound. The higher you go in pitch, the less power you may have to add: your ears are very much more sensitive to sounds in the 2000-4000 Hz range than they are to the fundamentals of the lowest notes of the guitar. So small changes in the high frequency range, where you can't tell what's going on, can make a big difference in tone. This is an old paradox: how is it that some people can make better guitars consistently, when they have no direct control over what makes them better?

I measured a lot of this stuff when I was doing the series of experiments on the 'corker' guitar, trying to figure out what effect a 'port' would have in different places. I compared the spectra below 1000 Hz, simply because that's the only part of the sound where I feel I have a prayer of figuring things out. What the guitar does above that often seems to be _related_ to what it does in the low end, if you look at the right things, but it's more a statistical sort of thing: so many peaks per octave, or a peak to dip ratio of so many dB. That may be the answer to the paradox, but (as always) 'more research is needed'.

When I looked at the data, what I saw was that the activity of the back overall seemed to correspond most closely with the sound pressure levels of the air in the box. Generally speaking, opening a port dropped the SPLs of most of the air modes, and the activity of the back was also reduced. Of course, a lot depended on where the port was, what inside air modes were changed, and in what ways (pitch, SPL, node locations) and how that all lined up with what the back _wanted_ to do.

As for where you feel the back moving: that's also a function of it's modes. Usually you only get the center of the back really rolling at a relatively low pitch: anywhere from about the 'main top' pitch (near the open G string) up to a fifith or so higher, depending on how the back is made. What exactly happens upward from there depends on too many things to enumerate, but in general the vibration 'breaks up' into smaller and smaller areas as you go up in pitch, usually in between the braces. Some of these little areas will be around the edges, but the edges themselves, the bindings, don't move a lot.

You also have to remember that, however the energy gets to the back, it has to come from the top. That's the only part that is being driven effectively by the strings. (Yes, the neck moves, but not nearly as much) The backs of most guitars are heavier than the tops, and stiffer, and they are not facing the audience. Overall, the back is just going to be a poorer sound producer than the top.

In fact, what I saw in my measurements of the sound output in front of the guitar was that there tended to be dips in the spectrum at pitches where there were strong back resonances. These modes are 'losers'! The one excption to that was the 'main back' resonant mode, the lowest pitched mode where the whole back was moving. The higher modes are made up of small areas that are out of phase with each other, so most of the air they move inside the box is just 'sloshing around' rather than moving in and out of the sound hole. The 'main back' mode can pump air through the soundhole, and add to the output if you can get it cooperating with the top. Otherwise, it seems to me, the main contribution of the back to tone is that those 'dips' in the spectrum contribute to 'tone color'.

I suspect that much the same goes for the sides most of the time: if they're vibrating they're probably 'wasting' power. That may be why stiffer sides seem 'louder' to some folks.

On the effect of bindings: remember that I said that the top is not 'hinged' around the edges. It's not 'stiff' either: it's a sort of 'stiff hinge' because the liners can flex. When you rout the binding rabbets you make the hinge less stiff, and the pitch of the 'main top' resonant mode drops because of that, or, at least, that's what I've seen. When I put the bindings on, the 'main top' pitch goes back up a bit, but it's not as stiff as it was to begin with. Again, all of this is going to depend on how you make your liners, how tall the binding is, how wide your purfling, and so on.

This is all pretty complicated; it gives me a headache sometimes to try to sort it all out. I'm still trying to paint in the big areas with a broad brush. I think most of us would agree that the sides are not the most important sound producing parts of most guitars, and that the details of what we do there are not likely to make or break a given instrument. Obviously, the details matter more and more as you get better and better. As Dante put it: "The closer a thing is to perfection the more it feels of pleasure and of pain'.