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I've been wondering how bridge height at the wings practically affects the stiffness to weight ratio of the guitar top as a whole. I know as you decrease the height of a brace you decrease the stiffness but bridges are generally made of ebony or rosewood and I think are very stiff. Is it possible that if we reduce the wings on a normal belly style bridge from say 10/64" to 7/64" we'd have a lighter top without sacrificing any practical stiffness? I'm just thinking the bridge could be so stiff that once you reach a certain point changes in height would impact the stiffness of the top as a whole in a negligible way.


I guess what I'm asking is, at what point are you saving weight without impacting the stiffness in a meaningful way versus making the wings of the bridge too thin? Or to put it another way... at what wing thickness are you just adding dead weight?

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Jeremy Douglas

I think like you Jeremy. My wings go down to around 4-5/64ths. With my brace design, which is close to Martin's advanced X, my wings are right over and around the x brace legs so I believe it not to be a big isssue taking them down thinner. And as you stated, most bridge materials we use are very stiff, many times moreso than the spruce under them. My guitars have a consistent "birth" tone when strung up. I believe the bridge, in location, size and style, makes a huge impact.

Goodluck.
Steve

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"We might not be big but we sure are slow"

I think taking the wings down that thin will cause a difference in stiffness and tone. That may be good for some designs and not for others, but I do think the change in stiffness would be meaningful.

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David White, Toronto

"All my favourite singers can't sing."

here is my belly bridge. Feel free to use, or improve! Forgive my rudimentary CAD skills.

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John Mayes
http://www.mayesluthier.com

Thanks guys. I'm a little surprised I haven't heard more about this topic. It seems like it would be kind of important but maybe the difference(weight or stiffness) in a few 64ths isn't that big. I guess someone could test it by gluing the bridge with the wings thicker then seeing if the resonant frequency of the top rises or falls as they shave the wings down.

The reason I posted this is because on my last(first) build I used a bridge I bought from cncguitarparts.com and I guess I didn't think much of it because it was a premade bridge but sometime after I strung it up I noticed the wings were thicker than on my other guitars. It was about 10/64ths, my Larrivee OM has a belly bridge with wings at about 8/64", and my Santa Cruz OM belly bridge is about 6/64". The resonant frequency of my top ended up a little low and I think too loose for its weight, so I wanted to improve that and wondered if taking the wings down to 6 or 8 64ths would raise the resonant frequency or lower it.

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Jeremy Douglas

If by resonant frequency you mean the 'main top' mode, cutting down the bridge wings will probably not change that by much, unless you really take them down a lot. I suspect that for that mode the stiffness and mass effects of the bridge wings just about balance out, so that the pitch goes neither up nor down by much. Thinning the wings might drop the 'cross dipole' mode, since they are in a the main bending area for that mode. On classical gutiars thinning the bridge wings is also a way to tune the 'cross tripole' mode, which is a major sound producer in the 450-500 Hz range in many cases. Thinning the wings usually drops that pitch.

There are LOTS of variables in this, of course. Ultimately the only way you'll know for sure is to try it.

Thanks Alan, I always appreciate your insight.

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Jeremy Douglas

For Alan;
What effect does the weight of the bridge ultimately have?
I've seen so many different styles over the years that it gets confusing to me .
I have a Southwell in the shop with a fan braced top,raised f.b. with adjustable neck angle.
The bridge is tiny!!!!
same length as a regular classical bridge(17.5mm-18mm)
but the width is half the normal width.
14mm wide.
With tapered wings.
The guitar is very bright,great trebles,strong but weak basses.

Thanks
Mike

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Mike Collins

in basic terms adding weight to the bridge would enhance the bass

Thanks Jody!
I'm just having a hard time digesting all the info lately!
There's so much to learn!! !!

Mike

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Mike Collins

Mike, I know what you are saying, its complicated , in subtle ways .probably what realy happens when you add weight is damping, which under the right circumstances would seem to enhance the bass, and somewaht inhibit the higher end . Jody

Jody writes:
"in basic terms adding weight to the bridge would enhance the bass"

I'd say that it would tend to cut down on the response all around, but more in the treble. I would also differentiate this from 'adding damping', although that is certainly the common meaing of the term.

One key concept that really helps is that of 'impedance' It took me a while to get my head around this one, but basically it's the ratio of driving force/velocity at a given frequency. If it takes a lot of force to get a given amount of velocity (and thus amplitude) then the impedance is high at that frequency.

Suppose you have a bar of spruce, and another of Indian rosewood. It would not be unusual for them to have the same stiffness at a given thickness, but the rosewood would be twice as dense as the spruce, or more. If you hold them up and tap them the rosewood bar will have a much lower resonant pitch because the stiffness is the same but the mass is higher. It makes some sense that if you were to set up a rig to drive the two pieces at their respective resonant frequencies, you'd get less amplitude from the rosewood bar with a given input of power. The rosewood bar would have higher impedance overall, due to it's greater mass. Obviously, the resonant pitch of each bar is the frequency where the impedance is lowest: that's one definition of resonance.

But note that if you tapped on the two bars the rosewood one would probably ring longer. This is because it dissipates energy more slowly through internal 'losses', and that's the physical definition of low damping. It has lower damping than the spruce, even though it's harder to drive.

It's easy to see that adding mass to something tends to raise the impedance more at high frequencies. Adding stiffness raises the impedance too, but has more of an effect at low frequencies. Impedance is a measure of how much force you're using, and that will depend on what you're pushing against.

Losses add to impedance, too. In fact, at resonance, what's happening is that the mass term in the impedance equation (the so-called 'mass reactance') exactly cancels the 'stiffness' term, and all that's left is the 'resistive' term of the losses. As much energy is stored in inertia as the thing moves as is stored in deflection when it's off to the side, and all the energy you have to make up to keep it moving is what's 'lost' to heating the thing up.

Yeah, I know, it's confusing.....

That's a really good post, Al, thanks.

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David White, Toronto

"All my favourite singers can't sing."

"Yeah, I know, it's confusing....."

Understatement of the year!
But thanks Alan, I'm "trying" to "get" this stuff!

Joe

Joe Sustaire wrote:
" I'm "trying" to "get" this stuff!"

So am I, man, so am I!

Alan,

Great post

is there a good reference source for this subject?

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Eddie Lee

As for wing thickness, a 30% change in thickness at the wings would effect stiffness way more that it would effect mass. One way to look at it is that the connection between the bridge wings and the X-brace is the crux of the whole structure. It's where the 2 stiffest braces meet. From a structural standpoint, if the bridge is flexible there, the top will be able to pull up more over time. If you've ever removed a bridge from an old guitar, you know that they deform over time. Thinner wings would let them deform more and/or sooner. Some bellying is good in my book but at some point, it causes problems.

From a sound standpoint, the bridge drives the top. The X brace spreads the movement of the bridge to a larger area of the top. The way the bridge relates to the X is a big factor in how the top moves. So drastically changing the stiffness at the wings makes a difference in the way the top moves.

Bridges are one of the easier things to experiment with. It would be simple to build one with your standard bridge, let it settle in awhile, then shave down the wings and see how it changes the sound. You won't know about long term stability though.

You can also add and reduce mass pretty easily too. If you don't like the end result, you can always remove the bridge and put on a new one.

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http://www.chassonguitars.com

Thanks Jeremy for asking the question. This is really interesting for me. Thanks John.


Al that is a really neat explanation.
Kent I realise this is probably better in a separate thread but could you please elaborate some more on the difference in sound as the stiffness is reduced.? I am also very curious about the position the X braces exit the wings and how this changes the way the top moves. Thanks.

Craig.

Kent is right about the relationship between wing thickness, stiffness, and mass. As with any brace, the stiffness goes as the cube of the depth: so reducing the height of the wings by about 25% would make them half as stiff, but would not drop the overall mass of the bridge by very much.

In many ways the bridge is just the heaviest, and one of the stiffest, braces on the top. Often the bridge will weigh nearly as much as all of the other top bracing put together. It's not as stiff for the weight as spruce, but the central part of almost any bridge can be considered as 'rigid' at frequencies in the range of the fundamentals of any fretted note. You'd have to reduce the height of a normal belly bridge a lot to make it effectively much less stiff, at least in the middle. The wings are another matter, of course.

That being the case, reducing the wing height is more likely to increase the bass response than anything else you could do to the bridge, short of replacing it with a heavier one. You've reduced the mass a little, which makes it easier to drive at high frequencies, but you've reduced the stiffness more. This will tend to lower the pitches of some of the resonant modes of the top, and make it a bit more active, and it lowers the 'stiffness' part of the impedance equation, which means it will be easier for low frequencies to 'leak' out of the strings and into the top.

Reducing the center height is far more likely to increase the relative amount of treble response, by reducing the overall impedance of the bridge, and especially the mass part. The stiffness is less, of course, but it's so high to begin with that the effect of the loss is not too pronounced in most cases. There is most likely a 'threshold effect'; at some point as you reduce the bridge stiffness in the center it gets to be within range of the top and bracing stiffness, and suddenly the sound starts to change. This is one of the things that makes generalized pontification on line hazardous: there's always somebody who took the middle of the bridge down and got more bass.

There's also the 'little' matter of your ears working differently from microphones and computers. Things don't always sound like what you'd expect from looking at a spectral plot. Maybe that guy really did get more treble output, but it _sounded like_ more bass to him. So take everything that's said from a 'scientific' perspective with a grain (or a pound) of salt; the math and physics guys can tell you what's _supposed_ to happen, and measurement people like me can tell you what _did_ happen, but you're the only one who can really say what it sounded like to you.

My experience is that stiffer wings give me more output across the spectrum. But I doubt that's universal. I'm of the belief that a change in one person's system can produce a different outcome in someone else's system. And why take my word for it when it's so easy to try

As for how it effects top movement, I can't prove any of this but I work under the belief that the more the bridge is coupled to the X, the more potential you have to drive a larger portion of the top as the X spreads the movement out to a larger area (and you may have more potential to completely kill the top by making it over-stiff). The less coupled they are, the more potential for the bridge to rock. Either approach can work but you have to think in terms of a system rather than individual parts. But again, that's just theory based on my intuition, observations and experience, not on any measuring.

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http://www.chassonguitars.com

Fantastic info guys. So now I'm thinking, does anyone else besides John want to share their wing thickness??

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Jeremy Douglas

Thanks Al and Kent. Much appreciated. That little piece of wood is sort of like the rudder in a boat(vivid imagination at work here)

Regards

Craig

My wing thickness tapers down to .125 for light guage strings.(steels)

Mike

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Mike Collins

Thanks Mike. That seems to be a common thickness.

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Jeremy Douglas