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surface area of bridge contact with guitar top

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surface area of bridge contact with guitar top

Postby Brian Evans » Sat Mar 04, 2017 2:07 pm

Apologies in advance, I am getting all theoretical again. I think I think too much... :roll:

In an archtop guitar (or any guitar for that matter - I think. Maybe.) you have a decision to make about the footprint of the bridge as it sits on the arched top of the guitar. Does that contact area make a difference - large, small or none - in the efficacy of energy transfer to the top? I think that inside of a window of possibilities, it does not make much difference at all, and smaller may be better.

1. Consider a traditional tuning fork with a ball on the ends of the handle. You tap the fork and touch the ball to the surface of the bridge and it creates a loud tone. Given a very stiff brittle wood in the bridge - ebony - and a perfectly round steel ball of exceptional hardness, the contact area is infinitesimally small yet the energy transfer is exceptional.

2. A large contact area requires a large bridge, therefore greater mass. To transfer energy the string must excite the mass of the bridge, so a large heavy wooden bridge will dampen the transfer of energy, or simply absorb some energy and not pass it to the top. Every time I've made a bridge heavy (not adding weights, which is a somewhat different thing, but made the actual bridge construction heavy) I lost volume, and making the bridge light increased volume. (I don't really mean "volume" but rather the whole concept of the sound of the guitar, maybe with an emphasis on increase harmonic richness, or shimmer. I don't know a word for that)

3.Transfer of energy between the top and the bridge. Greater surface area reduces the per-unit loading of the interface, smaller surface area increases the per-unit loading. I believe that if you stay within the elastic limits of the material, so that both the pre-load on the bridge (the downforce from the string break angle) and the added load of transferring the string energy don't deform either material, the same amount of energy can be transferred regardless of surface area. But...

4. Attenuation from the surface area of the contact patch due to material properties. It's possible that a quite large contact area with quite low per-unit loading will suffer from a greater ratio of energy loss than a small contact area with a high per-unit load with actual lacquered wood, which is a fairly soft and dampening material. In other words, the smaller area could be more efficient after transfer loss is taken into account. I'm thinking big soft rubber ball and small steel ball bearing, both dropped onto a steel plate. The ball bearing rebounds almost perfectly, while the soft rubber ball absorbs a lot of the energy into itself.

5. I know violins are different, but they have quite tiny feet on their bridges, the actual transfer path from the upper section of the bridge is very narrow, and they have quite thin extensions to spread the mechanical pre-load out a bit over the top so they don't dent it. The transfer area for the string energy is quite tiny.

To take this to bridge design, I think the surface area has more to do with not permanently denting the top after years of 35 pounds of downforce, but the actual sonic performance of the bridge has little to do with that, and may even be diminished. Tediously shaping the feet of the bridge to perfectly mate with the top is artistically valid, but doesn't affect the sound. In an adjustable bridge with the thumb-wheels, the surface area of the transfer point between the bridge base and the topper is (I had to work it out twice, I didn't believe it) 25 thousandths of a square inch (the area of the ends of the two threaded rods that get stuck into the bridge base). I believe that as long as the contact area between the bridge and the top is sufficient, more isn't better. What is "sufficient" I don't have a clue. My bridges tend to have a contact area of around 1.5 square inches between two feet, the feet landing over braces inside the guitar. And I do tediously shape the feet to mate perfectly with the top - a sign of good crafting, in my opinion...

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Re: surface area of bridge contact with guitar top

Postby Brian Evans » Mon Mar 06, 2017 10:24 am

What little information I've managed to find, and it's been extraordinarily difficult, seems to say that a large contact area is going to be less efficient than a small contact area at transferring the wave energy from the string vibration. Within the window of physical nature of the material, stiffness, plasticity, etc, a smaller cross-section transfers more energy and reflects less back into itself when interfacing with a larger cross-section. Hence the tuning fork analogy, and probably why strings (which really are quite small in the surface area contacting the bridge) can transfer energy effectively to the larger cross-section of the bridge, and why those tiny threaded posts of an adjustable bridge work at all, they really are quite good at transferring energy despite their tiny surface area. So a small, light bridge with delicate feet that match the surface with an area designed to minimally damage the top through compression would seem to be the compromise to reach for.
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Re: surface area of bridge contact with guitar top

Postby Beate Ritzert » Mon Mar 06, 2017 10:47 am

Brian, there is an older discussion on this subject here in this forum. Did You see it? Intuitively, i also prefer bridges with a small contact surface, but i was not overly happy with my attempts to use a narrow full contact bridge. So maybe even the cross section might have some impact.

BTW: Maccaferi-Bridges, which are occasionally also used on Archtops (by Stefan Hahl for example) are full contact but hollow, so the actual contact surface seems to be more on the smaller side.

My impression is that there are many other parameters which have an impact, simple ones like weight, less feasible ones dependent on the local distribution of stiffness in the bridge, and also the damping properties of the materials in use.

I find it hard to distinguish all these from each other.
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Re: surface area of bridge contact with guitar top

Postby Randolph Rhett » Mon Mar 06, 2017 12:42 pm

Tjere seems to be such a wide gulf between theory and practice in guitar building that rules based on theory seems counterproductive. Beyond that, if you are building for others rather than as a personal distraction there really is no such thing as "improvement". People want what they know, what their heroes played, or what they wanted when they had no money and now can afford.

That said, I prefer a solid wood full contact bridge. I just experimented and settled on something I like today. Tomorrow who knows? My least favorite is a brass TOM bridge with feet rather than full contact. That implies for me that I prefer the sound of a larger footprint. Guess which one potential customers seem to respond to best...
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Re: surface area of bridge contact with guitar top

Postby Brian Evans » Mon Mar 06, 2017 1:54 pm

"That implies for me that I prefer the sound of a larger footprint."

Thus is the essence of the thing stated! It does no good to fume and fuss over theories when a practical experiment can tell you what creates a sound that satisfies and you are happy with. Guitars are intensely visual as well as kinesthetic and audible. "you eat with your eyes first" is a cardinal rule of cuisine! I'm going to stop before I start trying to figure out the impedance of a bridge and it's effect on frequency response, and designing a bridge with a tone control! :) I build only to please myself, if someone else likes it well enough to buy it, then I am doubly pleased. But the former comes before the latter.
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Re: surface area of bridge contact with guitar top

Postby Alan Carruth » Mon Mar 06, 2017 4:24 pm

I find that some theory can be very helpful on these things, so long as you keep in mind that they're ferociously complicated. As Beate says, it's often hard to separate out the various aspects.

Please don't let's drag violins in here. Aside from the arched plates and the F-holes in homage, there's not a lot in common between archtop guitars and fiddles in the way they work. In particular, the whole bridge setup is wildly different. It's all related to the fact that guitars are plucked and fiddles are bowed.

One of my friends claims that it's impossible to make an archtop guitar bridge too heavy. That's the sound he likes.

"To transfer energy the string must excite the mass of the bridge, so a large heavy wooden bridge will dampen the transfer of energy, or simply absorb some energy and not pass it to the top."

'Damping' technically refers to the rate at which energy is dissipated. It's related to friction or other types of loss (the sound the guitar output is a 'loss' as far as the system is concerned, and counts as 'damping', even though it's what we're after), but not to mass. The concept to think of there is 'impedance'.

As the term implies, impedance includes anything that can impede the motion of the thing: technically mechanical impedance it the ratio of force over velocity at a given frequency. If you think about it the variables are mass, stiffness, and damping. As a simple system, think of a stick that has been clamped to the bench with the end hanging out. If you pluck the end it will vibrate. The pitch will depend on the mass and stiffness of the stick, as will the amplitude of vibration for a given input of force. Both of those are mostly determined by the mass and stiffness. If you shave the stick down in thickness you reduce the stiffness more than the mass. The impedance is lower, so it's easier to move at all frequencies, and the resonant pitch drops. If you take the original stiff stick and add some mass at the end, it gets harder to move at all frequencies, but the pitch still drops. In this case the amount of sound output is small, so that's not adding much to the impedance, but the internal losses are there. Lower losses will allow the thing to vibrate longer after it's been plucked: aluminum will go longer that wood, and most wood will vibrate longer than, say, Masonite, all else equal.

So, a heavier bridge gives the top and bridge system higher impedance. It's harder to move it, you get less amplitude for a given input, but if the top is the same the internal damping is similar, so that's not what's cutting down on the response. Technically, the heavy bridge doesn't 'damp' the vibration. If anything, the lighter bridge, with the lower impedance and greater amplitude, has higher damping, since there's more 'loss' from the system in the form of sound.

In any stringed instrument, the function of the bridge is twofold: to define the vibrating length of the string, and communicate the force of the string to the top where it can be converted to sound. These two functions are fighting each other. To really define the string length properly you want a massive and stiff bridge that doesn't move and keeps all the vibration in the string. With a bridge like that you can't hear the guitar. To transmit the sound from the string to the top you want to match the impedance of the two. The bridge needs to be as light and flexible as the string at the top, and as heavy and stiff as the top at the bottom. This is part of what the violin bridge does, but, of course, it's not a 'perfect' match, at least, most of the time. When it is too good a match you get a 'wolf' note: the string 'sees' the bridge as just a continuation of itself, so it doesn't 'know' how long it is or what pitch to make. Of course, the only thing that will have exactly the same impedance as a string at every frequency is another piece of the same kind of string. The bridge is not that, so it's only likely to match the string impedance at one frequency. If that's a partial of a played note that sound 'leaks' out of the string and into the top and gets turned into sound quickly. Thus is born the 'thuddy G' on flat top guitars, and the 'cello 'wolf' note.

Again, this is all quite complicated. It took a Nobel prize winning scientist (Raman) to figure out the 'cello wolf, so if you're busting your brain over this that's hardly surprising. Besides, the technical stuff is only part of it. What really counts is how it sounds to you. If, in your experience, bridges with larger footprint areas work 'better', then they work better. Somebody with a sufficiently complex model might be able to figure out what that's doing, if they had enough incentive. 'What it's doing' and 'what you like' can be only loosely related. Nobody will fault you for going with what you like.
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Re: surface area of bridge contact with guitar top

Postby Patrick Hanna » Sat Mar 11, 2017 5:28 pm

I have pondered this question long and hard in the past and I assure you I am NO expert. Alan always offers good advice and information. Before I had ever read any of Alan's postings, I read about D'Aquisto, who seemed to favor more massive, heavy bridge feet. I also read and watched Benedetto, who seemed to favor narrower, lighter, less massive bridge feet. Both seemed to favor full contact bridge feet. Many ready-made bridges with dual feet were (and still are) offered in catalogs. In short, there was a lot of contradictory information available, but all of these designs seemed to work for different people. I made my first arch top bridge with a rather heavy foot, ala D'Aquisto. I am now working on another guitar and will probably give it a somewhat smaller bridge foot. This won't be a good comparison, because they are two different guitars. However, if I decide that I like the lighter bridge, I will be tempted to make a second bridge for the first guitar, to see if I like that, too. My point in all this is that you could probably make two bridges -- or at least two bridge feet--and try them on your guitar to determine which one you prefer. I think all the options will work and the only thing that really matters is which one works best for YOU.
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Re: surface area of bridge contact with guitar top

Postby Mark Langner » Mon Mar 13, 2017 5:17 pm

As usual, Alan has covered some of the relevant theory quite clearly. I can only offer my own experience.
1) Early on, I tried one of the commercial, two-foot bridges. Based on some advice I found here, I added a full-contact, single foot to it, bridging the two feet. Result? More sound. Better sound, to my ears.
2) A couple guitars I have built, after some break-in time, Had a sound I was not happy with. Close inspection revealed some gaps in the contact between bridge and top. Some careful work to improve the contact resulted in more sound, and again - to my ears - better sound.
3) I went as far as doing this experiment: Lay a piece of waxed paper on the top, coat the bottom of the bridge with epoxy and place in exact location on the top (waxed paper between bridge and top, of course!). Cure. Remove waxed paper. I was amazed by the improvement in volume and quality of sound. I left that one that way.

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Re: surface area of bridge contact with guitar top

Postby Beate Ritzert » Mon Mar 13, 2017 8:42 pm

Mark Langner wrote:3) ...

Under string tension?
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Re: surface area of bridge contact with guitar top

Postby Mark Langner » Mon Mar 13, 2017 9:02 pm

Beate Ritzert wrote:
Mark Langner wrote:3) ...

Under string tension?


Yes, although not full tension. Judgement call.
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Re: surface area of bridge contact with guitar top

Postby Brian Evans » Mon Mar 13, 2017 9:44 pm

I've thought for a while now that thickened epoxy as the final foot for a bridge might be an excellent idea. I have in mind making an extremely light, hollow bridge out of spruce with a bone top, just to see what happens. Ken Parker mentions this idea in some published interviews. In the PITA category, this recent late winter cold snap coupled with very low outside humidity has my main guitar buzzing all over the place. I am looking forward to opening the windows and letting some actual air in soon...
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Re: surface area of bridge contact with guitar top

Postby Eldon Howe » Fri Jun 16, 2017 7:50 pm

I have had a similar idea.
I have left over Nomex core. I might laminate carbon fiber flat stock on each side of the core. And add a wood filler on the top side. I could use bone as well.
In this arrangement, the bridge would run at an angle.
I would think this bride would be very stiff and light.
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