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Bridge Design
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Author:  Darryl Young [ Wed Aug 04, 2010 8:43 pm ]
Post subject:  Bridge Design
I asked this question on the "Bridge Material Help and Wood ID" thread and Steve suggested I start a new thread for this discussion......so here it is.

I would appreciate input on how the stiffness of the bridge wings affects tone. What changes when the wings are made thinner and floppier? How about if I leave them thicker and thus stiffer? What if I make them longer or shorter?

I'm guessing the design of the wings affects tone in more ways than just increasing/decreasing the overall weight of the bridge. I've read that thinning the perimeter of the soundboard on the sides of the lower bout (where the bridge wings point) will increase the treble response. Will thinning the bridge wings give the same result? How about shortening the wings?

Appreciate your thoughts!
Author:  Robbie_McD [ Wed Aug 04, 2010 8:58 pm ]
Post subject:  Re: Bridge Design
Great question!
My thoughts would suggest the width and depth of the conventional bridge is a factor of gluing strength - why would we not have short bridges then?
Many of the cool bridge shapes I have seen still have a similar gluing area - thickness at the wings......?
Author:  Burton LeGeyt [ Wed Aug 04, 2010 9:37 pm ]
Post subject:  Re: Bridge Design
Darryl,

I think it matters just as much what is going on under the bridge. You may have already a lot of reinforcement and stiffness from braces and bridge patches. I have been really thinning the wings on my bridges but also running a brace all the way across the top under the front of the bridge so I think it evens out somewhat. Depending on where the x braces and finger braces fall it could really affect what thinning the bridge wings might accomplish.

For what it is worth I wish I could say that I have noticed this or that from thinning the wings but I can't attribute any one thing to it with any certainty. I wish I could! I can say that I would be more nervous thinning them as much as I do without the extra cross stiffness I put in that brace. They may still be fine but I would not be overly confident about it.

Here is a photo of a recent(ish) bridge. I think the wings are just about .1 for most of their width:

Attachment:
CL17-bridge.jpg
Author:  Daniel Minard [ Thu Aug 05, 2010 1:43 am ]
Post subject:  Re: Bridge Design
A few guitars back, I left the clamping cauls stuck under the wings of the bridge. (Oh those nasty deadlines...) When the customer brought the guitar back for its 3 month checkup, I noticed the cauls & removed them. The result was shocking. The guitar sounded beautiful, right from the get go. Exactly what I was trying to achieve for this customer.
After I removed the little plywood blocks, it sounded like crap. Harsh & "jangly" when chorded & no where near as sweet when fingerpicking. I put on a set of Tomastik Spectrums & got most of the mellow tone back, but it certainly gave me something to think about for future guitars.
I used to make the bridge wings quite thin, but now I am leaving more mass on the ends.
It's too soon to make any judgements about this decision, but it does seem to helping me get the results I'm after.
I don't know if this helps, Daryl, but i thought it might be food for discussion.
Author:  Haans [ Thu Aug 05, 2010 5:17 am ]
Post subject:  Re: Bridge Design
Well, Daniel, that sure flies in the face of convention. I've never had a problem with pyramid bridges...but then, I'm not much for reductionist thinking.
Author:  Darryl Young [ Thu Aug 05, 2010 8:15 am ]
Post subject:  Re: Bridge Design
Daniel Minard wrote:
A few guitars back, I left the clamping cauls stuck under the wings of the bridge. (Oh those nasty deadlines...) When the customer brought the guitar back for its 3 month checkup, I noticed the cauls & removed them. The result was shocking. The guitar sounded beautiful, right from the get go. Exactly what I was trying to achieve for this customer.
After I removed the little plywood blocks, it sounded like crap. Harsh & "jangly" when chorded & no where near as sweet when fingerpicking. I put on a set of Tomastik Spectrums & got most of the mellow tone back, but it certainly gave me something to think about for future guitars.
I used to make the bridge wings quite thin, but now I am leaving more mass on the ends.
It's too soon to make any judgements about this decision, but it does seem to helping me get the results I'm after.
I don't know if this helps, Daryl, but i thought it might be food for discussion.


Wow. That is interesting......thanks for sharing that Daniel.
Author:  SteveSmith [ Thu Aug 05, 2010 8:45 am ]
Post subject:  Re: Bridge Design
So this leads me to a question about making the bridge wings thicker or thinner: Is it changing the mechanical compliance of the bridge wings that has the effect or is it more the mass of bridge as a whole?

If I remember correctly from some of Al's posts over the years, changing the mass of the bridge will change the bass/treble response. Increasing the mass tends to damp out the higher frequencies which might track well with Daniel's experience.
Author:  DennisK [ Thu Aug 05, 2010 10:02 am ]
Post subject:  Re: Bridge Design
Daniel Minard wrote:
A few guitars back, I left the clamping cauls stuck under the wings of the bridge. (Oh those nasty deadlines...) When the customer brought the guitar back for its 3 month checkup, I noticed the cauls & removed them. The result was shocking. The guitar sounded beautiful, right from the get go. Exactly what I was trying to achieve for this customer.
After I removed the little plywood blocks, it sounded like crap. Harsh & "jangly" when chorded & no where near as sweet when fingerpicking. I put on a set of Tomastik Spectrums & got most of the mellow tone back, but it certainly gave me something to think about for future guitars.
I used to make the bridge wings quite thin, but now I am leaving more mass on the ends.
It's too soon to make any judgements about this decision, but it does seem to helping me get the results I'm after.
I don't know if this helps, Daryl, but i thought it might be food for discussion.

Interesting! Was that a pyramid bridge, by any chance? Those things have always puzzled me a bit, having the wings attached by a thin area so they can flop all over. My guess is that it's deliberate to get a different sound than a bridge that stiffens its full width on the soundboard, the wings just provide glue area, and the pyramids are mostly decorative since they're small and thus don't add that much weight or stiffen a large area.

Have you read Ervin Somogyi's books? As the theory goes, on fan braced classical guitars, cross dipole motion (side to side rotation) contributes to volume close to the instrument, and long dipole motion (rotation in line with the strings) contributes to projective power and high frequency sound. But for X braced guitars, the roles are usually swapped. I'm thinking the weight of the cauls attached under the wings was enough to significantly decrease the cross dipole, thus reducing the higher frequencies. "Harsh and jangly" sounds about right for an overactive cross dipole after freeing it back up. How were the cauls held in there, by the way? Do you think they could have had a stiffening effect in addition to just their inertia?


As for the original question of bridge stiffness, I have no hands-on experience, but my instinct is that a general arch shape across the width of the bridge would be best, to keep the whole thing as stiff as possible, with mass focused in the middle where it rotates more easily. The "scalloped" style of the standard Martin bridge wings makes sense as well, being pretty much the same as braces are usually shaped when tapering down to notch into the linings. Seems fitting, since it's not unlike notching into the legs of the X underneath :)

Certainly having thin wings mainly for glue area is an option as well though. But you have to keep in mind where the legs of the X brace are underneath, and whether you want a rigid triangle, or some floppiness between the bridge and the X.
Author:  Kent Chasson [ Thu Aug 05, 2010 11:15 am ]
Post subject:  Re: Bridge Design
Daniel Minard wrote:
A few guitars back, I left the clamping cauls stuck under the wings of the bridge. (Oh those nasty deadlines...) When the customer brought the guitar back for its 3 month checkup, I noticed the cauls & removed them. The result was shocking. The guitar sounded beautiful, right from the get go. Exactly what I was trying to achieve for this customer.
After I removed the little plywood blocks, it sounded like crap. Harsh & "jangly" when chorded & no where near as sweet when fingerpicking. I put on a set of Tomastik Spectrums & got most of the mellow tone back, but it certainly gave me something to think about for future guitars.
I used to make the bridge wings quite thin, but now I am leaving more mass on the ends.
It's too soon to make any judgements about this decision, but it does seem to helping me get the results I'm after.
I don't know if this helps, Daryl, but i thought it might be food for discussion.


Yes, I've also experienced guitars that are improved in the exact same way with more mass at the bridge. I even pulled a bridge on a brand new guitar once to replace it with a heavier one. Like Haans said, this seems to fly in the face of convention but it's true on some guitars. Fortunately, this is very easy to confirm. Tape a bit of weight on you bridge and see what happens.

My take on pyramid bridges is that the idea is to get the extra mass of the pyramids and maintain the flex at the wings. Making the wings uniformly thicker adds stiffness and mass. Both can work and it's best in my opinion to think in terms of what you want from the whole top as a system, including top, bridge, bridge plate and bracing.
Author:  Daniel Minard [ Thu Aug 05, 2010 11:25 am ]
Post subject:  Re: Bridge Design
To answer your questions...
My usual bridges are a fairly standard belly type. This one is Gabon ebony, though I usually use Macassar these days. The cauls were held on with a small tab of double sided tape. I think the stiffening effect would have been minimal. Strictly a matter of mass.
I had an opportunity to play that guitar this spring, while on a visit to Toronto. It has mellowed nicely & I'm very happy with it, overall. It still has Tomastic strings on it. Next time I get access, I'm gonna try a set of D'Adario lights on it again, just for fun.
This issue brings to mind Mario Proulx's Magic Tone Enhancer brace. It's a pity he's not hanging out here anymore. It'd be interesting to get his take on this.
Thanks for your comments folks!
Author:  Alan Carruth [ Thu Aug 05, 2010 12:59 pm ]
Post subject:  Re: Bridge Design
As has been said, changing the bridge mass usually effects the bass-treble balance, all else equal. A more massive bridge tends to keep the high frequency energy in the strings, so the guitar sounds 'bassier'. Adding weight to the bridge also tends to drop the 'main top' resonant mode pitch a bit, and that can also contribute to the bass.

On classical guitars without a 'Bouchet brace' the bridge wings are a major stiffness element, and reducing their height can drop top mode pitches noticably. The one most effected is usually the 'cross tripole', which is generally up around 500 Hz (B or C on the high E string, 8th or 9th fret), and, of course, reducing the stiffness also makes the mode more active. This is one of the ways classical makers fine tune the high E string response. The 'momopole' and 'crss dipole' modes are effected less, and the 'long dipole' is not usually effected at all by altering the mass or stiffness of a classical bridge.

The cross tripole is not usually as important on a steel string, and can be missing entirely, or shifted 'way up in pitch, due to the higher cross grain stiffness of X bracing. You might see more of an effect from making the bridge wings longer or shorter, but it's hard to say. Although I look closely at the mass, I don't pay as much attention to the stiffness of steel string bridges: maybe I should.

In terms of the roles of the 'long' and 'cross' dipole modes in the sound, it's interesting to note that one of the main differences between steel string and classical guitars is the pitch of the cross dipole mode. On most steel or nylon string guitars the 'monopole' or 'main top' mode will end up somewhere around 180-200 Hz, and the 'long dipole' around 350 Hz. On classicals the 'cross dipole' is often around 250 Hz, and may be as low as 220 on some Flamencos. The cross dipole mode on steel strings tend to be closer to 300-320 Hz, and can be high enough to 'mix' with the long dipole, yeilding a pair of 'diagonal dipoles' in some cases.

I believe it was Meyers who first pointed out that guitars that have a sharp 'main top' spectral peak with a narrow band width tend to have a 'cutting' or 'harsh' sound. Since the cross dipole mode is the next top resonance up from the 'main top' mode, and is much less efficient as a sound producer owing to 'phase cancellation', having the cross dipole close to the main top pitch limits the bandwidth of the main top peak, and contibutes to a 'cutting' timbre that is useful on classical guitars, and desireable on Flamencos. Moving the cross dipole up in pitch on a steel string helps to increase the 'fullness' of the sound, and balance off the greater treble energy of steel strings.
Author:  Jim Watts [ Thu Aug 05, 2010 1:51 pm ]
Post subject:  Re: Bridge Design
The resonate frequency of a structure is the square root of it's spring constant / mass or (K/M)^1/2. So by leaving the cauls taped in you essentially raised the mass of the structure with out changing the spring constant due to the poor bonding of the cauls. Thus lowering the resonate frequency of the top.
I am willing to take a leap here and say that tops with a lower resonate frequency sound "warmer" than tops with a high Resonate frequency.
Remember the bridge is part of the stucture, which defines K (spring constant). So, I think Kent is right about pryamid bridges attempt to increases the mass without effecting the stiffness too much.
Author:  Tom West [ Thu Aug 05, 2010 5:24 pm ]
Post subject:  Re: Bridge Design
Burton: Your bridge to me looks quite small especially in terms of gluing area. Wonder if you could tell us a bit about your experiences with it and maybe some sizes. Thanks. Apologies to Darryl for a bit of a high-jack.
Tom
Author:  Burton LeGeyt [ Fri Aug 06, 2010 11:19 am ]
Post subject:  Re: Bridge Design
Hi Tom,

The specs on the bridge are (app.):
6 7/8" long
7/8" wide wings
1" wide main body
1 1/4" at little half circle
the main body part is app. 3 3/8" long (it is slightly tapered)

The wings are about .11 on one I am about to put on a guitar. It depends on the kind of wood. Most rosewoods end up at about that thickness. Weights are usually just under 20 grams. Sometimes a touch over.

I have used it (or one very much like it) on about 5 guitars now and have not noticed any real problems. I do use a large spruce bridge plate with a small hardwood plate on top of that to anchor the ball ends so if you include the spruce plate as part of the top then the wood is about .15 thick or so around and behind the actual bridge. I have not been getting a crease style belly behind it. I have used heavier bridges and liked the sound too but I like the sound of these smaller bridges more. I don't feel like I am lacking for bass and I do feel like the guitar has more possibility in terms of responsiveness. I think it takes more control to focus it but you get more when you do. One thing people seem to like about ebony (and somewhat by default heavier) bridges is that they reduce string noise, which I feel they do. I think that is across the board high frequency loss though and if the player is skilled in coaxing sound out of the guitar you are losing some beneficial sparkle in that register which can really make a difference in some playing styles (especially a combination of fingernail/finger pad style hand picking).

I think there is a middle point in between getting good bass from a heavy bridge and active bass response from a small bridge in where it isn't that great. This is not a scientific opinion, rather just a hunch. I think at some point the thinner (and of course also lighter and less stiff) you go you get that bass back in a different manner. I hear it is a "airiness" in the sound that I love. I can't say this is from the bridge alone, of course. It has to work with the whole system but I don't think a heavier bridge on an otherwise similar guitar would give you that same sound. That is my thinking at the moment at least.
Author:  Darryl Young [ Fri Aug 06, 2010 12:25 pm ]
Post subject:  Re: Bridge Design
I appreciate the responses so far. Thanks everyone.

To Al (or anyone else), what would help me understanding your discussions on Chladni patterns is to be able to associate the correct name with each pattern. I have the monopole down and know which one that is. The dipole, cross dipole, long dipole, etc., etc. start to confuse me and I'm always questioning which pattern is being discussed.

Any chance someone could post a pic of all the patterns on the soundboard with each pattern labeled with the correct name?
Author:  Darryl Young [ Fri Aug 06, 2010 12:37 pm ]
Post subject:  Re: Bridge Design
I appreciate the responses so far. Thanks everyone.

To Al (or anyone else), what would help me understanding your discussions on Chladni patterns is to be able to associate the correct name with each pattern. I have the monopole down and know which one that is. The dipole, cross dipole, long dipole, etc., etc. start to confuse me and I'm always questioning which pattern is being discussed.

Any chance someone could post a pic of all the patterns on the soundboard with each pattern labeled with the correct name?
Author:  Tom West [ Fri Aug 06, 2010 3:31 pm ]
Post subject:  Re: Bridge Design
Burton: Thanks for the reply and info. The bridge is bigger then I thought it was via the picture. My bridges at one time were quite big and I gave myself a big surprise one day when I found one of my newly built guitars too much on the bassey side for my liking. I trimmed the bridge on the guitar with a small plane and that guitar came alive. Much more responsive, quicker attack,more treble response. It was then that I realized how sensitive the bridge area is to any change and have been taking advantage any chance that I have the opportunity. My bridges nowadays are a bit larger then yours. Room for experimentation.Thanks again.
Tom
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