Official Luthiers Forum!

The author of a new Classical guitar building book says that a lighter weight bridge is better than a heavier one.
I have always believed that lighter is usually softer producing a "damping" effect.
Am I wrong - or?

How about a Balsa wood bridge?

Ray

Dont know about the bridge, but a lot of great classical guitars have been made with Western Red Cedar tops, which are about as light and soft as a guitar could possibly tolerate I imagine.

Ray, like everything in guitarmaking, it's a balance. we have to use a hardwood for the bridge so it can stand up to the strings' pull, but if it's too chunky its ability to transfer vibrations will be impared.

Generally, lighter seems to be better; but don't confuse "light" and "dense." Rosewoods are fairly dense, but can be made reasonably light in service. According to previous discussions here, though, ebony is less efficient in transferring vibrations no matter how light it is. That's also why you don't see oak bridges. However, both ebony and oak can be used effectively as back/side material. Ain't lutherie fun?!!!

Ray-
'Lighter' doesn't necessarily mean 'made of less dense wood'. Reducing the bulk/mass/size of the bridge is one tactic that can be used.

Of course, the best material for bridges is unobtanium. Second choice, naturally, is Brazilian Rosewood. Some of the lighter hardwoods (like walnut) have been used as well, apparently.

Cheers

John

Ebony -has higher damping (filters out the high end) than rosewood. In part it's because of the extra mass, in part it's just a property of the wood itself. The more mass you have to move, the less high-end energy you are left with.

Tap some rosewood fingerboards and some ebony ones, and you should be able to hear what I mean. The rosewood will tend to resonate, ring out longer, fast attack, while the ebony is slower, duller, fades out faster.

Makes me wonder why African Blackwood isn't used more for bridges.

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I have used African blackwood for bridge plates (which is part of the bridge construction), and it certainly works well for that.   

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Rian Gitar og Mandolin

Think of it this way:

When you pluck the string you put in a limited amount of power. You use that power to move the bridge. Moving the bridge involves accelerating it from a stop, and then stopping (decellerating) it again so that you can acclerate it the other direction. High frequency response is all about acceleration. You know that if you want a lot of acceleration in a car, and the horsepower is limited, you need to keep the weight down. Same with guitar tops: if you want high frequency response you need to keep the weight down.

Of course, the top weighs a lot more than the bridge: three or four times as much, at least. Still, the weight of the bridge has a lot to do with the frequency response becuse that's the first thing the string 'sees': a lighter bridge will generally be more 'treble balanced' than a heavy one.

There's a price. One of the main 'jobs' of the bridge is to tell the string how long it is. 'Ideal' strings, ones that make perfectly harmonic sounds, have to have the ends fixed, immobile. Of course, if the bridge and top on your guitar didn't move at all you'd never hear it, so you can't have a 'perfect' systyem in that sense. You do have to provide enough stiffness and mass in the bridge to keep things under control, though. Too light a bridge will cause 'wolf' notes by feeding energy from the top back into the string. This can drive you wild.

IMO, and it's only my opinion, a thirty gram bridge on a classical guitar is too heavy, and a fifteen gram bridge is too light. I usually shoot for around 24 grams, more or less. It's hard to get that light with ebony, possible with Pau Ferro/morado, reasonably easy with Brazilian or Indian rosewood, and a piece of cake with walnut. It's hard to say what role the actual damping factor of the bridge material plays, but it probably doesn't hurt to pay some attention to that, too.

Alan-
Do you have any 'weight guidelines' for bridges for steel-string guitars (less pins)?

Also, what do you (or anybody else) think of the idea that the larger guitars (like dreadnoughts) 'need' a bigger area bridge to drive the top? Or is this just a cosmetic issue- big bridge for big top?

Thanks

John

I just got some bubinga that has a very consistant and clear tap tone.
Although it seems to be real close to the weight of ebony it rings much
bigger and clearer. It is so hard that the top I'm making I had to finish it
with a scraper because my plane wouldn't touch it. Might be a good choice
for an acoustic bridge It's what I use for archtop bridges.

Steel strings weigh more than classical strings and have higher tension, so the bridges are usually heavier. I'd say in the range of around 25-30 grams is OK for a steel string bridge, and it can be heavier. Less than about 20 would be too light, IMO. YMMV

The size of the bridge is an intersting thing to think about. Remember, the bridge is one of the biggest braces on the top, and usually weighs about as much as all the other top bracing put together. All of it. It adds stiffness as well as mass. Usually the mass effect 'wins': the tap tone pitches go down when you glue on the bridge. However....

A long bridge, one where the wings get closer to the edge of the top, adds a fair amount of crosswise stiffness to the top. Often the guitar ends up sounding 'brighter', so long as the bridge was not made too heavy in the process.

The height of the wings effects the bending stiffness in the middle of the top, the same as tall braces. Classical makers sometimes fine tune the response by thinning the wings down near the tieblock.

The deeper the bridge is along the line of the string pull, the less likely it will be to peel off any time soon. This depth cuts down on the maximum stress in the glue line along the back edge of the bridge. It's why belly bridges can hold the extra stress of steel strings even though they often don'thavwe any more gluing area than a classical bridge that would peel up in a few months.

If you're using something like a redwood top, that has low peel resistance, and you want to make a wide bridge but don't want too much weight, try a wood like walnut. Skinny ebony pyramid bridges belong on tough, straight grained, Red spruce tops.

Thanks, Alan!
John