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Bridge Weight
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Author:  DannyV [ Sun Mar 02, 2014 1:03 pm ]
Post subject:  Bridge Weight
A question I would like to throw out there. Feel free to speculate or better.

I think it's pretty common to try to keep the weight of bridges down as much as possible. I'm not 100 % sure why but then again I'm not sure why I scallop braces other than that it seems to work. :? Is there any reason not to use lighter, less dense woods? Longevity of the bridge possibly? I just did a run of bridges of traditional woods, Macassar, black Ebony and Brazilian. The less traditional and lightest being Kingwood and Pau Ferro. PF being the lighter of the 2. Both have a nice ring to them. More so than Ebony IMHO. Both of these seem to be nice candidates and I think aesthetics could work well with a matching FB. I have a PF finger board on a guitar that seems to be wearing well.

Any thoughts?
Author:  Imbler [ Sun Mar 02, 2014 1:19 pm ]
Post subject:  Re: Bridge Weight
I only build classicals, but I have always read that steel strings need a wood like ebony to damp a little of the high frequencies,
Mike
Author:  Hesh [ Sun Mar 02, 2014 1:22 pm ]
Post subject:  Re: Bridge Weight
Yeah... :D

Somogyi delves into suggested bridge weights for his style of building in his two book set. Perhaps Trevor does too, anyone know? I want to say the Somogyi number but I am not certain that I remember it correctly.

For me and after pulling bridges off early 20th century Martins where the bridge may only weigh 12 grams... I'm not sure that there is an answer here without considering the context of a specific instrument. Or more specifically some instruments may benefit from more mass on the top and some from less.

When we once discussed bridge pins here on the OLF likely back in 2006 or so I wrongly thought that some pin materials had an audible impact. Al C. corrected me (and taught us all) that it was more likely the change in mass and less likely the material which was in this case BRW.

Although I don't have the answer here what I suspect strongly is the case is that bridge mass needs to be considered in the context of the specific instrument in question as to if more or less is desirable. There is also the variable of the playing style and player as well. Some tops need more to drive them, some less. And materials too can make a difference.

At the owner's request we replaced a previously replaced ebony bridge on a 20's Martin and restored it to what it originally had, a BRW bridge. The BRW bridge was lighter and both the owner and I agreed that it seemed to open up the guitar as desired. One other in our shop heard no difference. I'm just happy that the guy who had to pay the bill was happy...

Danny you may be asking a question here that does not have a known answer but it should be a good discussion.

Interesting comment you made too about scalloping. For those of us who are not big fans of scalloping it's often because we have yet to hear a good argument as to how and why it may work and exactly what it does. I know all of the arguments, or likely most of them having followed this topic for years now with a sincere desire to gain and understanding but so far I can't find an argument without a few holes in it... I'm not against scalloping but I am reluctant to promote any idea that I don't have a good grasp of.
Author:  Tom West [ Sun Mar 02, 2014 2:32 pm ]
Post subject:  Re: Bridge Weight
There is only so much energy in the string when it is plucked. The energy has to start the bridge(top assembly) moving, if the weight is high the attack will be slower, the top will vibrate less. A less responsive guitar. If the weight is low the opposite is true. Weight in the top assembly is a tone and response killer. The bridge area of the top is also the most sensitive area to weight change. A few grams removed there has much more effect compared to the same weight removed elsewhere providing the stiffness remains the same. Just my opinion.
Tom
Author:  meddlingfool [ Sun Mar 02, 2014 2:36 pm ]
Post subject:  Re: Bridge Weight
I'm pretty sure Trevor mentions it in his book, but I don't recall the number either off the top of my head.

My 1x6" rectangular bridges in ebony weigh between 27 and 29 grams.

A rosewood of same size is about 20 grams.

Here's a little test I just did.

On a sloped dread that I have in progress, the top with no bridge is 192.3, or G3 -34 cents.

With the 20 gram rosewood bridge, it's 175.5, or F3 + 8 cents, a difference of 16.8 hertz.

A 28 gram ebony bridge is 166.4, or E3 + 16 cents, a difference of 25.9 hertz.

The difference between the two bridges is 9.1 hertz, more than a hertz per gram, basically a semitone difference between the two woods. Of course, glueing them on will skew those numbers 5-8 Hz upwards, but the relationship will remain somewhat constant.

What does that mean? Aside from the various timbre changes from damping differences, to maintain a set top freq, the ebony bridge will require extra stiffness in the bracing to make up for the extra mass. You therefore have both a stiffer and heavier top than if you had used a rosewood bridge, pitched to the same top freq.

As it happens, I prefer that timbre, a bit more sparkle, more overtones, sustain and headroom, but that what I like to make. If a customer wants a warmer guitar, rosewood is the choice.

Now, about scalloping...

I feel fairly definite about it as I have been doing some careful experimentation between scalloped and tapered bracing over the 35 guitars we made last year using each style.

Scalloping seems to affect the mids more than either the bass or treble.

Scalloped braces tend to emphasize low mids, tapered seems to give a more balanced sound, and I think it's logical why.

If I'm remembering correctly and not talking out my sound hole (always a distinct possibility), the cross dipole is largely responsible for the mid freqs.

If you take a tapered bracing top and look at how much wood there is under the bridge wings, it is fairly stiff.

If you look at a scalloped guitar in the same place, wood has been removed, meaning that it is more flexible in that area, allowing the dipole to drop in freq, and *possibly?* be louder in amplitude?

It would explain the general consistency in the tone difference I have heard between the two systems anyway. Unfortunately, I have no data to back this up as I only collected main top, main back, and main air measurements, which I now realize was a grevious oversight.

I'll know more over the next 50 guitars....
.
Author:  Alan Carruth [ Sun Mar 02, 2014 3:56 pm ]
Post subject:  Re: Bridge Weight
IMO, the main 'job' of the bridge is to present the string with a reasonably stationary end point, so that it will know how long it is, and what note to make. You do this by making the bridge area of the top more massive and/or stiff than the string, to produce an impedance mismatch. Mechanical impedance is the ratio of force/velocity at a given frequency, and is generally proportional to sqrt(mass*stiffness).

As you're driving the top in and out, you have to accelerate the mass of the bridge for every cycle of vibration. Since high frequencies mean more cycles per second, that's more acceleration that the limited force of the string has to accomplish. Thus adding mass tends to impede the top motion more at high frequencies than low ones. People often say that a heavy bridge gives 'more bass', but in point of fact it probably gives a bit less bass. It just seems like more compared with the big cut in treble power.

Stiffness tends to add more to the impedance equation at low frequencies. Of course, the bridge doesn't bend much itself, so most of the stiffness in the top/bridge system comes form the top and the bracing. Scalloped bracing allows the center of the top to move more, enhancing the 'bass reflex couple'. you get more sound at the 'main air' pitch (usually around G on the low E string), and also, and maybe especially, at the 'main top' pitch, near the open G string frequency. Enhanced mid-range makes some sense in that respect.

Resonant frequencies are simply those where you get the most motion for a given input of energy: they're impedance minima. If the top is moving a lot at some frequency you're trying to play the string can actually get confused and think it's longer or shorter than it really is. Notes near strong resonances can be displaced upward or downward in pitch, sometimes by a noticeable amount. In really bad cases you can even get a 'warble', as the pitch of the string drifts up and down for complicated reasons.

It seems to me that one reason for using heavy ebony bridges on scalloped Dreads is to avoid those sorts of problems. The scalloped bracing is not as stiff at the bridge location, and you need to do something to 'nail the top down' at the main air and top frequencies in particular, so adding some mass is a reasonable way to go. It costs a bit of power, of course. If you can site your top resonances between scale tones, that added mass might not be as important.

One of my students got a little carried away when carving her bridge, and it ended up at around 20 grams. The guitar had more treble than she liked, and we ended up adding a couple of grams of mass (with poster putty) inside the top to tame it. At that point the guitar had no finish on it. She brought it back a couple of years later to have me finish it: her medical practice and a new baby cut into her time. When I got the varnish on it we found that it no longer needed the added mass.
Author:  meddlingfool [ Sun Mar 02, 2014 4:20 pm ]
Post subject:  Re: Bridge Weight
Alan,

Any thoughts on my pet theory about scalloping lowering the frequency of the dipole?
Author:  DannyV [ Mon Mar 03, 2014 11:36 am ]
Post subject:  Re: Bridge Weight
Hesh wrote:
Yeah... :D


For me and after pulling bridges off early 20th century Martins where the bridge may only weigh 12 grams...

Was that back when they were experimenting with Pine. :lol:
Hesh wrote:
I know all of the arguments, or likely most of them having followed this topic for years now with a sincere desire to gain and understanding but so far I can't find an argument without a few holes in it... I'm not against scalloping but I am reluctant to promote any idea that I don't have a good grasp of.

I don't care what anyone says, it looks cool. And as an added bonus, carving those peaks and valleys and tapping truly brings out the inner luthier in me. Ohhhh, did I just say that.
Author:  DannyV [ Mon Mar 03, 2014 11:38 am ]
Post subject:  Re: Bridge Weight
meddlingfool wrote:
I'm pretty sure Trevor mentions it in his book, but I don't recall the number either off the top of my head.

My 1x6" rectangular bridges in ebony weigh between 27 and 29 grams.

A rosewood of same size is about 20 grams.

Here's a little test I just did.

On a sloped dread that I have in progress, the top with no bridge is 192.3, or G3 -34 cents.

With the 20 gram rosewood bridge, it's 175.5, or F3 + 8 cents, a difference of 16.8 hertz.

A 28 gram ebony bridge is 166.4, or E3 + 16 cents, a difference of 25.9 hertz.

The difference between the two bridges is 9.1 hertz, more than a hertz per gram, basically a semitone difference between the two woods. Of course, glueing them on will skew those numbers 5-8 Hz upwards, but the relationship will remain somewhat constant.

What does that mean? Aside from the various timbre changes from damping differences, to maintain a set top freq, the ebony bridge will require extra stiffness in the bracing to make up for the extra mass. You therefore have both a stiffer and heavier top than if you had used a rosewood bridge, pitched to the same top freq.

As it happens, I prefer that timbre, a bit more sparkle, more overtones, sustain and headroom, but that what I like to make. If a customer wants a warmer guitar, rosewood is the choice.


.

Thanks Ed! That really cleared things up for me. :? :lol:
Author:  meddlingfool [ Mon Mar 03, 2014 11:49 am ]
Post subject:  Re: Bridge Weight
If I can confound you further, please let me know....
Author:  Jim Watts [ Mon Mar 03, 2014 12:52 pm ]
Post subject:  Re: Bridge Weight
Ed, were the two bridges glued on or taped on, on you experiment?
Author:  JasonM [ Mon Mar 03, 2014 12:58 pm ]
Post subject:  Re: Bridge Weight
Some observations to add to the thread:

I built a pair of identical (as much as possible) parlours, using tops from the same billet, side by side, same spruce bracing, profiled the same etc. On one I used my lightest EIR bridge, 16 grams, and on the other I used a 40 gram cocobolo bridge. I fully expected to, but could not hear a difference between the two. Then I installed a UST under saddle on the coco bridge on customer request. I was expecting some degradation, due to the now "spongy" coupling with the saddle and bridge. Again, no difference, as I had the other guitar to compare to. I now no longer worry about bridge weight. I'm sure electronic measurements would show a difference, but not to my human ear.
Author:  meddlingfool [ Mon Mar 03, 2014 1:27 pm ]
Post subject:  Re: Bridge Weight
Yes, they were taped on. The numbers would certainly change with glue-up, but the general relationship remains fairly true. I have found a constant 5-8hz raise between taping and glueing. Constant enough to rely on it during building.

Nowadays, I just use magnets of appropriate weight and it works just as well, with less hassle.

Jason,

Bridge weight makes a huge difference to the sound of my guitars, that's why I worry about it.
Author:  JasonM [ Mon Mar 03, 2014 2:10 pm ]
Post subject:  Re: Bridge Weight
Easy enough to stick 20 gams of putty to your bridge, and have somone else play your guitar 10 times randomly, with putty on and off, while you face the other way. If you can call each one right, then its worth the fuss.
Author:  Goodin [ Mon Mar 03, 2014 2:25 pm ]
Post subject:  Re: Bridge Weight
Here you go. Read this by Frank Ford:

http://www.frets.com/FretsPages/Musicia ... ndobr.html
Author:  Alan Carruth [ Mon Mar 03, 2014 2:51 pm ]
Post subject:  Re: Bridge Weight
meddlingfool asked:
"Any thoughts on my pet theory about scalloping lowering the frequency of the dipole?"

Some thoughts, but not much data to back them up. I've only used scalloped bracing a couple of times. It does seem to me that the peaks of the scallops are in areas where the cross dipole bends a lot, so I would imagine scalloped tops would tend to have relatively higher cross dipole pitches. Again, it's hard to say much for sure without data.
Author:  meddlingfool [ Mon Mar 03, 2014 4:51 pm ]
Post subject:  Re: Bridge Weight
I need to get into Chladni testing I think so I can actually see what's going on...
Author:  Trevor Gore [ Mon Mar 03, 2014 5:00 pm ]
Post subject:  Re: Bridge Weight
Bridge mass makes a huge difference on my guitars. I've used SS bridges as light as 14 gm and rarely go higher than 25 gm.

A 14 gm bridge on a lightly built guitar gives you the equivalent of a SS flamenco sound (cutting and somewhat raucous) whilst higher mass bridges slow down the attack and smooth things out.

Between bridge mass and saddle material there is huge scope for changing sound. If you want to experiment, put a low mass bridge on your next guitar. It's really easy to increase the effective bridge mass using denser bridge pin materials.

meddlingfool wrote:
What does that mean? Aside from the various timbre changes from damping differences, to maintain a set top freq, the ebony bridge will require extra stiffness in the bracing to make up for the extra mass. You therefore have both a stiffer and heavier top than if you had used a rosewood bridge, pitched to the same top freq.

Exactly. If you increase both the stiffness and the mass of the top, the monopole mobility (a measure of responsiveness that correlates well with perceived loudness) decreases.

Of course, there's no answer that suits everyone. So a custom builder needs to understand how these variables affect the sound.
Author:  DannyV [ Mon Mar 03, 2014 5:09 pm ]
Post subject:  Re: Bridge Weight
JasonM wrote:
Some observations to add to the thread:

I built a pair of identical (as much as possible) parlours, using tops from the same billet, side by side, same spruce bracing, profiled the same etc. On one I used my lightest EIR bridge, 16 grams, and on the other I used a 40 gram cocobolo bridge. I fully expected to, but could not hear a difference between the two. Then I installed a UST under saddle on the coco bridge on customer request. I was expecting some degradation, due to the now "spongy" coupling with the saddle and bridge. Again, no difference, as I had the other guitar to compare to. I now no longer worry about bridge weight. I'm sure electronic measurements would show a difference, but not to my human ear.

That's pretty amazing Jason. I've wondered about bridge weight and how it would affect different size guitars. My simple logic would tell me that a smaller guitar would have a harder time driving a heavier bridge. Maybe not so.
Author:  DannyV [ Mon Mar 03, 2014 5:17 pm ]
Post subject:  Re: Bridge Weight
Trevor Gore wrote:
Bridge mass makes a huge difference on my guitars. I've used SS bridges as light as 14 gm and rarely go higher than 25 gm.

A 14 gm bridge on a lightly built guitar gives you the equivalent of a SS flamenco sound (cutting and somewhat raucous) whilst higher mass bridges slow down the attack and smooth things out.

Between bridge mass and saddle material there is huge scope for changing sound. If you want to experiment, put a low mass bridge on your next guitar. It's really easy to increase the effective bridge mass using denser bridge pin materials.


Thanks Trevor. I see know reason not to try a Pau Ferro bridge. It's very stable and doesn't appear to be prone to cracking. Pin density? Wood, bone, brass? Saddles? Wood, bone, Corian?
Author:  Trevor Gore [ Mon Mar 03, 2014 5:31 pm ]
Post subject:  Re: Bridge Weight
DannyV wrote:
Pin density? Wood, bone, brass? Saddles? Wood, bone, Corian?

The cheapo plastic pins are the lowest mass. You might also consider Tusq for a saddle material.
Author:  Jim Watts [ Mon Mar 03, 2014 6:58 pm ]
Post subject:  Re: Bridge Weight
meddlingfool wrote:
Yes, they were taped on. The numbers would certainly change with glue-up, but the general relationship remains fairly true. I have found a constant 5-8hz raise between taping and glueing. Constant enough to rely on it during building. ...


Thanks Ed, makes perfect sense as a glued bridge will provide more stiffness than a taped bridge.
Author:  Rodger Knox [ Tue Mar 04, 2014 12:55 pm ]
Post subject:  Re: Bridge Weight
DannyV wrote:
I see know reason not to try a Pau Ferro bridge. It's very stable and doesn't appear to be prone to cracking. Pin density? Wood, bone, brass? Saddles? Wood, bone, Corian?


Pau Ferro (aka morado, bolivian rosewood) work fine for a bridge, I've used it several times. It may be a bit heavy, but is usually less dense than ebony.
It is an extreame sensitizer, I can't use it anymore.
Author:  sdsollod [ Tue Mar 04, 2014 1:34 pm ]
Post subject:  Re: Bridge Weight
Not to digress or hijack the thread, but would Granadillo work well for a fingerboard and bridge. Woodcraft has some on sale...
Author:  RusRob [ Tue Mar 04, 2014 3:07 pm ]
Post subject:  Re: Bridge Weight
So here is a interesting point.

If lighter weight produces more volume why don't more builders use pinless bridges?

Since you are eliminating the weight of the pins and drilling holes horizontally through the bridge that would reduce a considerable amount of weight.

But you rarely see them... idunno

Bob
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