Bridge conundrum

[Earl]

I've been puzzling for days trying to understand this, & then for days more over how to express it. Before I continue, let me say that I've spent a good deal of time looking through archives here trying to find relevant information.

What I'm trying to understand (most simply put) is the relationship between bridge mass & tone. Also how top mass & stiffness relate to bridge mass.

I recently completed octave mandolin #7. It, like all my others, is a flat top (induced arch). When I first strung it up, the sound seemed quite muffled & thin: higher overtones or harmonics seemed to dominate. The bridge mass was relatively low, ~9g. I made another more massive bridge (~16g.) The result was that the tone was improved considerably but still rather "trebly". So I made yet another bridge even more massive, ~33g. Now it sounds pretty good but it looks like it has a railroad cross-tie with an inlaid walrus tusk for a bridge.

Next I made a more massive bridge (~27g.) for another OM which I already considered to sound quite good. This made it sound even better. More bass, more sustain, & clearer trebles. I did the same for a third OM which suffered from a much too thin top. Once again the result was much improved tone.

Most of what I read in the archives seems to extol the virtues of lighter bridges. So I'm wondering what it is about my instruments that seems to necessitate more mass instead of less. All of these OMs use a double X brace. Is it likely that excessive stiffness (I currently have no method of objectively measuring stiffness) requires a more massive bridge?

I'll add pictures later if anyone is interested.

Thanks for any input - Earl Tyler

[fscotte]

It's impossible for anyone to know "what's wrong with your" builds. Your definition of "clear" tone may not be what mine is. All I can say is that it's much easier to attach a heavy magnet or clay or even pieces of ebony, if you want to know how extra mass effects the tone.

[Earl]

Yes, I have tried attaching weights to bridges, But I don't think it has the SAME effect as increased mass in the bridge itself. It certainly has an effect, but it seems more to serve as a mute. I'm just interested in finding out more about the relationships between top mass & stiffness and bridge mass.


thanks - Earl Tyler

[Tom Wright]

Assuming your different-mass bridges have the same footprint/contact area, it is reasonable to think the resonances of the bridge itself matter. Also, even if some frequencies are absorbed the result may seem louder if the sound you want is now dominating.

Adding mass that is not perfectly coupled would not replicate the circumstances, so it would be hard to compare. The larger piece of wood will have both a lower resonant frequency and a stronger one. The smaller piece has less to vibrate so its resonance will be small and probably more spread (low Q value). The larger piece would be more like a xylophone or marimba bar, with a definite note. This could easily add to the high-mids.

The larger mass, while inevitably resisting movement somewhat, might add to the top's resonant behavior by lowering its resonant frequency, or more accurately, the resonance of the coupled system. It might line up to reinforce one particular frequency more than without it, so making the low end stronger.

Testing by adding mass would not replicate the resonant behavior of the bridge, and any compliance or flex would dissipate energy that might resonate otherwise.

Of course, a larger footprint complicates the question, and probably would encourage the top to move more as a unit, with less energy lost to local deflection.

[Earl]

Tom, you described the situation in more precise terms than I could muster; thank you for that. The question which remains for me is whether any factors of construction can be identified which might cause an instrument to require a more massive bridge to achieve "optimal performance". Could it be excessive stiffness of the top plate? Top plate mass or thickness? Factors of back construction? I realize this is an extremely general inquiry which cannot be addressed with great specificity, I'm just looking for any guidance which might point in useful directions as I go forward. I did not intend to imply that there's anything wrong with my builds, I'm just looking for information that will help me to improve future builds.

- Earl Tyler

[Bertram Henze]

Could it be excessive stiffness of the top plate?

Pure physics I can contribute: there are two resonance frequencies determined by the instrument body:
1 - the Helmholtz resonance frequency determined by volume and total hole area of the body
2 - the base (0,0) oscillation mode of the top plate
In an ideal world, both should be approx. the same. The behavior with a heavier bridge seems to indicate that indeed the second of the above frequencies is considerably higher than the first. Of the two, the second one is also the one which is easier to change - experiments are in order to determine the truth (less stiffness is just one option).

[fscotte]

Earl I just know from past threads that the answer you seek isn't here. Nobody understands why a mandolin sounds good or not. But that's not to say that some don't know HOW to make one sound good, as there are many luthiers who build fine instruments.

I try to make the plates as light and stiff as possible. My bridges are around 10 to 15 grams. I don't see any evidence to indicate a few grams here or there makes any difference at all. I do know that when my plates were too loose, a heavier bridge provided a more focused, pleasing sound. But that can be rectified by making the plates stiffer.

[Earl]

Thanks, Bertram & fscotte. My hunch is that the parameter to change is the top plate stiffness. If I need to lower the frequency of the (0,0) mode, I guess I need to decrease the top stiffness. That should offset the necessity of the more massive bridge. The Helmholtz resonance is between D & D#, but I have no idea about the (0,0) mode. I'm guessing it must be higher.

What perplexes me most is that I've built 6 OMs with the same basic bracing. All of them are quite similar in tone except one. That one borders on "tubby" sounding while the others tend to be "bright" sounding. That is until I added the massive bridges to the "bright" ones & they became more "bassy", balanced, & had more sustain. I have not been able yet to determine what is the critical difference with the "tubby" one. The "tubby" one has a very low mass bridge, btw. I have tried to reproduce the effect, but with no positive results. I know it seems that it should be trivial to understand the difference, but I have had no success.

One remaining difference to be investigated is that Tubby has no truss rod & a very thick neck. I built it as a test & didn't feel like wasting a truss rod. All the others have 2-way truss rods. I cannot believe that this could cause that much difference, but I'm running out of other options to consider.

Even if I never figure out what's going on With Tubby though, I still need to solve the original problem.

Thanks for your time & consideration - Earl Tyler

[Inklings]

Please, please, for heaven's sake, do an inlay pattern of a walrus on Tubby's headstock. 8 posts in and that instrument has character already!

[peter.coombe]

I think with most cases like this it is suck it and see. Trying to understand what is going on is likely to end in frustration. Note that Dave Cohen has already stated that his measurements have shown that mandolin bridges are so stiff that they do not have any resonances of any consequence. This is quite different from violin bridges. My take (i.e. guess) on this is that a heavier bridge will change the main top mode(s) (i.e. lower the frequency) and maybe it couples to the back mode(s) better. Whether that is actually what is happening or not is unknown. Without actually making some measurements it really is a guess.

[Bertram Henze]

Yes, all we can give is guesses, sprinkling probability on what to try first and what to leave for later. But nature is the ultimate judge on what helps.

[Earl]

Yes, all we can give is guesses, sprinkling probability on what to try first and what to leave for later. But nature is the ultimate judge on what helps.

Yes, & that's all I can ask. Your guesses have corroborated what I had been thinking, & that makes me feel a bit better moving forward. I think it was you, Bertram, who said in a post some while ago that it's all about finding the right spot between "tubbiness & thinicity". That has stuck with me & the quest continues....

Thanks to all - Earl Tyler

[Bertram Henze]

Thanks Earl, but don't nail me to it
The real man in the know is Dave Cohen; he seems to have stepped out of plenary posting on scientific topics but may still be reachable by PM.