4 Bridge System Madness.

[Killian King]

Heres a shaot of a Hamlett tail piece. Never seen masking tape used to dampen the strings between bridge and tailpiece. Dead on inline with bridge.

What a piece of work.

[Eric Oliver]

To encourage Mr. Pete in his search to reduce crosstalk or phase additions or subtractions...as if I know what that is...is there a way to have a mandolin top that would have separate top segments for each bridge string post?
I am thinking of the cracked Charlie Sawtelle Martin guitar top.

[Bertram Henze]

...is there a way to have a mandolin top that would have separate top segments for each bridge string post?

Since the neck would contribute a little coupling as well, it would be best to have 4 different mandolins with one bridge post each...

[Dave Cohen]

To encourage Mr. Pete in his search to reduce crosstalk or phase additions or subtractions...as if I know what that is...is there a way to have a mandolin top that would have separate top segments for each bridge string post?
I am thinking of the cracked Charlie Sawtelle Martin guitar top.

You can put all of the cracks or slits in the top you want. It will not alter the main normal modes of the top. If you look at holography (aka interferometry) data, you will see that the shapes of the normal modes go right through the soundhole areas as if they weren't there. Holes or cracks are not a barrier to the modal motion of a plate. To create a barrier that will impose a node (and hence a phase reversal across the nodal line), you need something massive, like f'rinstance a very large brace. Even that won't change things much for the very lowest frequency modes. So the direct answer to your question is that there is not a practical way to "separate" the strings. It would just be contradictory to what top plates actually do, or for that matter, need to do.

[Pete Jenner]

An 40 fret mandolin with one pair of strings might work.

Dave, did you see my question?

[Dave Cohen]

An 40 fret mandolin with one pair of strings might work.

Dave, did you see my question?

You mean w/ regard to ideas about measurement? Yes, but I forgot to answer. Sorry. How you measure depends on what you want to measure, and what you want to measure depends on the question you want to answer. If you look in the sections I referenced in the Fletcher & Rossing text, there are a couple of different things you might want to look at. One is string frequencies. Section 4.10 points out that a string coupled to another string through a bridge (saddle) will have it's pitch (aka frequency) thrown off very slightly by the interaction as the pitches of one string approaches that of the other. The problem with just plucking the strings and doing an ft spectrum is that the combination of your sensor, computer, and software probably don't have sufficient resolution - mainly the computer and the software. Those measurements don't work well enough for that experiment with ordinary 44.1 K resolution. Maybe w/ 196 K resolution, if you have that. The other thing to measure based on sections 4.9 and 4.10 is impedance. You could also measure something like accelerant, as the two are related (see the Appendix to chapter 4). You would do that at your various bridge saddle points. May want to damp the strings for that, or maybe not.

[Timbofood]

This is so cool! I am virtually a moron with respect to the physics but, the thought behind all this is fascinating!

[Dale Ludewig]

Dave, I've seen the interferometry and so forth, as you know. I hope this is related to this thread. What would happen if a crack or a narrow saw kerf were to run the entire length of the top, rim to rim lengthwise. I know the plate doesn't seem to notice a hole in it, like ff-holes or an oval hole, but the plate is still a 'whole', with a hole in it. Assuming one could build a top that would support the string tension, do you think it would still act the same being coupled only at the rim? I would think that at that point, it's no longer a plate, certainly not what it was before and the behavior would be drastically changed.? Just thinking.

[Dave Cohen]

Dave, I've seen the interferometry and so forth, as you know. I hope this is related to this thread. What would happen if a crack or a narrow saw kerf were to run the entire length of the top, rim to rim lengthwise. I know the plate doesn't seem to notice a hole in it, like ff-holes or an oval hole, but the plate is still a 'whole', with a hole in it. Assuming one could build a top that would support the string tension, do you think it would still act the same being coupled only at the rim? I would think that at that point, it's no longer a plate, certainly not what it was before and the behavior would be drastically changed.? Just thinking.

In my post #106, "accelerant" should have been "accelerance". Damn software thinks it is smarter than me. Not that it's all that difficult to be smarter than me, except that the @#$ software is actually dumber than me.

Your question is one I hadn't thought about before. Can't say it is something I would want to try. What you are describing is two independent half plates. They wouldn't be completely independent, because they would be coupled at the tail and the heel of the body. Assuming you had independent half bridges to drive each half plate, you would have twin plate modes at a higher frequency, since modal frequencies depend on plate size, among other things. Not a good thing. Also, both half plates would be coupled to the same air mass in the body cavity, and also to the same back plate motion(s) through both the cavity air mass and the ribs. It is an oddity that someone might well want to try (not me!), but it opens up some cans of worms.

[Timbofood]

I am in complete accord about spell checking software!

[Jim Adwell]

Dave, I've seen the interferometry and so forth, as you know. I hope this is related to this thread. What would happen if a crack or a narrow saw kerf were to run the entire length of the top, rim to rim lengthwise. I know the plate doesn't seem to notice a hole in it, like ff-holes or an oval hole, but the plate is still a 'whole', with a hole in it. Assuming one could build a top that would support the string tension, do you think it would still act the same being coupled only at the rim? I would think that at that point, it's no longer a plate, certainly not what it was before and the behavior would be drastically changed.? Just thinking.

What a bizarre idea. I'm tempted... But how could one brace the two tops without joining them somehow other than with the rim?


EDIT: a longitudinal brace on each side at the edges nearest the center might work.

[sunburst]

What would happen if a crack or a narrow saw kerf were to run the entire length of the top, rim to rim lengthwise.

I saw such a mandolin once, many years ago. I don't think I knew who built it even then, and if I did I surely would have forgotten by now anyway (most likely). The top was made as two separate halves with about 1/4" gap between them. I didn't see any "accelerant" spectra or anything like that, but it sounded a lot like an oval hole mandolin, so whatever goes on in that top, my guess is that it is something like what goes on in an oval hole top.
It seems that anything we can think of, someone has already tried it...

One of my silly ideas is to do a similar thing but make the top 3 separate pieces with two slots about where the f-holes usually go. Maybe I'll get to that in my "spare time"... or maybe whomever has already tried it will chime in here soon and save me the trouble.

[Jim Garber]

A lot of this is flying way over my head. I know this prob has nothing to do with anything we are talking here, however... take a look at this oddball Gelas mandolin:

[Pete Jenner]

A lot of this is flying way over my head. I know this prob has nothing to do with anything we are talking here, however... take a look at this oddball Gelas mandolin:

I'd actually thought of making a bridge like that for my 12 string guitar to see if it reduced bellying.

[Astro]

As usual I can plead total ignorance on this thread. But even if this type design has no effect on tone, doesnt it at least make it easier for set ups ? Especially long term after notches get worn, sound boards settle, preferences change with different players, ect.

I have always wondered if it wouldn't be a design improvement to have the tuners and end pin hooks all lined up straight with the string spacing so that such a sharp lateral turn of the strings at both ends would be diminished. Would that effect tone, sustain, string life, tuning ease?

And btw Pete, where's the whammy bar ?

[belbein]

Mr. Jenner, I admire your creativity. I have no opinion on whether the current bridge technology is "as good as it gets" or isn't. I will just observe that the history of science teaches us that every time we say "it's as good as it gets" we find out that we left something out of our equations. And throughout history, it's been the lonely old inventor, working in his pajamas and down-at-the-heels slipper, with a half-drunk bottle of dark ale on his workbench in his dank workshop in Kiwiland, who pushes technology forward.

OK, well maybe not always in Kiwiland.

[Pete Jenner]

Mr. Jenner, I admire your creativity. I have no opinion on whether the current bridge technology is "as good as it gets" or isn't. I will just observe that the history of science teaches us that every time we say "it's as good as it gets" we find out that we left something out of our equations. And throughout history, it's been the lonely old inventor, working in his pajamas and down-at-the-heels slipper, with a half-drunk bottle of dark ale on his workbench in his dank workshop in Kiwiland, who pushes technology forward.

OK, well maybe not always in Kiwiland.

Thanks Bob but I sleep in the nude and always finish my Dark Ale. TMI?

[Bertram Henze]

Well Pete, that adds a few injury hazards to those already present in any workshop.

[Pete Jenner]

What can I say Bertram? I like to live dangerously.

[Timbofood]

Peter, don't go there, PLEASE!

[JeffD]

Is this the kind of thing that one would use COMSOL Multiphysics software to model? I mean, is enough known that an accurate acoustic model could be developed, which would predict what effect radical design changes would have?

[Bertram Henze]

Is this the kind of thing that one would use COMSOL Multiphysics software to model? I mean, is enough known that an accurate acoustic model could be developed, which would predict what effect radical design changes would have?

Any model must do two things:
- produce an audible result so we know "how it sounds",
- validate itself against a known design (e.g. an A5), so we can believe what we hear.

And the effort must be cheaper than just building wooden prototypes (some of which would have to be built anyway, in the end).
The big advantage of such a model would be that whatever the result, we would be perfectly able to explain it and thereby rule out any interesting speculation in endless Cafe threads (did I say "advantage" )...

[Dave Cohen]

Re models: Models are "good" to the extent that they "work", and they "work" if they give the same result as experimentally observed behavior.

There are two types of models possible for string instruments. Both have been done, and are occasionally revisited and refined. One type is the simplistic model, employing a few masses as oscillators. The other is the discretization of an actual instrument shape, employing powerful software such as FEM (aka "Finite Element Matrix") or BEM ("boundary Element Matrix) programs, very expensive. There is some freeware FEM out there, but it is much more limited than the pricey stuff.

The simplistic models use a single mass for each vibrating part of an instrument. The two- and three-mass models (Christenson & Vistisen, Caldersmith, etc.) were done in the late 1970s and early 1980s. The two-mass model used one mass on a spring for the top plate, and a second mass for the mass of air, also on a spring, in the soundhole region. The three-mass model used a mass for the top plate, another for the soundhole air mass, and a third for the back plate. These experienced early but limited success. The two-mass model predicted two peaks for the first or main body resonance, while the three-mass model predicted three peaks for the main body resonance. What we actually see is two main peaks in some guitars (and mandolins), three in others. Depends on whether the instrument has what Trevor Gore calls an "active back" (plate). John Popp derived a simple four-mass model in 2010. Wasn't peer reviewed, just presented at an ASA meeting. Iirc, Trevor Gore also derived a four-mass model, but didn't see profound differences from the three-mass model. Both of the four-mass models used a mass for the top plate, a second mass for the air in the soundhole region, a third for the back plate, and a fourth mass for the ribs. Imo, the limitations for both Popp's and Gore's models were in experimental data. All of the simplistic models show limitations w/ regard to higher body resonances.

FEM or BEM models break a CAD representation of an instrument into a bunch of discrete rectangular elements - the more the merrier. Each of the elements is governed by Newton's laws of motion. Each element is also "coupled" to its' neighboring elements by simple spring relationships. The overall behavior of the instrument is computed by "brute force" computational power. The problem with those models is that they need some experimental parameters for input, which are best gained from actual experiments with actual instruments. The alternative is to just try "best guess" values for those parameters, then keep iterating the values until the results converge to agreement w/ experiment. So we are back to the limitations of experiment once again. Some early discretized modeling of a classical guitar was done by Antoine Chaigne, and a few more attempts by others since then. Much more of that stuff in the violin world than for plucked strings.

[Pete Jenner]

I won't be doing any modelling. I've barely got the time to do the other measurements.

Dave, how do I measure impedance and accelerance?

Can you come here for a couple of weeks?

[sunburst]

I won't be doing any modelling.

Me neither. I'm afraid the days of that possibility are long past for the both of us...