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Bridge blanks: Fascinating tap tones
http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=4192		 Page 1 of 1
Author:  Mattia Valente [ Wed Dec 21, 2005 1:58 am ]
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Three new threads in a day, eh? Anyway...

I'm doing one of my periodic 'inventory reshuffles' (ie, going through the stuff I have, checking there's no weird humidity/damage/whatever going on, and mostly just an excuse to play with the pretty tonewood), and I'm just sorting through my 15-odd oversized madagascar rosewood bridge blanks. These are all plenty large enough for two bridges (14-1/4" x 2" x 9/16". They're consistently sized, within about 1/8", 1/16", mostly width variation if any. They're also very pretty. And ring very much like bells. So I set about tapping 'em, and I notice the wide variety of notes I get out of them.

All are musical, clear (holding the piece lightly between thumb and middle finger at about 1/4 of its lenght) tones, but the tap tones are dinstinctly different. I don't have a scale, or I'd weigh them, but the lighter pieces seem to have a lower frequency tone than the heavier ones (the difference between the highest pitch and the lowest is 6 half-stems: highest pitch is an E, octave on 1st string, lowest is a B.). Out of the 15, 2 each are at the extremes (high and low pitch), 2 and 1 one step in from each extreme, then 5 and 3 (lower half/upper half). Slightly left-leaning normal distribution. There seems to be little to no correlation between colour and tone; all of the lowest tap tone pieces are dark and ink-lined, and the incidence of lighter coloured pieces does increase as the tap tone gets higher, but there are chunks just as dark coloured in the 'upper register'. It's also mildly entertaining that the frequencies match very, very closely. Many match exactly (might've been the same board/tree, although intra-group variability is still high), some are off by a few cents (I guess due to size differentials).

I have no clue what this means, or which ones will make better bridges, or even which bridges to use in what situation, but it's still fun stuff. But I am tempted to label 'em by tap tone now, rather than merely based on aesthetics (more ink = pretty).

Thought I'd share that impressively trivial revelation :-)
Author:  Tim McKnight [ Wed Dec 21, 2005 2:22 am ]
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HI Mattia:
I conducted an experiment at a Jam this summer and had lots of people tap fingerboards and tops and then I had a series of subjective questions that they would anser and rate each one. Each FB and top had a piece of tape placed in the center so each person was holding the wood by the tape and not the wood whicjh eliminated the holding point and grip pressure variable. I had three ebonies (East Indian, African Gaboon & Macassar) The first two were fairly high dampening and had very little sustain compared to the Macassar boards which I found interesting. I had 9 different species and the overwhelming favorite was Osage with Macassar tied with Braz for 2 & 3rd places. Pau Ferro was the highest dampeing with a very fast decay and Rock Maple was rated as the brightest tone. Osage had the most sustain. Lots of interesting data was collected.
Author:  Jim Watts [ Wed Dec 21, 2005 2:32 am ]
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Mattia,
One thing of interest to note is that within a species a denser piece of material will typically have a higher modulus of elasticity. Which I believe is what you've discovered here as you noted the "lighter ones have lower frequency"
Your right, interesting stuff!
Author:  Mattia Valente [ Wed Dec 21, 2005 2:44 am ]
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[QUOTE=Jim_W] Mattia,
One thing of interest to note is that within a species a denser piece of material will typically have a higher modulus of elasticity. Which I believe is what you've discovered here as you noted the "lighter ones have lower frequency"
Your right, interesting stuff![/QUOTE]

That means the denser ones are more elasitc/stiffer, So the lighter, lower frequency ones are less elastic/less stiff than the others, right? Care to speculate what this means in terms of sound per bridge?

Tim: interesting stuff! Did you have several samples per wood, or just the one? I think the thing that struck me here was the fairly huge variation between the pieces (in terms of pitch, not so much the nature of the ring, which was loud, musical, sustained). I'm fairly unsurprised at the Ebony results; none of the ebony fingerboards (gaboon, all of them) I've got ring worth a dang. The macassar ebony back/side sets, however, have a pleasing resonance to them. Mattia Valente38707.4492361111
Author:  Brock Poling [ Wed Dec 21, 2005 4:40 am ]
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Incidentally Jim... I really like that logo you are using in your avatar. I presume that is your headstock logo?

Author:  Alan Carruth [ Wed Dec 21, 2005 1:07 pm ]
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Are all of those blanks _exactly_ the same size? For a direct comparison of pitch to mean anything they would have to be, since stiffness varies as the cube of the thickness. Much the same goes for the length; if they aren't the same direct comparisons don't mean too much. There are ways of testing rectangular blanks like that so that you get numbers that can be plugged into a spreadsheet to find things like Young's modulus and damping factor.

Once you've got those numbers it's not entirely clear what they mean, though. Offhand you'd think that low damping would be a good thing, and most poeple tend to agree on that, but without a lot of real data to back it up. Stiffness and density are harder to specify, though. After all, you want the bridge to have a certain amount of both of those, if only so the string will know how long it's supposed to be.

I'm not trying to discourage anybody from developing data to guide them in the construction process; quite the contrary. I am suggesting that the data you've got there is sort of rough, and could be refined. Then maybe we could figure out what some of this stuff means.   
Author:  Jim Watts [ Wed Dec 21, 2005 4:53 pm ]
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Alans absolutly right about the thickness needing to be identical and this why I always contend that flexing a top is no way to judge initial stiffness. A little change in thickness goes a long way! Length and width are linear however and not as critical, they do matter however.
Too many factors to speculate, just pick the one that sound the most musical to you. I've found that the bridge weight can have an important impact on the overall sound, as I'm sure you know. Any way just keep track of all stuff and some day we'll know what all the data means (maybe).
Brock thanks for your compliments on the logo. it is my head stock logo. Here's a pic.
Author:  Mattia Valente [ Wed Dec 21, 2005 7:06 pm ]
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[QUOTE=Alan Carruth] Are all of those blanks _exactly_ the same size? For a direct comparison of pitch to mean anything they would have to be, since stiffness varies as the cube of the thickness. Much the same goes for the length; if they aren't the same direct comparisons don't mean too much. There are ways of testing rectangular blanks like that so that you get numbers that can be plugged into a spreadsheet to find things like Young's modulus and damping factor.[/quote]

No, they're not all exactly the same size; they're within less than a mm in length and thickness terms, vary a little more in width, although the majority, again, are within 1mm of the same. There also wasn't any correlation between slightly varying size and frequency (ie, not all of the slightly smaller ones, which was about 4 of them, were skewed towards one end of the spectrum)

I'm tempted to wait for the other 16 I've got coming in a few weeks, clean them all up a little (so they're exactly the same size, or as close as I can get them) and do some slightly more definitive testing (ditto with the equal number of fingerboard blanks I've got).

[quote]
Once you've got those numbers it's not entirely clear what they mean, though. Offhand you'd think that low damping would be a good thing, and most poeple tend to agree on that, but without a lot of real data to back it up. Stiffness and density are harder to specify, though. After all, you want the bridge to have a certain amount of both of those, if only so the string will know how long it's supposed to be.

I'm not trying to discourage anybody from developing data to guide them in the construction process; quite the contrary. I am suggesting that the data you've got there is sort of rough, and could be refined. Then maybe we could figure out what some of this stuff means.   [/QUOTE]

Indeed. And this is sort of a simpler thing to test than a fully braced top or back. I mean, I doubt deflection testing on pieces this short and thick is going to yield much in the way of easy-to-evaluate figures, for example. I'll definitely get some firmer numbers together when I find some time to do it. Interpreting their effects on instruments I build (being an amateur with practically no experience) is going to be a bit tougher.
Author:  Tim McKnight [ Thu Dec 22, 2005 2:14 am ]
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[QUOTE=Jim_W] Alans absolutly right about the thickness needing to be identical and this why I always contend that flexing a top is no way to judge initial stiffness. A little change in thickness goes a long way! Length and width are linear however and not as critical, they do matter however.
Too many factors to speculate, just pick the one that sound the most musical to you. I've found that the bridge weight can have an important impact on the overall sound, as I'm sure you know. Any way just keep track of all stuff and some day we'll know what all the data means (maybe).
Brock thanks for your compliments on the logo. it is my head stock logo. Here's a pic.[/QUOTE]

Not to hyjack the thread but man-o-man is that one elegant headstock design and logo! Even though it is a bit "Ryan-esque" I still think it works well.
Author:  Jim Watts [ Thu Dec 22, 2005 6:49 am ]
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Thanks very much Tim.
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