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PostPosted: Tue Nov 19, 2013 9:55 am 
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I'm quite new and probably getting ahead of myself (finishing #2 and have begun #3). But as I start to think about tap tuning, how do you account for the mass of the bridge while tap tuning? The method of assembly I've been taught is to glue the bridge on long after the box is closed (at the end of applying the finish). So most tap tuning will have been done without the bridge on. Doesn't that change everything?

I just ordered the Alan Carruth free plate tuning DVD and I've purchased the Responsive Guitar book so perhaps the answers will be there. But does anyone wish to chime in with their method or advice?


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PostPosted: Tue Nov 19, 2013 10:40 am 
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Koa
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On my guitars it will drop the main resonant pitch of the top by roughly a 1/2 to a full semitone. But there are many variable factors; bridge mass, bracing, and top thickness to name a few. Alan's DVD and The Responsive Guitar will lead you in the correct direction.

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PostPosted: Tue Nov 19, 2013 11:32 am 
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put the back on last. it allows you to tune the top with the bridge on instead of guessing.


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PostPosted: Tue Nov 19, 2013 12:23 pm 
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arie wrote:
put the back on last. it allows you to tune the top with the bridge on instead of guessing.

So you apply your finish after you put the bridge on? Or before you've closed the back?

I struggle to get a good finish without having to work around the bridge and fretboard extension.

I wondered if it was worthwhile putting the bridge on (before applying finish) tune the top. Remove the bridge, finish the guitar, then reapply the bridge.


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PostPosted: Tue Nov 19, 2013 12:55 pm 
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Brazilian Rosewood
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IMO there is no good way to account fully for the bridge. It's fairly easy to add some mass: poster adhesive works fine, for the most part. The problem is the stiffness: the bridge is, after all, a brace. I've tried a number of things over the years, from double stick tape to simply gluing the darn thing on. None of the temporary things I tried, like tape, really did the job: they were not as stiff as a glue line. Anything that came close enough to be reasonably stiff was far from 'temporary'; I'd have problems getting the bridge off without damaging the top. Simply gluing it on at the start is a pain in the neck in any number of ways.

In the end I opted for the 'Alfred E. Neuman Solution': "What, me worry?" Actually what I found was that the mass and stiffness effects seemed to more or less cancel out, at least in terms of mode shapes, which is what I'm mostly interested in. At any rate, since this is ultimately an empirical method, looking for things in the mode shapes that correlate to some extent with the final tone, it's no harder to justify leaving out the bridge than, say, tuning the top without gluing it to the rim. The assumption is that any bridge you use will have much the same effect on the final outcome, and so long as you stick with a fairly standard bridge, it seems to be reasonably justified. I wish there were a better way, but it's hard to come up with one.



These users thanked the author Alan Carruth for the post (total 3): EddieLee (Wed Nov 20, 2013 6:01 pm) • Linus (Tue Nov 19, 2013 4:41 pm) • Tom West (Tue Nov 19, 2013 2:50 pm)
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PostPosted: Tue Nov 19, 2013 1:53 pm 
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Thank you very much for the clear, articulate reply. Even as a newbie I was able to follow along. I appreciate it.


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PostPosted: Tue Nov 19, 2013 2:12 pm 
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Linus wrote:
Thank you very much for the clear, articulate reply. Even as a newbie I was able to follow along. I appreciate it.


That's a sign of effective teaching. [:Y:]

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PostPosted: Tue Nov 19, 2013 7:36 pm 
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Alan Carruth wrote:
IMO there is no good way to account fully for the bridge....

...but some ways are better than others...

The bridge mass and stiffness makes a big difference to the sound of a guitar. For an extreme example, just swap out some plastic bridge pins for brass ones and listen to the difference (which is primarily due to the change in mass rather than anything esoteric about the nature of brass).

Also, if you have a guitar that is responsive enough for it to matter, pitching the main resonances between scale tones is pretty important, too, to avoid wolf notes.

As Alan points out, it's hard to predict on a one-off basis (which is most of my building) but relatively easy on a standard build. It also depends very much on the bracing pattern you use. For example, fan and falcate bracing offer little by way of cross grain stiffness other than that supplied by the bridge (and bridge plate) whereas X and lattice bracing provide a lot of cross grain stiffness before the bridge is applied. So for fan and lattice bracing you have both mass and stiffness to take into account when adding the bridge and the answer is that main top resonance can come out both higher and lower in frequency than before, with a good measure of dispersion. For lattice and X bracing, the bridge predominantly acts as added mass, so it's easier to predict the effect and the main top resonance usually drops in frequency.

So if you're doing "standard" or "standard-ish" builds, with a few guitars you can measure the change in top resonant frequency per added gram of bridge mass. Then you build your data and can predict out with a good degree of accuracy for future guitars. If you've got some guitars that you have already built, you can still use them in your data, a little less effectively though, but OK for X and lattice bracing. Just add a little bit of mass (a single brass bridge pin) and measure the change in top resonant frequency and, as before, draw yourself a chart of change in frequency per gram of added mass. There's some examples in the usual place.

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These users thanked the author Trevor Gore for the post: EddieLee (Wed Nov 20, 2013 6:06 pm)
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PostPosted: Wed Nov 20, 2013 2:05 pm 
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Brazilian Rosewood
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Trevor is right, of course: on the assembled guitar the bridge mass and stiffness do make quite a difference in the sound. I've measured, and heard, differences in tone from changes of as little as a couple of grams, which is about the difference between plastic and wood bridge pins. I was mostly talking about the effect the bridge has on free plate mode shapes and frequencies, which is the way I approach 'tap tones'.

In fact, even a fairly heavy or stiff bridge may make little difference in either the mode shapes or frequencies on an assembled instrument top. If the stiffness and mass are in some sense 'balanced' you could see very little change, even though the sound could be altered quite noticeably. The heavy, stiff bridge has, of course, considerably raised the impedance of the string termination, which changes both the frequency response characteristics and the amplitude of the response.

Tap tones and/or mode shapes and frequencies are useful things to know, IMO, but they don't tell you everything you might want to know. As always, to get a useful answer you need to start by asking the right question...



These users thanked the author Alan Carruth for the post: EddieLee (Wed Nov 20, 2013 6:07 pm)
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PostPosted: Wed Nov 20, 2013 5:56 pm 
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Alan Carruth wrote:
In fact, even a fairly heavy or stiff bridge may make little difference in either the mode shapes or frequencies on an assembled instrument top. If the stiffness and mass are in some sense 'balanced' you could see very little change, even though the sound could be altered quite noticeably. The heavy, stiff bridge has, of course, considerably raised the impedance of the string termination, which changes both the frequency response characteristics and the amplitude of the response.

All very true and I'll add a little more. Monopole mobility is proportional to 1/SQRT(K*m), where K is the effective stiffness of the top and m the effective mass. A heavy, stiff bridge adds significantly to both K and m and so monopole mobility reduces. Monopole mobility correlates very well with perceived loudness, so it's important to get the balance right and it points to why the bridge is so important in shaping the volume and tone of a guitar. It is far from being just a place where the strings terminate and/or a styling statement.

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These users thanked the author Trevor Gore for the post: EddieLee (Wed Nov 20, 2013 6:08 pm)
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