Tap tuning as a rough guide when shaving the braces

Construction and repair of Classical Guitar and related instruments
Rosewood
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Tap tuning as a rough guide when shaving the braces

Post by Rosewood » Tue Jan 02, 2018 4:48 pm

I’m currently shaving down the braces, and thought I’d try to pay more attention to tap tuning this time around. Roughly what sort of frequency should I be aiming for?

I realise there’s lots of variables and unaccounted factors in tap tuning a free plate, but I’m not expecting anything too precise/scientific and I only want to try it as a rough guide.

To confirm, the bridge is not glued on, and I am holding the top with my thumb and index finger between the soundhole and join, and tapping in the centre at the bridge position.

Jon Gillard
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Re: Tap tuning as a rough guide when shaving the braces

Post by Jon Gillard » Tue Jan 02, 2018 7:26 pm

I had this conversation with Kevin Aram, and expected a long and complex tap tuning explanation, only to hear he does no tapping, but relies purely on the change in flexibility as material is removed. I suppose we all have to find our own way to the end point.
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Rosewood
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Re: Tap tuning as a rough guide when shaving the braces

Post by Rosewood » Tue Jan 02, 2018 7:52 pm

I’ve tried using flexibility on a previous couple of soundboards, but only the ‘feel’ in my hands as I never attached any actual numbers using deflection testing. I guess it’ll come with experience, but I don’t think I’ve yet got the ability to feel the flexibility and confidently know if it needs more material removing (or, regrettably, less!). I don’t want to get so absorbed in numbers so that I’m ignoring everything else, but I was hoping that tap tuning would help identify the rough ballpark and then I can associate how the flexibility feels at particular frequencies.

amezcua
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Re: Tap tuning as a rough guide when shaving the braces

Post by amezcua » Tue Jan 02, 2018 8:30 pm

Would it be safer to leave plenty of wood in place and do the tapping after the top is attached? Or if you glued the top with a weak hot hide glue without the bindings you could see how the taps sound . Hide glue is very obliging in that way . If you write down everything possible as a record beforehand you might use that as a safety net or indicator . Allow for a change when the final finish is applied . That`s easier to calculate. If you have long slim hands practice planing through the hole ? I always wonder how the internal volume is worked out too , plus what exact size the hole should be. And what happens with the rosette already glued on . Basically this is what making a guitar is all about rather than French Polishing. That bit is very beneficial for sound but it would make me even more nervous . See what every other maker has ever written about it first .
Or--Make another guitar shaped box --make a copy of the top as it stands and then fit a bridge on the inside and try out braces on the outside . Shave them or move them when you like .That will be your test box .

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Beowulf
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Re: Tap tuning as a rough guide when shaving the braces

Post by Beowulf » Tue Jan 02, 2018 10:16 pm

John Bogdanovich describes adjusting the braces in his book on classical guitar making. He notes two techniques:

Selecting braces by relative stiffness and then using the stiffest at the centre of the fan and then graduating to the least stiff at the outer edges. Scalloping is done by testing with tap tones at the bridge area and continuing to scallop until the tone is right on a note (any note) from a chromatic tuner.

The back braces are scalloped to produce three distinct tones just above each brace.

Though not a luthier myself, I think his book is excellent and well worth a read
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bftobin
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Re: Tap tuning as a rough guide when shaving the braces

Post by bftobin » Wed Jan 03, 2018 2:07 am

If there was a tried and true 'tap tuning' method, every guitar would be world class. A better way of looking at this would be 'voicing' the soundboard.
You can find some info on YouTube. If you follow a really good drawing, you will make a good guitar. Then, as you make more, keep flexing and tapping.
Keep a record of the changes that you make and you should develop your own 'tap tuning' method.
Then people will be asking YOU how to 'tap tune'.

Rosewood
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Re: Tap tuning as a rough guide when shaving the braces

Post by Rosewood » Wed Jan 03, 2018 10:42 am

Thanks for the suggestions. Most of the books I've read that reference tap tuning seem to offer a more comprehensive approach - at least further than I’m looking to go for now.

I was more hoping to use it so that when I start taking down the braces I can every so often measure the peak tap tone, and, when I get a certain number (e.g. XXX Hz), I know that I’m roughly getting into the right zone where I want to slow things down and pay more attention.

Alan Carruth
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Re: Tap tuning as a rough guide when shaving the braces

Post by Alan Carruth » Wed Jan 03, 2018 6:32 pm

I've been using the Tech' version of tap tuning, Chladni patterns, for a long time. From what I can tell the actual frequencies of the modes on the 'free' plate seem less important than the shapes of the patterns. If you think about it, the frequencies of the modes are a rough indication of the ratio of stiffness to weight of the top. What we're trying for, broadly, is 'the right' stiffness with as little weight as possible. A lighter weight top will have a higher tap tone pitch at the correct stiffness than a heavier one, all else equal. Of course, all else is never equal.

If you don't want to go the whole route to using Chladni tests you can still learn some things simply by tapping. As a general rule, you want to adjust the bracing to produce 'clear' tap tones: with a more or less well defined pitch and long ring time. You also want to get a lot of different tap tone pitches when you tap and hold in different places. Some makers try for particular pitches, such as getting the tap tone on specific scale degrees, or into relationships, such a clear thirds or fifths. I'm not so sure about that.

The best thing you can do is to get as much information as you can and write it down. Deflection testing is reasonably easy to do, and can tell you when you have the 'right' stiffness, although it may take a few tries to figure out what that is for your builds. Similarly tap tones can help you figure out when the top is working well as a system. Noting the weight of the finished top and back is also good. Keeping track of the deflection numbers and the tones you hear, and correlating that with how the top flexes in your hands in different ways, gives you an awful lot of information. You might also want to note some set-up information, such as how high the strings are off the top, and how far the bridge rotates forward when you put on the strings. I've never been sorry over having too much information, but it's a drag when you don't know something that turns out to be important. I keep my data for each guitar in a folder, so that I can go back and look things up. It's a real help when something turns out very well, or very badly. It takes a little time, but it's worth it in the long run.

Douglass Scott
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Re: Tap tuning as a rough guide when shaving the braces

Post by Douglass Scott » Wed Jan 03, 2018 10:32 pm

Like Al and others have suggested, there isn't a definitive goal when tap tuning, or deflection testing, or to any other way of monitoring your progress for that matter. When the top is free only data from your own previous instruments will help you. Once the guitar is completely assembled you can start comparing resonant frequencies etc. with other instruments.

While shaving braces measure and record your progress in as many ways as you can - for example 1) flex the top in your hands, 2) record pitches when the top is suspended and tapped in different spots, 3) measure deflection, 4) Chladni patterns... The more ways you pay attention to changes the fewer guitars it'll take for you to learn what's going on and how certain results on a free top translate to the finished instrument.

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Trevor Gore
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Re: Tap tuning as a rough guide when shaving the braces

Post by Trevor Gore » Fri Jan 05, 2018 2:59 am

Rosewood wrote:
Tue Jan 02, 2018 4:48 pm
I’m currently shaving down the braces, and thought I’d try to pay more attention to tap tuning this time around. Roughly what sort of frequency should I be aiming for?

I realise there’s lots of variables and unaccounted factors in tap tuning a free plate, but I’m not expecting anything too precise/scientific and I only want to try it as a rough guide.

To confirm, the bridge is not glued on, and I am holding the top with my thumb and index finger between the soundhole and join, and tapping in the centre at the bridge position.
The resonant frequencies of the free plate are very poor indicators of any resonant frequency in the completed guitar - which are ultimately what you listen to. The reason for that is that unless you know a whole lot about what you are going to glue to the top and the acoustic performance of those parts, you can't, in practice, determine with any precision where the resonances on the finished guitar will fall. The back, the air in the box, the side mass, the sound hole size, the mass and stiffness of the bridge (and more) all affect the resulting coupled resonant frequencies in the finished instrument.

I've had more predictable results by working to the measured material properties of the top and bracing to put me in the ball park and then using the coupling effects of the other components to trim the resonant frequencies to where I want them.
Trevor Gore: Classical Guitar Design and Build

amezcua
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Re: Tap tuning as a rough guide when shaving the braces

Post by amezcua » Fri Jan 05, 2018 10:30 pm

If you asked one current violin maker who has won several prizes for his instruments he might sound cagey about what makes a great instrument. But his basic attitude is not concentrated on single items and his usual answer is ---"It`s everything". He was always thorough and tested all the glues and materials in detail . Just do as many things as you can and hope for the best since it`s all so unpredictable .
How about a stiff outer frame in plywood to mimic the restriction that sides and back would create. If that gives a useful final result it will be worth repeating . That would not need any glue if you clamped the top in the frame .

printer2
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Re: Tap tuning as a rough guide when shaving the braces

Post by printer2 » Fri Jan 05, 2018 11:39 pm

On AGF a student showed his setup for tapping the tops, his results were a bit skewed as he had a heavy frame and hardware to hold the tops. Much heavier than the body of the actual guitar. I am thinking of doing the same thing but with a lightened structure. Will it be better? Who knows.
Fred

amezcua
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Re: Tap tuning as a rough guide when shaving the braces

Post by amezcua » Mon Jan 08, 2018 10:47 pm

The first top will have a difference in tone and frequency using a frame. Then you can compare that with the sounds after making it into a guitar. After that you will know a bit more about what the target is . You might dispense with the frame afterwards or use it when you start making lattices .
Part of your question was the note to aim at . I read one recommendation last week of F# for Flamenco and G for Classical .
Which way will the note move when you clamp the edges ? And how much ? If you shave near the edge the note will drop . That`s the main note at the bridge There will be other notes in different places depending where you tap . But there is still going to be a main note .The other areas can be changed and they might not affect the main one. One violin maker has a theory that plates can be tuned to give a sort of chord ,or at least not a dischord . Area Tuning is the theory title. It`s not so far fetched as some would claim . It`s going to be a musical instrument and all those parts will vibrate and be heard .

amezcua
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Re: Tap tuning as a rough guide when shaving the braces

Post by amezcua » Thu Jan 11, 2018 5:56 pm

There is a discussion about area tuning on a violin site . One maker dismisses it because he believes the plates should have a certain structure . He is talking about arching . This is a weak objection as the tuning areas can be adjusted to give the right notes with a scraper. You can do that after a violin is constructed before varnishing and you will not notice where those scraped areas are Visually it makes no difference and certainly will not interfere with the structure . The idea came from examining many famous Italian violins and noticing certain makers used their own patterns of tuning . The other way is to shove every guitar top through a sander --Job done! Maybe not a great guitar but quick and easy . You pays your money and you takes your choice .

Alan Carruth
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Re: Tap tuning as a rough guide when shaving the braces

Post by Alan Carruth » Fri Jan 12, 2018 12:09 am

I looked into the relationships between plate mode frequencies and arch shapes back when I wrote the series of articles on 'free' plate tuning for 'American Lutherie' magazine back in '91-'92. It's impossible to cover all the bases in a short series like that, but the principles are clear.

Basically, arched plate stiffness is a function of the arch height, arch shape, and wood thickness, along with the actual properties of the wood, of course. Curvature of the plate surface adds bending stiffness, as we all know: just roll a sheet of paper into a tube and try to bend it. Differing arch shapes alter the frequency relationships between the low order modes somewhat. Overall arch height primarily affects the frequency of one particular mode. Changing the thickness of the plate in a specific spot can change things too, depending on whether a given resonant mode is bending in that spot or moving a lot without bending much. Local changes in thickness of small areas will most likely tend to change higher-order modes that vibrate as groups of smaller areas, and this can be difficult to see or hear directly, although it almost certainly affects the timbre of the instrument. The one brace that's used on the violin, the 'bass bar', seems mostly to compensate for the stiffness that's lost in the top when the f-holes are cut in, at least, in terms of the low order resonant modes. Any adjustment you make in this system will alter the timbre in some way, but the relationships are complex.

None of this is particularly relevant to most Classical guitar makers or players. Although there are Classical guitars with carved arched backs, and even a few with carved arched tops, they are rare. It's difficult to make an arch top Classical that has a timbre that works well for the standard repertoire, and since it's a lot of extra work, and Torres didn't do it, not many people make them. ;)

Although most Classical guitar tops are domed to some extent, it's not really enough of a curve to make much difference in the way the top vibrates. Classical guitar makers do adjust local thickness of the top to some extent, but most of what violin makers do with arching we do with bracing.

Keep in mind, too, that on violins the tension of the strings is taken up by a tailpiece that attaches to the end block; the bridge is pretty much simply down loaded, and the leverage of the neck is carried by the entire top and back, which are relatively much thicker than guitar plates. I can't recall ever seeing a successful Classical guitar that did not have a fairly substantial transverse brace on the top above the sound hole to help keep the neck from pitching forward. I have read of various 'flying brace' schemes that do away with the upper transverse brace, but these have not caught on widely. That brace, in itself, has a major effect on the way the top of the guitar works acoustically, producing a much different appearing set of resonant modes from those seen on an arched plate, and altering the significance of the various modes.

All of which is just to say that violins and guitars are vastly different systems, and it's risky using one as a guide to the other. I do find 'area tuning' of violin tops to be interesting in some ways, and see how some aspects of it might transfer to the guitar. However, the way I understand the physics of plate motion doesn't agree well with the subjective descriptions I've read in the articles I've seen on area tuning. It's often very difficult to translate that sort of 'tacit knowlegde' into 'explicit knowledge' that can pass physical muster. They're hearing something, and claim to have found ways to use what they're hearing to control the tone of their instruments, but the claims are hard to establish with any rigor, and the causal chains are unclear.

Of course, much the same can be said of any system the purports to control the tone of a guitar.... ;)

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