Tap tuning as a rough guide when shaving the braces

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

Post by amezcua » Sun Jan 14, 2018 10:33 pm

I suppose there is some sound effect coming from the top itself . That is to separate the top from the bracing. Is any tuning done to the bracing or is most of the tuning done to the bracing ? Is a brace mainly or partly for tuning or for strengthening ? Just make the top standard and adjust the braces ? What I am curious about is how on earth can a lattice braced double top etc be tuned ? The lattice tops are like wet lettuce on their own. Or maybe that`s a left over impression from how people describe them .

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

Post by printer2 » Mon Jan 15, 2018 3:53 am

Bracing is for strength, they allow for the top to be lighter than if it were a single plate dimensioned for the same stifness/strength. The braces have a limited effect on the low frequency response. Courtesy of Trevor,

Image

The guitar is going to vibrate as physics and the guitar shape allows more than what we do to try and tell it what to do. The bracing can nudge things around a little for the lower end, a bit more for the midrange and some for the highs. It really depends what you are working with and how much modification you are doing.

The lattice top retards the breaking up of the top into individual section (as we go up in frequency) making it behave like a piston longer. Other than that I am not too familier with the upper range of the lattice top.
Fred

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

Post by Alan Carruth » Mon Jan 15, 2018 6:05 pm

IMO the frequency range where the top transitions from the monopole to higher order modes is more a function of the brace profiles and the way they're distributed than the actual scheme. Most lattice tops use what is often called a 'tapered' profile, where the braces are tallest at the bridge and become lower toward the edges of the top. That's one extreme in a continuum, with the other end of the line being 'scalloped' bracing, which is lowest near the bridge and usually has higher peaks near the edges. In between is 'straight' bracing, which is actually approximated often by the Fan bracing in a Classical guitar.

In addition, I'll note that, at least in the Smallman design, the top itself hardly contributes and stiffness at all: it's simply a membrane that fills in between the lattice elements to move air. In that case things like the density and long- to cross-grain stiffness ratio of the top wood, which matter on a 'normal' top, hardly enter into consideration. This also alters how the top will transition from the monopole to the low order multipoles and eventually the higher order stuff. All of this influences the timbre of the guitar, of course.

Differing stiffness can also be obtained by the way the bracing is distributed. Fan bracing, such as Torres used (but did not invent) generally converges in front of the bridge, making the top stiffer in what seems to be the critical area between the bridge and sound hole. Altering the fan angle can fine tune the timbre. Flamenco guitars tend to leave out the outer two fans, decreasing the cross grain stiffness of the top , and lowering the pitch of the cross dipole mode relative to the monopole. This alters the timbre in a way that may help the sound to 'cut' through in noisy settings. The mode frequencies shown for the Gore guitar are much more typical of steel string instruments, with X bracing, where the higher cross stiffness raises the cross dipole pitch. This produces a 'fuller' timbre, I think, with less emphasis on the high frequencies. Steel strings already have lots more energy in the high frequencies than nylon or gut strings do, and so are built to bring out a different part of the spectrum.

I strongly suspect that the Gore guitar shown actually has his 'falcate' brace pattern, which is not common, and would act much more like a carved arched top than a normal Classical guitar in the Torres pattern, or an X-braced steel string. I will also note that in theory you will not see the same mode at more than one pitch; if you do it's a sign that it's 'coupling' with other parts or resonances. In the case shown, the three 'monopole' modes are evidence of coupling between the top, the internal air, and the monopole mode of the back. In a case like that it's not strictly correct to speak of any of the observed patterns as a resonant mode of the top. All of this may seem like nit picking to players, but it does make a difference how you understand the way the guitar works. Think of it as being on the level of a discussion of rest stroke and free stroke in playing. ;)

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

Post by prawnheed » Mon Jan 15, 2018 6:33 pm

Alan Carruth wrote:
Mon Jan 15, 2018 6:05 pm

... I will also note that in theory you will not see the same mode at more than one pitch; if you do it's a sign that it's 'coupling' with other parts or resonances. In the case shown, the three 'monopole' modes are evidence of coupling between the top, the internal air, and the monopole mode of the back. In a case like that it's not strictly correct to speak of any of the observed patterns as a resonant mode of the top.

...
Very interesting. One question: Is it not always the case that the coupling between top, internal air and back/sides are always coupled such that all the resonances are really only relevant in the assembled guitar? Or is there enough of a correlation for tuning of the top alone to be meaningful?

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

Post by amezcua » Mon Jan 15, 2018 6:57 pm

Good question prawnheed and in the guitar diagrams I assume all those odd shapes will happen even if a top is just fed through the (mincer) er sander . (My little joke ). In the violin forum the mention of area tuning brought a wary comment in case it was treading on toes and revealing trade secrets . That gave me an uneasy queazy feeling . Do guitar makers have that secretive side as well ?

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

Post by printer2 » Tue Jan 16, 2018 1:44 am

Some are protective on how they do things. You would be hard pressed to find a picture of the bracing of a Somogyi guitar or of one of his apprentices. On the resonances, if you adjust your top for the same target as you had in a previous guitar and you build the back to match the resonance of the previous one you would may get the same response when the box is closed. At least on the low end.
Fred

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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 16, 2018 3:19 am

amezcua wrote:
Mon Jan 15, 2018 6:57 pm
Good question prawnheed and in the guitar diagrams I assume all those odd shapes will happen even if a top is just fed through the (mincer) er sander . (My little joke ). In the violin forum the mention of area tuning brought a wary comment in case it was treading on toes and revealing trade secrets . That gave me an uneasy queazy feeling . Do guitar makers have that secretive side as well ?
Well Santos Hernandez was certainly "secretive", as he did not want to train apprentices...oops, I mean competition. Had he done so, "Hernandez" might have become as widely known as "Ramirez". :wink:
1971 Yamaha GC-10 (Hideyuki Ezaki)
2017 Yamaha GC82S (Akio Naniki/Naohiro Kawashima)

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

Post by Alan Carruth » Tue Jan 16, 2018 7:16 pm

There is not a lot of secretiveness among American makers, but I'm told the Europeans do tend to be more so.

prawnhead:
There are two things to keep in mind about 'free' plate tuning. The first is that 'free' plate mode frequencies are only of very limited utility in predicting the assembled modes of the guitar. If you make two instruments that are 'the same': use the same wood, patterns, thicknesses, and so on, and the 'free' plates end up having the same mode frequencies, then the assembled instruments will have the same frequencies in the lower order modes as well. If you make any changes, such as using rosewood for the B&S of one guitar and, say, walnut, for the other, the modes will be different. This depends on how different the woods themselves are: if you match the density and stiffness the results will be more similar.

Second: the low order mode frequencies of the assembled guitar are very far from being the whole story. They establish the basic character of the tone, but may not tell you much about the quality. In other words, you can't tell just from those low order modes whether you have a good guitar or not. Most of the difference in quality seems to have to do with what goes on at higher frequencies, and that is both harder to control and harder to measure. My argument is that the high frequency behavior may be related to the free plate mode shapes, and that the utility of 'free' plate or 'tap tone' tuning has to do with that. But, again, it's hard to look at what's going on in that range, so it's difficult to demonstrate any links.

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

Post by prawnheed » Tue Jan 16, 2018 8:00 pm

Alan Carruth wrote:
Tue Jan 16, 2018 7:16 pm
There is not a lot of secretiveness among American makers, but I'm told the Europeans do tend to be more so.

prawnhead:
There are two things to keep in mind about 'free' plate tuning. The first is that 'free' plate mode frequencies are only of very limited utility in predicting the assembled modes of the guitar. If you make two instruments that are 'the same': use the same wood, patterns, thicknesses, and so on, and the 'free' plates end up having the same mode frequencies, then the assembled instruments will have the same frequencies in the lower order modes as well. If you make any changes, such as using rosewood for the B&S of one guitar and, say, walnut, for the other, the modes will be different. This depends on how different the woods themselves are: if you match the density and stiffness the results will be more similar.

Second: the low order mode frequencies of the assembled guitar are very far from being the whole story. They establish the basic character of the tone, but may not tell you much about the quality. In other words, you can't tell just from those low order modes whether you have a good guitar or not. Most of the difference in quality seems to have to do with what goes on at higher frequencies, and that is both harder to control and harder to measure. My argument is that the high frequency behavior may be related to the free plate mode shapes, and that the utility of 'free' plate or 'tap tone' tuning has to do with that. But, again, it's hard to look at what's going on in that range, so it's difficult to demonstrate any links.
That makes a lot of sense. Is it not possible to produce the same kind of Chladni patterns for the higher modes and frequencies? One may still not be able to tell what the guitar would actually sound like, but a comparison between the free plate and assembled guitar patterns would maybe reveal something. Or do the "patterns" just dissolve into a undiscernable mess at the interesting frequencies?

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

Post by amezcua » Tue Jan 16, 2018 8:34 pm

This is a very valuable source of knowledge . Is it possible to tidy up the business of flexing the top ? I thought of a leather tension strap across the widest part . A stiff support along the centre and a weight suspended on the strap loop beneath. So for a safe deflection from straight you could measure how far it bends with that exact weight.Comparisons would be more reliable than using your hands . The thickness could be from the Tatay advice ; Centre 1.95mm and Edges 1.35 (Dime and Nickel ). I hope I got that right way round .I`m not familiar with those coins .
My Tatay is remarkably light in weight compared to other guitars . Is there any advice about the benefits of lightness while the guitar is being thinned down ? I suppose the weight of every part should be recorded too .

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

Post by amezcua » Wed Jan 17, 2018 10:52 am

After sleeping on the strap idea it seems that bending the top plate across the width is going 90 degrees the wrong way . Surely the important direction is from the sound hole to the bottom block. The bending in a guitar matches the forces that pull a bridge off . So how about a clamp at the bridge position with a bar poking out to attach a weight to measure deflection ? The plate would need to be vertical and upside down I suppose . Gravity would act in the string pull direction . So we would be in the area of design belonging to the braces , which is stiffening .
In a way bending sideways is the area belonging to early ladder braced guitars . Torres chose to stiffen the top in a modern way to counteract bridge pulling forces . A normal bridge is wide and narrow which makes the top stiffer sideways. Torres left that alone and just stiffened the insides. How would a pair of separate vertical bridges with a cross piece work ? A H bridge ? Nice and bendy sideways . Plenty of stiffness endways. Differences in bass and treble could increase with a less rigid centre . Plenty of scope for experiment there . With two separated centres of forces there could be positive and negative interference as in bowed instruments which would increase tonal complexity . And still no need for any sound post . So still a guitar sound . Maybe even throw away all the braces .

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

Post by amezcua » Wed Jan 17, 2018 11:41 am

I just looked up Guitar H Bridge and found a Hahn bridge made of solid steel . ( For solid body guitars ). Unfortunately it looks so rigid between the two feet it seems to cancel out any benefits . In an acoustic / classical guitar the flexing bar ( raised slightly above the surface ) between the two feet should give a tonal change . Mechanically it would reduce the constant worry of a bridge ripping lumps off a delicate spruce top . It`s called a bridge at present . What kind of bridge stays in contact all the way across without a gap beneath it ? A guitar bridge .

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

Post by Alan Carruth » Wed Jan 17, 2018 6:51 pm

prawnhead:

There are several reasons why it's hard to get useful information on the higher order modes. The most basic is simply the power required to generate Chladni patterns at higher frequencies. As you go up the plates tend to break up into smaller and smaller vibrating areas. In order to generate a pattern you need to get enough amplitude so that the glitter (or whatever) is actually bounced off the plate. Getting that kind of acceleration on a small area takes a lot of power. This is especially true since we use a loudspeaker to drive the plates, avoiding adding any sort of load from a more direct driver. The coupling between the speaker and the plate is not all that efficient, so there's a lot of wasted power. In other words, IT GETS LOUD.

Another issue is that these resonances don't just show up at one frequency, but can be driven over a more or less broad band. When we speak of, say, the 'main top' resonance as happening 'at' 200 Hz, in fact it can usually be driven fairly effectively between, say, 195 and 205 Hz: 200Hz is just the peak. If there is another part of the guitar that has a resonance that's active anywhere within that band it will 'couple' with the 'main top' mode, sharing energy. This actually modifies the frequencies of both modes. As you go up in frequency there get to be a larger number of vibration modes of the different parts of the guitar. Somewhere around 600-800Hz or so there get to be so many that they overlap; you hit a 'resonance continuum'. At that point it is impossible to say what causes the shape you see: it's simply the whole instrument cooperating (or not).

There are ways to look at what's going on in the higher frequency regime, but they are tricky. Martin Schleske in Germany holds a PhD. in physics and a Master's rating in the violin maker's guild. He uses a laser setup to help him in making 'tonal copies' of fine violins. The laser is first used as a tool to measure the exact shape of the violin, so that he can copy that. It can also be used as a 'Doppler interferometer' to see how each point on the violin vibrates at different frequencies. A computer program is used to sort out the 'operational deflection shapes' into likely resonant modes of the parts, and he tunes these to match as closely as possible to the model instrument. He says that the output of the computer depends strongly on what you tell it to expect: if you think there will be a strong resonance at, say, 2410Hz, it will give you one. If you decide it's more likely to be at 2250Hz, it will change everything else and give you that. His copies are said to sound remarkably similar to the originals, but not exactly the same. It's a difficult problem...

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

Post by prawnheed » Wed Jan 17, 2018 7:23 pm

Alan Carruth wrote:
Wed Jan 17, 2018 6:51 pm
prawnhead:

There are several reasons why it's hard to get useful information on the higher order modes. The most basic is simply the power required to generate Chladni patterns at higher frequencies. As you go up the plates tend to break up into smaller and smaller vibrating areas. In order to generate a pattern you need to get enough amplitude so that the glitter (or whatever) is actually bounced off the plate. Getting that kind of acceleration on a small area takes a lot of power. This is especially true since we use a loudspeaker to drive the plates, avoiding adding any sort of load from a more direct driver. The coupling between the speaker and the plate is not all that efficient, so there's a lot of wasted power. In other words, IT GETS LOUD.

Another issue is that these resonances don't just show up at one frequency, but can be driven over a more or less broad band. When we speak of, say, the 'main top' resonance as happening 'at' 200 Hz, in fact it can usually be driven fairly effectively between, say, 195 and 205 Hz: 200Hz is just the peak. If there is another part of the guitar that has a resonance that's active anywhere within that band it will 'couple' with the 'main top' mode, sharing energy. This actually modifies the frequencies of both modes. As you go up in frequency there get to be a larger number of vibration modes of the different parts of the guitar. Somewhere around 600-800Hz or so there get to be so many that they overlap; you hit a 'resonance continuum'. At that point it is impossible to say what causes the shape you see: it's simply the whole instrument cooperating (or not).

There are ways to look at what's going on in the higher frequency regime, but they are tricky. Martin Schleske in Germany holds a PhD. in physics and a Master's rating in the violin maker's guild. He uses a laser setup to help him in making 'tonal copies' of fine violins. The laser is first used as a tool to measure the exact shape of the violin, so that he can copy that. It can also be used as a 'Doppler interferometer' to see how each point on the violin vibrates at different frequencies. A computer program is used to sort out the 'operational deflection shapes' into likely resonant modes of the parts, and he tunes these to match as closely as possible to the model instrument. He says that the output of the computer depends strongly on what you tell it to expect: if you think there will be a strong resonance at, say, 2410Hz, it will give you one. If you decide it's more likely to be at 2250Hz, it will change everything else and give you that. His copies are said to sound remarkably similar to the originals, but not exactly the same. It's a difficult problem...
Very interesting. I imagine a speckle pattern interferometer is beyond the budget of most guitar makers and, as you've confirmed, would probably only yield what we already knew i.e., it's complicated.

One day, I'm sure someone with enough programming skills will come up with a machine learning model that will be able to look at the free plate intereferometry results and predict how the guitar will sound, but we are not there yet.

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

Post by printer2 » Wed Jan 17, 2018 9:28 pm

Worth a quick look, I think I posted it here before but my memory is hazy about it.

https://sites.google.com/site/classicalguitardesign/
Fred

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