My first Chladni patterns

Construction and repair of Classical Guitar and related instruments
Costantino Proietti
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My first Chladni patterns

Post by Costantino Proietti » Fri Apr 05, 2019 9:46 pm

I took note of the modes I found on my Engelmann braced top. It seems to me looking at the modes at 210, 246,290 hertz, that could be some crosswise stiffness at the center of lower bout. It can be an overbuilt central brace. So my idea is to carve a little this brace. But as the top is quite light I will proceed with extreme caution. Any sugggestion?
Ps : sorry for the bad drawings. I post a pictures of 115 hertz mode just for curiosity as it tells me nothing.
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A fact is the most obstinate thing in the world. Bulgakov

Marshall Dixon
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Re: My first Chaldni patterns

Post by Marshall Dixon » Sat Apr 06, 2019 1:00 am

Hello Constantino,

Good work. I can see how this will be useful. Please tell me what kind of speaker you used and what you used for signal generation and amplification. I'm wondering if I have enough technology sitting around to be able to do it.

Thanks,
Marshall

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Trevor Gore
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Re: My first Chaldni patterns

Post by Trevor Gore » Sat Apr 06, 2019 1:57 am

Marshall,

As a minimum you need a sig gen (numerous free and pay apps for phones/laptops) a power amp and a 4" speaker. If you want to go more exotic, here's a way to do it.
Trevor Gore: Classical Guitar Design and Build

Marshall Dixon
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Re: My first Chaldni patterns

Post by Marshall Dixon » Sat Apr 06, 2019 3:54 am

Trevor Gore wrote:
Sat Apr 06, 2019 1:57 am
Marshall,

As a minimum you need a sig gen (numerous free and pay apps for phones/laptops) a power amp and a 4" speaker. If you want to go more exotic, here's a way to do it.
Thank's Trevor. I'll look for a decent speaker second hand. I have Audacity on my computer that I think will work as the sig gen.

Costantino Proietti
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Location: Rome

Re: My first Chaldni patterns

Post by Costantino Proietti » Sat Apr 06, 2019 4:04 am

I use the speaker in the picture, I don't know if it is the right one as it becames hot, but for now it works. For a signal generator I use an app for my smartphone ( Simple tone generator) and an amplifier with a microphone jack.
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A fact is the most obstinate thing in the world. Bulgakov

Frederich
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Re: My first Chladni patterns

Post by Frederich » Sat Apr 06, 2019 4:42 pm

It looks cool. But after the coffee goes and you’ll glue the bridge, and glue the top to the sides and then, glue a fingerboard down to the sounhole, I am wondering what’s left from these cool paterns ? The same thing happens to the carved plates of a cello, violin etc. Some luthiers are tuning the plates (top and back) to a specific frequency and a specific interval between the plates. When glued together, all these parts belongs to a new system that works different than elements of the system. At least, this could be a empiric way to keep the top and the back in the same weight and flexibility range from instrument to instrument..... Saying this, I know the storm will come.

Costantino Proietti
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Re: My first Chladni patterns

Post by Costantino Proietti » Sat Apr 06, 2019 5:39 pm

Frederich I am not the one who can talk about Chladni at all. I just made my first attempt to found the patterns of my braced top, hoping them could somehow help me. I completely agree with you about experience and tap tuning and flexing the wood etc., ...that I haven't got. I hope one day I will be able to discover a few wood secrets. The modes shown above don't tell me too much but they incite my curiosity for a different approach only to discover, through trials and errors, if they can be useful for me. I have to say that I spent a lot of time with the stuff and in order not to frustrate the time spent I decided to carve the braces a bit, looking for the so-called a ring and a half, (?) starting from the mode at 290 hertz, and after a lot of very slight scallops I found a round shape in the lower bout. But an ultimate shot of chisel and the things went completely wrong. The mode is back as before. So, for now I'm on the errors side! :oops: :oops: :D :D
I am aware that after closing the box the things will be completely different. I don't expect anything special. The Chladni shoud be just a visual tap tuning, so being able to interpret the modes could help also a traditional luthier.
A fact is the most obstinate thing in the world. Bulgakov

Frederich
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Re: My first Chladni patterns

Post by Frederich » Sat Apr 06, 2019 6:36 pm

Ciao, amico. Non dico che e sbagliato, ma....I am not saying that’s something wrong with doing that, I am wondering how it might help someone to do the right top, bracing, and ultimately guitar, just by looking to the way a top responds to a certain frequncy or frequencies. I wish it would be an answer to the question: how can I fix this C note on the 1st string that is short in sustain comparing with every other sound on all the register? It would be nice to know that changing the position or the dimension of a brace will be the fix, or just to learn how to not repeat the mistake. For now my answer was to plane the fingerboard down to the neck, remove the bindings, cut the top off, make a new top, install bindings, glue a new fingerboard, frets, finish, etc, and hope that this time it will be right. I believe in science, I was a mechanical engineer for most of my work time, but, unfortunately most of the acoustic studies of the musical instruments, and they are many, can’t come up with practical solutions to the problems. They are to many variables in the system. For now i let the process of building a guitar or a cello ( i do that sometimes) to be more artistic than scientific. In this way a guitar could be good, very good or oustanding. When is just only ok, than cut the top off and start all over again. If somebody can tell me very specific how to solve problems that sometimes arise (fortunately not to often), i will be extremely grateful. I wish you luck and let us know where your experiments lead you.

Costantino Proietti
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Re: My first Chladni patterns

Post by Costantino Proietti » Sat Apr 06, 2019 8:10 pm

Frederich thank you for the Italian word amico. And thank you for the wishes.
On last March in Rome it was held a three days convention of 30 Italian luthiers. Special guest was Josè Romanillos that was accompanied by his lovely wife. At the end of his brief conference he has been greeted by an interminable standing ovation. The Maestro epitomizes the artistic luthiery, as depositary of the art of give life to wood. And I can only admire this kind of luthiers. The Chladni maybe don't solve problems but is not in contrast, but you have not stated it, with classic liuthiery. I think that after tons of records the luthiers that use, in addition to their knowledge, also, this mean, are able to find the correlation between free top, braced top and finished guitar.
I don't know how much it's worth. As I'm about to assembly my third guitar and just thinking to my fourth, I hope to discover it soon. :D :D :D
A fact is the most obstinate thing in the world. Bulgakov

Marshall Dixon
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Re: My first Chaldni patterns

Post by Marshall Dixon » Sun Apr 07, 2019 8:41 pm

Costantino Proietti wrote:
Sat Apr 06, 2019 4:04 am
I use the speaker in the picture, I don't know if it is the right one as it becames hot, but for now it works. For a signal generator I use an app for my smartphone ( Simple tone generator) and an amplifier with a microphone jack.
Thanks Constantino,

I imagine speakers generate a bit of heat considering the movement of the voice coil but I've never really felt one in use. It shouldn't be too difficult toofind a second hand speaker.

Marshall

Frederich
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Re: My first Chladni patterns

Post by Frederich » Mon Apr 08, 2019 2:36 am

If you brace the top for a certain mode, what happens when you play another sound and the way the top vibrates changes dramatically? I think the problem of the guitars is the bracing. If you can make a top without braces, light enough to be responsive, but strong enough to support the string tension
than you won’t have dead notes or unbalanced strings, etc. One fan brace too tall, or too heavy, or in the wrong place could kill a certain note without an easy way to fix it. In my way to build tops, i try to have a very even stiffness and flexibility on the whole surface. In this order, my tops are very similar to a thin lens, thicker in center and thinner around the perimeter. Also the center is the lighter but stiffer part of the top, while the edges are more flexible like a speaker. A speaker is designed to respond equally to all kind of frequencies. The same should be a guitar top. So, I don’t speak about art, but about mechanics of the construction of the guitar. The mechanics, unfortunately, are too complex to be solved by just looking at those patterns. If I am wrong, i hope somebody will help me understand.
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Alan Carruth
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Re: My first Chladni patterns

Post by Alan Carruth » Mon Apr 08, 2019 4:47 pm

What's interesting is that unbraced tops do vibrate in very similar patterns to braced ones. You could make a solid spruce top thick enough to take the load of the strings, but it would be too heavy to sound well. Bracing is there to add stiffness without adding too much mass.

Bracing a top 'wrong' can really kill the sound. Makers have devised, by trial and error, several bracing patterns that work well to produce sounds that people like. Even using one of these 'standard' patterns won't guarantee a good sounding instrument, particularly if you make the bracing too heavy. That's what techniques such as 'tap tone' tuning try to do; to figure out where to remove material from braces to make the top work well. Chladni patters are just a 'tech' way to make tap tones visible, and home in more precisely on the pitches. It seems to some of us that the shapes of the patterns are more important than the pitches. The pitch of a resonance tells you something about the ratio of stiffness to mass, and that's helpful, but in the case of a guitar the absolute stiffness is more important than the ratio. A very low density top could end up quite light and still have a resonant pitch in the 'right' range, but could fail to have the requisite stiffness.

Frederich's 'sandwich' top is, in a sense, another approach to the problem. The top itself is a sort of 'extended I-beam' which derives it's stiffness from thickness, and eliminates most of the wood in between by replacing it with a light weight honeycomb material. If the layup is made thick enough there will be little or no need for other bracing, with the possible exception of an upper transverse brace to take up the bridge torque. This is critically dependent on good and long-lasting glue joints between the top and the honeycomb. It's easy enough to fix a bad joint once you know where it is, but hard to find it,if the experiments I've done are any indication. Time will tell.

The other main 'modern' design that does much the same thing is the Smallman 'lattice'. A balsa/CF I-beam lattice takes the loads, and veneer thickness top is simply a membrane across the structure to move air and produce sound.

In both the 'sandwich' and 'lattice' tops mass has been significantly reduced without losing the needed stiffness. This tends to shift the modes of the completed guitar upward in pitch, which gives a different timbre to the sound from the traditional top. Some folks don't like it, others do. There are other methods being tried (such as Gore's 'falcate' bracing) to accomplish similar ends. Moving the other way, into more 'traditional' methods of construction, some makers have used carved arched tops and back to gain surface area to produce sound without increasing mass out of proportion. Again, time, and the market, will sort it all out eventually.

To get back to the OP: those are reasonably good looking modes. Your mode at 270 Hz (?) looks like the 'ring+' mode, which is one I tend to concentrate on. What bracing system are you using? In general, what I've found is that the more 'good looking' (that is, well defined and reasonably symmetric) modes I can get on a top, the better the guitar tends to sound. One thing I wish I could get more people to do is to explore the modes of a simpler system first, to get an understanding of what the modes you see really are. If you can get a square (or several!) of expanded polystyrene bead board (not the more homogeneous rolled insulating board) it's a good thing to experiment with. Expanded bead board has the same Young's modulus in both directions, where most materials have a 'grain' introduced by the way they're made that makes them stiffer in one direction than the other. Glass is another material that is 'isotropic' like that, but it's harder to work and harder to find the modes on. Bead board is easy (but messy!) to cut up, and you can explore different shapes. Most of the lower order modes on a guitar top are versions of what you'll see on a square plate of bead board, and you can add masses and/or stiffness elements easily to see what effect things have without messing up an expensive piece of spruce.

Costantino Proietti
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Re: My first Chladni patterns

Post by Costantino Proietti » Mon Apr 08, 2019 8:11 pm

Thanks Luthier Frederich and Carruth for your interesestin speeches.
Alan Carruth wrote:
Mon Apr 08, 2019 4:47 pm
To get back to the OP: those are reasonably good looking modes. Your mode at 270 Hz (?) looks like the 'ring+' mode, which is one I tend to concentrate on. What bracing system are you using? I
I'm only able to give further informations on my soundboard. Unfortunately I did not tried the Chladni on the free plate. For my Engelmann I used the Hernandez y Aguado bracing plan with a thickness of 2.5 mm at bridge and upper bout and 1.9 and 2.1 at the bases/trebles sides. I did not glue the bridge patch. At the first view I thought the 290 Hz was the ring+ but now you pointed out it should be the 270 Hz I recognize this embryonal shape.
As I said I concentrated my first try to the 290 Hz mode but after few attempts on carving the braces I found myself at the starting point. In addition I lost the 75 and 115 Hz modes, only improving the shape of some higher modes at 315 and 356 Hz that seem neater.
A fact is the most obstinate thing in the world. Bulgakov

Alan Carruth
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Re: My first Chladni patterns

Post by Alan Carruth » Tue Apr 09, 2019 12:43 am

The modes you got at 75 and 115 Hz are actually counterparts of each other. On a square isotropic plate (such as that bead board) they show up as two modes with three diagonal lines, 'dexter' and 'sinister', at the same pitch. On a rectangular wooden plate 'tuned' so that the time it takes a wave to travel along and across the grain is the same (by manipulating the ratio of length to width) they show up as six pointed stars in 90 degree rotations, which may or may not be at the same pitch. On the braced guitar top plate the upper transverse brace distorts one of them into the shape you saw at 75 Hz, and the 115 Hz mode is the other. It's not distorted because the UTB doesn't need to bend for that mode, as it does for the other. Generally, when you lose one of those you lose the other. This is not necessarily a bad sign: we don't know what all of these modes 'mean' in the sense of how they affect the final outcome of the assembled guitar. Again, what seems to be more important is to have a lot of well-formed and active modes, rather than matching some 'ideal' set.

Costantino Proietti
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Location: Rome

Re: My first Chladni patterns

Post by Costantino Proietti » Tue Apr 09, 2019 4:53 am

"You’ve made your bed, now lie in it!”. :D
Mr Carruth you made all possible to make the matter simple, but as I'm a novice it remains obscure and impalpable. But at the same time intriguing as it seems to give more visual informations then hand tapping. Decode them will require lot of time and lot of trials.
Thank you for the explanation.
A fact is the most obstinate thing in the world. Bulgakov

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