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Chladni meets laser imaging
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=32421		 Page 1 of 1
Author:  Daniel Minard [ Fri Jun 03, 2011 12:16 am ]
Post subject:  Chladni meets laser imaging
If you haven't seen this yet, it's worth a peek.
http://www.bbc.co.uk/news/science-environment-13573631
Dan
Author:  Don Williams [ Fri Jun 03, 2011 5:57 am ]
Post subject:  Re: Chladni meets laser imaging
Interesting that most of the upper bout appears to be inactive. (ducking)
Author:  muthrs [ Fri Jun 03, 2011 7:02 am ]
Post subject:  Re: Chladni meets laser imaging
Ahh, but notice that the area around the soundhole and even into the soundhole is active. Technically the upper bout or a I guess the lower upper bout. That is why I feel it is important not to leave the upper legs of the X-brace too stiff, but I do have substantial structure above the soundhole. You can also hear this by tapping.
Author:  Chris Pile [ Fri Jun 03, 2011 8:16 am ]
Post subject:  Re: Chladni meets laser imaging
I'd be wondering what guitar they'd be using for research... something of quality, or a plywood nightmare borrowed from some student?
Author:  theguitarwhisperer [ Fri Jun 03, 2011 8:20 am ]
Post subject:  Re: Chladni meets laser imaging
Don Williams wrote:
Interesting that most of the upper bout appears to be inactive. (ducking)


What frequency were they checking? That would determine which regions were active or not, I would think.
Author:  Dave White [ Fri Jun 03, 2011 9:46 am ]
Post subject:  Re: Chladni meets laser imaging
Don Williams wrote:
Interesting that most of the upper bout appears to be inactive. (ducking)

Don,

You say that but run the slide show and look at the sequence of three pictures of the classical guitar top at low, high and higher frequencies and watch the activity creep up to the upper bout - if only those two massive transverse braces just above the soundhole weren't on the top who knows where it would have stopped.
Author:  Pat Foster [ Fri Jun 03, 2011 10:40 am ]
Post subject:  Re: Chladni meets laser imaging
If the exciter—looks like a transducer of some sort— is in contact with the top, it seems that could skew the modes and frequencies where they appear.
Author:  Darryl Young [ Fri Jun 03, 2011 10:43 am ]
Post subject:  Chladni meets laser imaging
Wouldn't you thick they just plucked a string?
Author:  Pat Foster [ Fri Jun 03, 2011 11:04 am ]
Post subject:  Re: Chladni meets laser imaging
Possibly, but they do state that the top is "actively vibrated." I took that to mean more than using the strings, which would introduce many more harmonics.

Work like this was done about 40 years ago. I wonder what is new in this project.

http://www.speech.kth.se/prod/publications/files/qpsr/1969/1969_10_2-3_036-041.pdf

Pat
Author:  Burton LeGeyt [ Fri Jun 03, 2011 11:05 am ]
Post subject:  Re: Chladni meets laser imaging
One thing I have noticed in my guitars where I use the buttresses and loosen up the upper bout is a lack of bite in the trebles, it rounds off the top end and makes it richer but not nearly as "crisp". I have attributed (tentatively) the extreme stiffness of the upper bout in a traditionally braced guitar to more trebley trebles, to use the scientific term. I always wondered why classical guitars seemed to have almost the same upper bout support as steel strings with so much less tension and I think now it is maybe to accentuate the trebles in the nylon strings. Could all be hooey too, the more I see the less certain I am. Seeing the nodes creep into the upper bout as they fragment into smaller and smaller shapes as the frequencies increase made me think about that. I haven't noticed anything substantial in my Chaldni notes though concerning the buttressed vs. non buttressed guitars. Little things, but not enough to form a real opinion.

The link is cool, Daniel, thanks for posting it!
Author:  JoeUlman [ Fri Jun 03, 2011 1:03 pm ]
Post subject:  Re: Chladni meets laser imaging
What I glean from the associated abstract (link below) is that the modes are electro-mechanically excited by one or sometimes two drivers attached to the instrument by double sided tape. This ensures that one mode dominates, limiting any misleading influence from multiple modes, a problem with some conventional methods. The driver placement is chosen to be near an anti-node and also near nodes of adjacent modes.

Some interesting guitars in the study include an early guitar of Josef Pages of Cadiz, a Torres style guitar, a Paul Fisher lattice braced guitar, a guitar with nodal bar, a guitar with and without the bridge to study the effect of the bridge [design], and five different bracing patterns.

The abstract can be found here:

http://viennatalk.mdw.ac.at/papers/Pap_ ... ardson.pdf

Joe
Author:  Pat Foster [ Fri Jun 03, 2011 1:35 pm ]
Post subject:  Re: Chladni meets laser imaging
Great Joel. I looked around but couldn't find it. Thanks for posting.

Pat
Author:  Darryl Young [ Fri Jun 03, 2011 3:49 pm ]
Post subject:  Re: Chladni meets laser imaging
Burton LeGeyt wrote:
One thing I have noticed in my guitars where I use the buttresses and loosen up the upper bout is a lack of bite in the trebles, it rounds off the top end and makes it richer but not nearly as "crisp". I have attributed (tentatively) the extreme stiffness of the upper bout in a traditionally braced guitar to more trebley trebles, to use the scientific term. I always wondered why classical guitars seemed to have almost the same upper bout support as steel strings with so much less tension and I think now it is maybe to accentuate the trebles in the nylon strings. Could all be hooey too, the more I see the less certain I am. Seeing the nodes creep into the upper bout as they fragment into smaller and smaller shapes as the frequencies increase made me think about that. I haven't noticed anything substantial in my Chaldni notes though concerning the buttressed vs. non buttressed guitars. Little things, but not enough to form a real opinion.


Interesting Burton.

In his book, The Responsive Guitar, Somogyi discusses this topic somewhat. His take is that classical guitars are braced with several vertical braces (meaning running roughly parallel to the top grain) which provides a lot of stiffness resisting the bridge rocking back and forth (the long dipole mode). Due to the stiffness, the higher notes excite the vibration mode. He says some folks will remove too much material when voicing the top thinking this will free up the rocking movement of the bridge and find they loose their high end treble voice which is a mistake. He stated that the bridge bouncing up and down (monopole mode) corresponds to the low end frequencies. The bridge rotating side to side (the cross dipole mode) corresponds to some of the mid-range frequencies. And the bridge rocking back and forth corresponds to the upper frequencies.

On an X-braced steel string, things change. The X-brace provides a lot of stiffness to the cross dipole mode (the bridge rocking side to side).......to the point that it may be stiffer than the bridge rocking front to back (long dipole mode). The wider the X-brace angle, the more stiffness that's added to the bridge rocking side to side so that mode is responsible for producing the highest frequencies.
Author:  Don Williams [ Fri Jun 03, 2011 4:26 pm ]
Post subject:  Re: Chladni meets laser imaging
Dave White wrote:
Don Williams wrote:
Interesting that most of the upper bout appears to be inactive. (ducking)

Don,

You say that but run the slide show and look at the sequence of three pictures of the classical guitar top at low, high and higher frequencies and watch the activity creep up to the upper bout - if only those two massive transverse braces just above the soundhole weren't on the top who knows where it would have stopped.


Yes, I figured I would spark a little debate on that... but the closer you get to the neckblock, the less and less activity that there seems to be. That whole area is braced pretty heavily, so it's hard to give the top much leave to move.
Author:  Blanchard [ Sat Jun 04, 2011 10:35 am ]
Post subject:  Re: Chladni meets laser imaging
Interesting article, but it is my experience that very little can be learned from doing this type of analysis on completed guitars. In the end they all look about the same. Whether the guitar is concert grade or a piece of junk, It will show the same modes in the same order. The frequencies of the modes and the power required to drive them might vary, but the mode will all look about the same.

To really see the subtleties that effect the tone of an instrument, that type of analysis needs to be done on the individual parts. The top before bracing, the top with bracing, the back with and without bracing, etc. Two guitars that sound different and can have similar modes when completed, but will have very different modes sets if the individual parts are compared.

The majority of Chladni work that I do on my guitars happens with the unbraced top and back plates. I have found that if I get mode sets right at that stage, the rest just falls into place. If I get it wrong, there is little I can do with the bracing or body depth or bridge design or anything else to make it right.

M
Author:  Bob Garrish [ Sun Jun 05, 2011 10:35 am ]
Post subject:  Re: Chladni meets laser imaging
By the sounds of it, their gear uses some expendables (there's talk of developing each image with an emulsion) which would make it pretty expensive and slow to use it for the sort of iterative tuning we do with glitter patterns.

I wonder how long it'll be 'til there's a semi-affordable real-time system to do the same thing? I'm sure it'll come from some other industry, though...it seems the number of guitars a builder or company is putting out is inversely proportional to their interest in real research :)
Author:  JoeUlman [ Sun Jun 05, 2011 11:34 am ]
Post subject:  Re: Chladni meets laser imaging
Bob Garrish wrote:
By the sounds of it, their gear uses some expendables (there's talk of developing each image with an emulsion) which would make it pretty expensive and slow to use it for the sort of iterative tuning we do with glitter patterns.

I wonder how long it'll be 'til there's a semi-affordable real-time system to do the same thing? I'm sure it'll come from some other industry, though...it seems the number of guitars a builder or company is putting out is inversely proportional to their interest in real research :)


My take is that the high resolution images produced using photographic emulsions are for construction of the phase-modulating holograms. For real time visualization they use a speckle interferometer (as when positioning the drivers near the anti-node of the target mode and also near nodes of adjacent modes.)

Joe
Author:  ChuckB [ Sun Jun 05, 2011 12:41 pm ]
Post subject:  Re: Chladni meets laser imaging
Mark, are you saying that determining the thickness of the top (by using Chladni patterns), for the size and shape of guitar body, is paramount to final shaping of braces for more acceptable mode shapes? If so, this is something that I am interested in learning more about. Can you direct me to any literature/books/dvds (already have Alan Carruth's) to research this.

Thanks, Chuck
Author:  nickton [ Sun Jun 05, 2011 1:34 pm ]
Post subject:  Re: Chladni meets laser imaging
I my very unscientific way I have been observing or just noticing how a guitar feels when I pluck a string and what kind of damping effect if any occurs when I put my hand on different areas. Most vibration happens in the lower bout and almost none on the upper, and grasping the sides (or upper bout) seems to have no noticeable damping ( or is it "dampening"?) effect. This leads me to think that placing a sound hole or holes on upper bout areas might allow more top under the strings to vibrate freely... I hope that makes sense. Very interesting thread by the way.

I wonder what the patterns would be on a guitar with cantilevered or raised fret board extension.
Author:  nickton [ Mon Jun 06, 2011 12:04 am ]
Post subject:  Re: Chladni meets laser imaging
Aha of course, or should I just say duh: Tacoma "wing series" guitars evidently go with that same sound hole concept. I just did a drawing for my next guitar that looks a lot like theirs (completely by accident I promise). After initially using an oval I figured something a bit more like an opened up f hole might be better, so I scrounged around and found my "french curve" scraper and traced it on the upper bout. It ended up looking similar to a Tacoma "Wing" or "Paisley" sound hole. Interesting.... :mrgreen:

http://www.tacomaguitars.com/resources/ ... atalog.pdf
Author:  Blanchard [ Tue Jun 07, 2011 10:42 pm ]
Post subject:  Re: Chladni meets laser imaging
ChuckB wrote:
Mark, are you saying that determining the thickness of the top (by using Chladni patterns), for the size and shape of guitar body, is paramount to final shaping of braces for more acceptable mode shapes? If so, this is something that I am interested in learning more about. Can you direct me to any literature/books/dvds (already have Alan Carruth's) to research this.

Thanks, Chuck


Chuck

Essentially, yes, that's what I'm saying. When It comes to the shape of the modes and and relationships between their frequencies, the top itself will have a much greater affect than the bracing. Assuming, of course, that the guitar is not grossly over braced or under braced. I have found that getting the long grain to cross grain stiffness ratio of the top in balance with the length and width ratio of the guitar body shape is the single most important factor is getting the mode shapes of the braced top and, ultimately, the tone of the guitar the way I want them.

I don't know of any literature that talks about this. In most of Al Carruth's work he started with a braced plate. I do, however, have a rough copy of a talk I did at the Healdsburg show a few years ago that discusses my Chladni methods in general terms. It would provide you with a good starting point to get your own ideas rolling. Contact me privately if you are interested.

Mark
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