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As I work my way through my first finish, I've become curious about the science behind using different types of wood.

How does wood impact the sound?

Doesn't finishing the wood impact how sound goes through the wood? I imagine that all the poor filler and sealer and epoxy and 10 coats of nitro would create a barrier that would limit the variations in different woods and sound?


I know this isn't necessarily true but why wouldn't the wall of finish impact sound?

Yes every thing you put on the wood has an effect. It may be miniscule but it has an effect as does every thing you do, thinning plates, type and size of bracing you name it.

Simple answer the heavier, and or less flexible the the finish the more it dampens the amplitude of the wood. This is why we add such a thin coating.

The best finish in terms of least dampening affect per mil thickness of finish that I know of is French polish (hand rubbed on shellac in the French polish application method) This is due to its elasticity and weight.

I should also have said some hand rubbed varnish type finishes are also very low damping on hard woods but may penetrate the top too much and dampen the precious tops response.MichaelP39034.6124652778

turbo411 asked:
"How does wood impact the sound? "

How long do you have to talk about this?
;)

Mostly we're looking at bending waves; sort of like ripples on a pond, except that for most of the sounds we're dealing with the 'pond' is small relative to the length of the waves. It's sort of hard to visualize the riples traveling around, since they just get started and run into something to bounce off of. Hence, we usually talk about 'standing wave patterns', or 'resonant modes'. Every guitar has a lot of different resonant modes, each one happening at a particular frequency. For a top or back that has high stiffness relative to the mass, and thus fast bending waves, the resonant mode frequencies will be high. It's all just different ways of looking at the same things.

The speed of the bending waves traveling through the wood is set by the density and elastic modulus (Young's modulus) of the material, and how thick it is. Another important property of the material is the 'damping factor'; a measure of how much energy it dissipates as it flexes. The pitches of the resonant modes are set by the speed of the waves, and the size and structure of the guitar.

Braces add a lot of compication to things. At low frequencies, where the braces are much closer together than the length of the bending waves, they just add a lot of stiffness for their mass. Of course, the braces add stiffness most along their length. At higher frequencies the braces can 'look' like heavy, stiff 'lumps' to the waves traveling through the thin wood, and they can break up the vibration patterns. Think of a shallow pond with a lot of rocks just below the surface. The speed of the waves in the pond depends, in part, on how deep the water is, if the height of the waves is something like the depth of the pond.
Where the rocks get close to the surface they break up the wave patterns, and cause ripples, even when the rocks never break through the surface.

This is all a bit of a digression from a question abbout the effect of finish, I know, but I think it helps sometimes to have a good general picture of what's going on.

Different finishes have various effects. All finishes add mass, of course, and thus slow down the waves. In terms of the resonant mode patterns, added mass tends to shift them downward in pitch. However, many finishes also add stiffness, which can help raise the pitches. Different finishes have different 'damping factors'; some have more damping than the wood, and some less. And, of course, different woods also have different damping factors, and _each_ wood can have a different damping factor depending on the direction the wave is traveling, the frequency, and things like the relative humidity. Did I mention that this gets complicated?

A violin maker in Germany, Martin Schleske, did a study on the effects of finshes on wood vibration properties several years ago. In very general terms, here's what he found:
1) Oils, such as linseed and walnut oils, have high damping. Any finish that contains oil tends to add damping to the wood it's put on, depending on how much oil there is. Straight oils that you wipe on are the worst. Not only do they add a lot of damping, they also add a lot of weight, and they don't really protect the wood well, since they don't form a waterproof film. Traditional oil and resin varnishes are better, since they don't penetrate deeply the way plain oils do. If they have high resin content ('short oil' or 'rubbing' varnishes) they can add more stiffness than they do weight, at least across the grain on spruce. 'Spar' varnish has lots of oil and little resin; it's a 'long oil' varnish.
2) Resins vary. He tested a lot of the usual violin makers things; gamboge, sandarac, elemi, and so on, as well as shellac and nitrocellulose lacquer. Shellac and nitro were the two 'best', in the sense that they had _lower_ damping than the spruce they were put on, and added the most stiffness for the weight. Nitro was about 1/3 stiffer and lower in damping than shellac.

Now, as for the larger question about wood and sound.....


Wait......


Why are you looking so funny, Lance?


OK, OK, some other time......


   

Al Carruth.... you are the man. I was interested in this topic as well. I think I need to study up a little more on this kind of stuff. Thanks for the explanatory post. I like the way you explain this. Not so technical that I need a college degree to understand.
Thanks,
Homeboy

[QUOTE=Alan Carruth]
This is all a bit of a digression from a question abbout the effect of finish, I know, but I think it helps sometimes to have a good general picture of what's going on.

Different finishes have various effects. All finishes add mass, of course, and thus slow down the waves. In terms of the resonant mode patterns, added mass tends to shift them downward in pitch. However, many finishes also add stiffness, which can help raise the pitches. Different finishes have different 'damping factors'; some have more damping than the wood, and some less. And, of course, different woods also have different damping factors, and _each_ wood can have a different damping factor depending on the direction the wave is traveling, the frequency, and things like the relative humidity. Did I mention that this gets complicated?
   [/QUOTE]

I don't know how relevant this is, but it just occured to me that if a finish isn't fully cured when a guitar is first strung up. Wouldn't it's 'dampening factor' change as it cured? As I said, I don't know how relevant it is. I've been wondering about new guitars 'opening up' over the first few weeks and what factors drive that. A curing finish might make a contribution to this. Hmmm....Just trying to understand....

Morton Hutchins looked at a lot of violin-type finishes over time, and found that most of them were more or less fully cured in about three months. Shellacs, 'spirit' varnishes, oil varnishes, what have you. I suspect the same is true of nitro. As they cure the stiffness certainly gets higher, the mass a little lower in general (outgassing), and my gut says the damping drops. I have no data on UV cures and waterborns, but my small experience with the latter suggests to me the possibility of a longer cure time.

There's enough there to have an effect on immediate 'playing in' all right. However. I'll note that at least some aspects of playing in do seem to require actual playing, whereas those finish effects don't. I'd put finsih curing down as a 'secondary' part of playing in, at least for now. As always, I'm willing to be taught different if anybody can come up with the data.

[QUOTE=Homeboy] Al Carruth.... you are the man. I was interested in this topic as well. I think I need to study up a little more on this kind of stuff. Thanks for the explanatory post. I like the way you explain this. Not so technical that I need a college degree to understand.
Thanks,
Homeboy [/QUOTE]

I'll second that. Carruth could describe what he ate for breakfast, and it would be one of the first posts I read. I, among many I would guess, greatly appreciate all your posts Mr. Carruth...thank you.Bill Greene39034.8905439815

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Alen when you get a free decade or three I want to set down and have a conversation about wood and sound waves. I have a question or 500, OK? Oh and remind me to bring my note book

I second and third the appreciation given. This was very helpful! After I digest it for a day or two I may have some questions :)

     Not only for the great informative post, but for proving that I'm not the
only one who can and will write epic length posts.

Good stuff and.....that's exactly what I was going to say.

Regards,
Kevin Gallagher/Omega Guitars

I'm afraid that Alan has over-simplified the case of wave motion through the material of the guitar.

When a seismic event takes place in any material (plucking the string) a series of different wave forms are set up that propogate through the material. These take four main forms:

P waves or pressure waves, that cause a compression/dilation effect as they propogate.

S-waves or shear/transverse waves that cause a motion of the material as the waves passes through it at right angle to the direction of wave travel (either vertical of horizontal).

Love waves, which like S-waves involve shear displacement, but differ from S-waves in that the displacements are only horizontal and that the amplitude decreases rapidly following the event.

Rayleigh waves, are rather like ocean waves in that the pressure form within the medium through which the wave travels passes in vertical ellipses. As with Love waves, the amplitude rapidly decays.



The problem of wave propogation through the material of the guitar is further complicated by the fact that the meterials used in its construction are not homogeneous either within themselves or from material to material. This will cause both reflections and refractions of the traveling waveform both at the junction of differing material and at density boundaries within each material (grain lines), particularly for S and P waves and their complex sets of harmonics. This is not dependent on wavelength, but will effect each part of the wave as it crosses a density boundary.



Diffractions of the propogating wave will also occur at point junctions within the material causing interference with the propogating wave, and some loss of amplitude.

So basically if you wanted to build a guitar, you wouldn't start with a non-homogeneous material like wood. In fact the best sounding guitar I've ever played was the polymer guitar made by Loughborough University.

Interesting subject that you could spend 30 or more years studying and still not get to the end (Oops I have!)

Colin

Colin S39035.2131481481

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Alan,

I always love reading your stuff - I learn loads and loads.

Colin,

Fascinating stuff too! I'd like to think that my guitars are filled with lots of Love waves, but I'm just a sad old hippie at heart

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

[QUOTE=Colin S]

The problem of wave propogation through the material of the guitar is further complicated by the fact that the meterials used in its construction are not homogeneous either within themselves or from material to material. This will cause both reflections and refractions of the traveling waveform both at the junction of differing material and at density boundaries within each material (grain lines), particularly for S and P waves and their complex sets of harmonics. This is not dependent on wavelength, but will effect each part of the wave as it crosses a density boundary.

Colin

[/QUOTE]

Colin, does that mean that the glue you use to glue down braces etc will have a bearing on the sound of the guitar, if you think of the glue line as a "density boundary"? Or is the effect too small (between different glues and on a the scale of a guitar) to pick up?

In terms of the velocity of a wave through any material the density is a greater influence than the modulus of elasticity. The latter will slow down the wave but an increase in density will result in the wave travelling faster with less loss of amplitude. Indeed the glue line will be two boundaries, as the wave moves from wood to the glue and back to wood etc. So yes the type of glue will make a difference, you want one that with a low elasticity. Hot Hide in fact! (LMI white is pretty good as well)

Now must get back to some 3D tomography interpretation of the Yellowstone magma chamber, boy I'm glad I don't live near there!

ColinColin S39035.2534375

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Thanks Colin that jibes with what I almost remember from my physics lectures, as Mario is always saying it's a hundred little things rather than just one or two big things (see Mario we are listening ). Another argument for HHG.

Thanks also to Alan, it's nice to have a light bulb moment occasionally that's confirmed by someone who knows, even if mine are of the low wattage variety.

Now Colin, isn't that yellowstone thingy called a caldera or some such? Saw it on discovery channel. Glad I'm a long way away from there too! Even if I do have to pay huge shipping bills. I bet that thing could send out a wave or two.PaulB39035.2732060185

Hey guys, am i safe here in Quebec?

Serge are you near water?

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Yeah, the Ottawa river!

If the Yellowstone volcano blows full bore again nothing west of the Mississippi is safe from major ash fallout in the short run and nothing pretty much hemisphirical in the long run. Give up it all over Serge

Sorry to hijack the thread, but you guys do know it happens every 6-700,000 years don't you? Guess when it last happened, that's right, 6-700000 years ago!

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Wow! So much to do. So little time.

_________________
Beautiful and unusual tone woods at a reasonable price.
http://www.rctonewoods.com/RCT_Store
The Zootman
1109 Military Rd.
Kenmore, NY 14217
(716) 874-1498

Bob, if it does start to go you are very welcome to come and stay at my place in England. Oh, just make sure to bring all that wood over with you.

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Colin, you're scaring me man! Not the Yellowstone thing... but you brought back some extremely traumatic memories of my college physics class. Now I'm gonna need some serious therapy. I hope you have a good lawyer!


Michael, I was watching some docu-drama about the Yellowstone thing called "Supervolcano". From what I gathered, the whole country and for that matter, the whole planet could potentially be majorly affected by it.

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