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  #1  
Old 11-19-2014, 01:24 PM
myersbw myersbw is offline
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Default Inner soundboard smoothness...?

To all you luthiers out there (I am not one)...

I just ended up with a Martin GPCPA4 and like the tone. I'm looking at pickup system upgrades and a photo made me realize...the inner soundboard surface is not at all sanded to the level the top surface is. Not even close.

I'm not a luthier, but I'm also not oblivious to surfaces, reflections, coatings, etc. So, the question is...would there or could there be a reason not to have that interior quite as smoothly sanded? Does a lesser sanding stop unwanted grain reflections that could cancel? (like treating a wall in a bare, lively room does?)

Or, could a little extra smoothness add to the the reflections and enrich the tone...i.e. being a freer surface to vibrate. ? I've a tendency to believe lack of "better" quality control may be the reason...but, I can't be certain.

Thoughts on this?

BradM
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Old 11-19-2014, 02:00 PM
KevinLPederson KevinLPederson is offline
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My thoughts would be, that in the production setting, there is more attention paid to the side of the top you see. They just run top after top through a thickness sander and then prep the face side of the top. Most likely nothing more than that.

I personally would not leave the inside top rough as it comes off the sander, but probably not as finish sanded as the face side.

Kevin.
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Old 11-19-2014, 02:22 PM
Kent Chasson Kent Chasson is offline
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I've heard numerous (and contradictory) theories over the years about the level of sanding required inside the guitar. Some people think a smoother surface leads to a glassier tone. Some people believe a smoother surface gives a better glue joint, some think you need some "tooth" to get a better glue joint. Some people think a smoother surface is less hygroscopic, some think it's more hygroscopic. Sorting out the facts, if there are any, would be pretty difficult.

The one thing that I think would be hard to argue with...

A majority of the finest guitars throughout history are rougher on the interior than the typical guitars of today. Old school builders rarely worried about how the inside looked. For better or worse, that is a modern phenomenon.
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Old 11-19-2014, 02:41 PM
myersbw myersbw is offline
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I sort of figured this might be a debatable and non-provable topic in regards to "best scenario". In the same sense that out of phase reflections will cancel, I get the need for differing surfaces to break up canceling "standing waves" so to speak. (From my amatuer radio / electronic eng technology lingo...so bear with me.)

Thanks for the responses guys! I'll just leave well enough alone inside that box. As I said above... ...I like the tone I'm getting...why mess with it.

Cheers!
BradM
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Old 11-19-2014, 04:01 PM
printer2 printer2 is offline
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Coming from the electronic engineering side of things (sort of), the wavelengths of sound that are generated in a guitar will not notice the roughness on the inside. Hygroscopically, I could see a polished wood surface absorbing water at a slower rate as you would have a greater surface area, but realistically a couple percent increase in rate will not make much difference.

Kind of like a car I guess, nice smooth metalwork on the top side to give the impression of a sleek machine. That is until you look at the underside of car.
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Old 11-19-2014, 04:29 PM
Rodger Knox Rodger Knox is offline
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Quote:
Originally Posted by printer2 View Post
Hygroscopically, I could see a polished wood surface absorbing water at a slower rate as you would have a greater surface area.
Polished wood would have less surface area, which would absorb at a lower rate.
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Old 11-19-2014, 05:28 PM
myersbw myersbw is offline
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I get the point, but let's say moisture content is ideal...which make the more ideal sonic resonation...the smooth or the lightly irregular surface?
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Old 11-19-2014, 11:57 PM
tadol tadol is offline
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No idea about the sound - but the smoother the surface, the "cleaner" it will stay. Roughness allows dirt and other contamiants to adhere more easily, which will then hold moisture more easily, leading to the easier growth of molds, fungus, etc. I'd leave a note and make sure someone checks the inside of that guitar in 50 years to make sure there isn't a problem -
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  #9  
Old 11-20-2014, 12:25 AM
tysam tysam is offline
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In an interview in The Big Red Book of American Luthiery (Vol 3) Jean Larrivee was quoted as saying that the inner top and bottom of a guitar need only be sanded down to 80 grit.
in this day and age , with the money that is being asked for high end and custom built guitars, care and attention to the inside of a guitar as well as the visible aspects, is being equated with quality of workmanship. Does it affect sound? Mr Larrivee, at the time of his interview, didn't seem to think so and frankly, I'm not in a position to argue with him.
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Old 11-20-2014, 07:45 AM
printer2 printer2 is offline
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Quote:
Originally Posted by Rodger Knox View Post
Polished wood would have less surface area, which would absorb at a lower rate.
Oops, reworded the sentence and screwed up. Yes that is what I meant. Thanks.
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Old 11-20-2014, 11:09 AM
LouieAtienza LouieAtienza is offline
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If you liked the tone of that Martin, then does it really make a difference whether the inside is sanded glass-smooth or not? I suppose if you really wanted to test this out you can sand the inside surface and see if you detect a change in sound. Likely the change would come from a thinner soundboard rather than the smother/rougher surface.

In a production setting I don't find that objectionable. Here in the Custom Shop section I believe most all builders here would strive for a better inner surface since that's what a client would expect and that is one touch that separates the custom builders from the manufacturers. Also many individual builders are using hide glue for bracing which works best with a smoother surface. Yellow glues have better "gap-filling" ability and can work with a rougher surface. Also it's a misnomer that furniture makers of yore used toothed blades in their planes to abrade a surface to promote adhesion when veneering; the toothed surface allowed the glue to displace which leads to a smoother more even veneering.
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  #12  
Old 11-20-2014, 11:13 AM
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Tim McKnight Tim McKnight is offline
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Way back in a previous lifetime I used to build 2 stroke race engines. Interestingly your questions are similar to the ones we used to address as we modified and ported engines. Back then, there were two schools of thought:
1) Smoother port surfaces are better and flowed air/fuel faster and more efficiently.
2) "Specific" surface texture on the ports and head dome were important to improve atomization of the fuel/air mixture leading to efficient and complete burn rates.

Both paths could lead to great engine performance but other factors had to be considered equally important. Port timing, reed thickness, reed material, fuel, lubricant ratio, compression ratio, expansion chamber design, muffler design and so on...

So, what has this to do with your question about air flow dynamics inside a guitar? I have inspected great sounding vintage guitars with buzz saw marks still on the inner surfaces of the sides, back and top. I have also seen equally great guitars with machine sanded and slightly smoother interiors. There are many other factors that are probably more important than the surface texture of the wood IMO, ie the Q of the top itself, thickness and graduation of the top plate, the brace mass, the way the top is voiced, bridge plate thickness, size and material ... and so on....

I do go to great lengths to sand and seal the interior of my guitars and have a hunch (no proof mind you) that surface texture / smoothness does factor in but how big that factor is is anybody's best guess.
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Old 11-20-2014, 11:14 AM
myersbw myersbw is offline
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Yeah, my conclusion is to not touch it. Especially as I don't have small hands and it's not worth risking mucking around in the cavity when, as you know, I do love the tone of the guitar. And, once it's through the house system...does it really matter? Not a chance.

But, if my hands were smaller and time was on my side...I might've ventured to take some 0000 steel wool and smooth it a bit more than what it is. Just too much careful work is needed given the risk of smacking harshly into a brace, etc.

bradM
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  #14  
Old 11-20-2014, 11:26 AM
myersbw myersbw is offline
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Quote:
Originally Posted by Tim McKnight View Post
... There are many other factors that are probably more important than the surface texture of the wood IMO, ie the Q of the top itself, thickness and graduation of the top plate, the brace mass, the way the top is voiced, bridge plate thickness, size and material ... and so on....

I do go to great lengths to sand and seal the interior of my guitars and have a hunch (no proof mind you) that surface texture / smoothness does factor in but how big that factor is is anybody's best guess.
THAT's the perspective I was looking for, Tim. Thanks! Basically, you've given enough proof (observation of different surfaces vs. tone generated) that inner surface attention may fall into that (electronically speaking) 'less than a 10% difference' where most ears will detect no noticeable difference.

Scientifically, could you study and prove the concept out? Likely to a degree, but when it comes to pleasing tone to the ear, is it worth pursuing? Your remarks tell me it's not worth the effort on my part.

But, you raised another interesting concept and that...sealing the inside. Surely that has to have a more significant effect on hydration and stability? Or, are the sealants still porous enough that dehydration can happen quickly?

I kind of liken it to the example of take two pieces of 1/2 board and paint one on on side only...the other one on both (including ends)...leave it in a damp garage for a length of time and watch which one warps quicker and/or to a greater degree.

Lol, looks like I've hijacked my own post into another topic path! Go figure... :P

bradM
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  #15  
Old 11-20-2014, 11:44 AM
LouieAtienza LouieAtienza is offline
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Quote:
Originally Posted by myersbw View Post
Yeah, my conclusion is to not touch it. Especially as I don't have small hands and it's not worth risking mucking around in the cavity when, as you know, I do love the tone of the guitar. And, once it's through the house system...does it really matter? Not a chance.

But, if my hands were smaller and time was on my side...I might've ventured to take some 0000 steel wool and smooth it a bit more than what it is. Just too much careful work is needed given the risk of smacking harshly into a brace, etc.

bradM
Steel wool is probably the worst thing you can use on unfinished softwood. At the least, it will follow every peak and valley of the surface and take more away from the softer grains of the wood, leaving an even rougher surface. At worse, you embed steel fibers into the soft wood that will rust and stain the wood.
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