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#1
10-18-2014, 04:21 PM
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What is actually making it sound different?
For the purpose of this question let's use tow identical factory made guitars, one with rosewood back and sides, the other with mahogany back and sides.
Although these two guitars are built to the same specs, most will conclude that they sound different. One perhaps is more bright. But, was is actually causing this difference in sound? When a string is plucked, the sound board goes into motion, sending sound waves into the body of the guitar that bounce off the back wall and exits the sound hole, repeatedly until there is no more energy from the string. So .... is it the reflective nature of the back board or wood used for the backboard that varies what you hear? Does the selection of wood used for the backboard have different absorptions ... hindering some sound waves from reflecting out? Is it the back boards ability to move that varies the sound? What if the back board was not wood, but a thin piece of aluminum or plastic, how would that change the sound you hear? What if you shellacked the inside of both woods ... would they then sound the same? 
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#2
10-18-2014, 04:48 PM
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With the leap of assuming all else equal, stiffness, mass, and damping account for the differences no matter the material.
And the tone fairies. And maybe our eyes and preconceptions.
__________________
Chasson Guitars Web Site
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#3
10-18-2014, 05:08 PM
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yes,yes,yes,yes and no. Even if you take two back and sides from the same billet. two tops cut one after the other and build to the same specs, those two seemingly identical guitars will sound different. That is assuming you have a qualitative way of measuring the sound difference. A builder can, within a reasonable parameter, know what type of sound will be produced by a certain combination of woods and build technique but there is no way to perfectly reproduce the "same" sound in subsequent builds. Wood is an imperfect medium and builders are only human .
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#4
10-18-2014, 05:08 PM
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This is a huge can of worms. On the one hand there is a pretty well defined answer that's fairly simple, but on the other hand there's so much going on that we're not entirely sure how it all works. Add in the fact that there's no agreed on meaning for tone descriptors like 'bright', and it gets contentious fast.
The simple answer: As with a stretched string, every guitar has certain well defined ways that it will vibrate. As you know, if you use a heavier or lighter string, of steel or nylon, or build it up with windings of Pbronze or nickle or whatever, you get somewhat different sounds. Guitars are far more complicated structures than strings are, and vibrate in more complicated ways that are much harder to predict. Still, if you tap a guitar it will 'ring' for a little while (yeah, it sounds like 'thud', but nonetheless...), and that sound actually contains signals from all the different sorts of vibrations that it can make when you tap it at that place. This 'spectrum' is characteristic of the particular instrument; it's very unlikely that you'll ever find another one that will work in just that way, although you can get close. That's because the pitches, and relative strengths, of the various 'resonant modes' are determined in some part by the exact properties of the materials used, and wood varies a lot, so that even with careful selection it's difficult to make all the parts exactly the same. Thus every guitar will have different resonant pitches at different levels. It's possible to 'see' those resonances, using Chladni patterns or laser holography. What you'll see is that different parts move more or less at different pitches. Thus, the lowest resonant mode of the top is usually that where the lower bout moves like a loudspeaker cone, producing sound in the same way. The exact area and configuration of each mode makes a difference in the sound, too. Large areas tend to be more effective at moving air and making sound. Many modes (most, in fact) have more than one area moving at the same time, generally out of phase with each other, and these will more or less cancel out to some degree. They can also add up in odd ways, particularly at high frequencies, to 'beam' sound in a particular direction. This can be very sensitive to small details of wood properties and structure, like the way the braces are placed and profiled, so even if you have two guitars with resonances at the same pitches they may sound different because they work a little differently. There's one other property that comes in in this 'simple' model, and that's 'damping'. Basically, it's a measure of how fast the energy is dissipated when you pluck a string, say. Damping seems to be one of the main differences between nylon and steel strings, and it can be argued that all of the differences between the guitars flow from that. Damping is both a material property and a structural one. Rosewood tends to have much lower damping than, say, maple, and that's probably one reason rosewood and maple guitars sound different. But you can end up making a rosewood back with higher or lower damping simply by the way you brace it, so the material property is not destiny by any means. So in theory, if you know everything about the frequencies and configurations of the resonant modes of the guitar, and what the damping factors are for each of them, you can describe the sound. If you can copy them exactly, you should be able to copy the sound. The problem is that wood varies so much, and our hearing is so sensitive to small changes in some ways (even when it misses large ones in other ways), that in practice it's very difficult to describe all the resonances that might matter, and darn near impossible to control them well enough to purposely make 'identical' guitars. At this point I don't have time for the complicated answer.... 
Last edited by Alan Carruth; 10-18-2014 at 05:15 PM.
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#5
10-18-2014, 05:17 PM
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rosewood is denser then mahogany, making it reflect more sound as opposed to absorbing.
__________________
build, learn, grow
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#7
10-18-2014, 06:10 PM
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How much does the choice of wood for the back make a difference? What is the role of the backboard in the guitar. Is it to reflect the sound waves out of the sound box through the hole? Does it have another purpose?
What if the back was made of foam? Would you lose almost all of the sound? What about metal? Would the sound waves bounce off that so much as to increase volume?
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#8
10-18-2014, 06:21 PM
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Quote:
__________________
Martin 0-16NY Emerald Amicus Emerald X20 Cordoba Stage Some of my tunes: https://youtube.com/user/eatswodo
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#9
10-18-2014, 06:38 PM
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Quote:
Tom
__________________
A person who has never made a mistake has never made anything
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#10
10-18-2014, 07:05 PM
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Quote:
There are so many ways in which to ask the question: Another way may be to simply ask, what is the purpose of having a back side on an acoustic guitar? Maybe the answer to this question might lead us to it's MAIN purpose.
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#11
10-18-2014, 07:21 PM
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If you take two- or a bunch - of identical factory guitars (I've found those to be the most 'idendicalest') some with mahogany, some with rosewood and all else being equal, they'll have a subtle difference, in my experience. So the simple answer to your question is the property of a given wood. The complicated answer comes from understanding the complex differences between them.
And then there are reflective backs and resonating backs...
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#12
10-18-2014, 08:30 PM
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it completes the air cavity, focuses and directs sound through the soundhole. Pluck an electric string....not much volume. Put that same electric over a wooden box with a port in it and, even with the relative massive thickness of the electric body there is an increase in volume. Or....it is to stop overweight players bellies from blocking the soundport. It's one of the two, I never get that one right.
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#13
10-18-2014, 09:11 PM
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The IP's concept for how a guitar works does not overlay my own concept very well. It is good to ask these kinds of questions and I support it, but cannot address it at this level.
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#14
10-18-2014, 09:18 PM
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The back and sides can reflect or absorb (dissipate energy within the wood) soundwaves coming from the top. Does mahogany absorb, and absorb more especially at the higher frequencies, than does rosewood? Sure sounds like it. That's about it IMO.
Could not find much on the internet regarding tests on wood, but here is one paper http://www.acoustics.asn.au/conferen...apers/p746.pdf
__________________
Derek Coombs Youtube -> Website -> Music -> Tabs Guitars by Mark Blanchard, Albert&Mueller, Paul Woolson, Collings, Composite Acoustics, and Derek Coombs "Reality is that which when you stop believing in it, doesn't go away." Woods hands pick by eye and ear
Made to one with pride and love To be that we hold so dear A voice from heavens above Last edited by rick-slo; 10-18-2014 at 09:28 PM.
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#15
10-18-2014, 09:44 PM
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The IP did not give a concept of how it works. Only Suggestions and mostly Questions. Does Bruce agree with Alan, there's so much going on that we're not entirely sure how it all works?
Rick says sound waves get absorbed or lost in the wood, or the wood can reflect the sound waves. What is best, what do we want the back panel of the sound box to do?
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