How does back & side wood color the tone - absorb and reflect?

[PTL]

Is there a primer anywhere online about how the different types of for the body color the tone. I know that there is a range within any specific species, and that various species overlap. I am speaking of generalities.

Would I be correct that it comes down primarily to absorption and reflection of the frequencies that the top is producing?

So, for example, would Mahogany absorb a lot more upper partials than Rosewood, which would reflect more upper partials in comparison?

And say if we had a 1/2" thick steel body (yes, too heavy to hold but bear with my illustration) to which we glue a guitar top and attach strings, then will most of the frequencies from the top be mostly reflected? And then, in comparison, all the tonewoods would be absorbing particular frequencies to give its particular 'tone'?

And of the available B/S woods, which ones generally absorb the least energy off the top, and which the most?

And would the tonewood that is the least absorptive be perceived as the loudest (though not necessarily the most pleasant).

Thanks.

[mercy]

I believe the harder woods taake less of the energy you put into a string. I also believe that each species has a general tone with a given top which is the main tonal factor. But the interior has a lot to do with a guitars sound as well.
All the different rosewoods are the best for fingerstyle in my opinion but the softer woods such as hog and walnut are better for strumming, again for my ears and preference. So I think youve got it right, softer woods eat energy and harder woods eat less. The word often used is damping. Of course there are many other factors in the sound of a guitar

[WordMan]

The top is the speaker.

The back and sides are the cabinet.

The cabinet colors tone with its dimensions, and because the material used dampens some frequencies.

The differences in dampening properties are what affect which overtones remain, and the sustain, but only in a way that supports the top’s primary role.

[tadol]

Very true, but it also depends on how the builder chooses to build - on many guitars, the back and sides are little more than a solid box, while some builders make the back and sides an active part of the system. The weight of the guitar can be an indication of the build style, but the easiest way to see how active the back and sides of a guitar are is to hold it really tight to your body and compare it to holding it away from your body. I won’t specifically say one is better than the other, but the more active the back and sides are in the instrument, the more you’ll notice how much the choice of wood influences the tone of an instrument -

[vindibona1]

Originally Posted by WordMan

The top is the speaker.

The back and sides are the cabinet.

The cabinet colors tone with its dimensions, and because the material used dampens some frequencies.

The differences in dampening properties are what affect which overtones remain, and the sustain, but only in a way that supports the top’s primary role.

Well put. So if we all stipulate to this as truth, we can take it a step further. How we influence the top influences the sonic properties that are reflected in the back and sides. We all know that we can influence a lot of the guitar's character by the position, thickness, scalloping (I suppose that affects brace density within it's given position)... and bracing material. What the top gives the back and sides is critical. And if you can maniuplate the top in various forms, bracing position, material and construction and even bridge pins, then it cannot help but affect the contribution of the back and sides.

Case in point: Some months ago I did a blind experiment and challenged people to identify which was a rosewood Taylor and a maple Taylor; both GA/ce models. Half those responding to the poll got it wrong. I suspect that what closed the gap was that the rosewood (814ceDLX) had adi bracing, which tends to dry up of the overtones which tend to reverberate more in rosewood guitars and that an opened up maple 614ce with a bone saddle provided more sustain in the harmonics within the body that was generated by the top.

Furthermore, I can take that 614 and make it more rosewood-like by putting a solitary buffalo horn bridge in under the low E. Conversely, I can dry up a rosewood guitar that has too much harmonic sustain by substituting ebony bridge pins with bone. But that same guitar with different strings (the other outlier component) those same bone pins in that same rosewood guitar made it too dry sounding for my taster, bringing it very close to what I'd expect from mahogany.

The real bottom line is that no individual component of a guitar, even as prominent and critical as tonewoods are, are islands unto themselves. And so I sort of have to grin a bit when we talk about tonewoods as if they are the primary thing that matters when selecting guitars.

[bufflehead]

Originally Posted by PTL

Would I be correct that it comes down primarily to absorption and reflection of the frequencies that the top is producing?

I'm not sure that absorption/reflection is a useful metaphor, because it may oversimplify the role of tonewoods.

That said, I would suggest that rosewood helps emphasize overtones, thus adding a bit of reverb to a guitar, while mahogany helps emphasize fundamentals, adding warmth.

[Alan Carruth]

This gets extremely complicated very fast.

In the low range the guitar acts like a 'bass reflex' cabinet. The 'main top' resonance acts like the speaker, and the 'main air' resonance is the Helmholtz resonance of the cabinet. By capturing some of the energy from the back of the 'speaker' you end up with a more efficient system than you would with an 'infinite baffle', where the cabinet was closed and rigid. That said, there are several differences between the guitar and the usual bass reflex system.

One is that the 'free' air (Helmholtz) and 'speaker' (top) resonances are not tuned to the same pitch on a guitar; typically they're a fifth or so apart, with the 'air' pitch lower. Coupling between the two drives them further apart, so that they end up spaced out by about an octave. This 'bass reflex' range accounts for most of the actual power output of the guitar.

Usually the back also gets into the act here, unless it's extremely rigid and heavy. If the back is made of a dense wood, such as a rosewood, that also has low damping, it can act as a 'flywheel', storing energy in this range and pumping air through the sound hole. Thus an 'active back' can actually help increase the output of the guitar, but, it seems, only in this range; normally between G on the low E and the open G string pitch, more or less.

Basically, all of the energy from the strings has to go through the top to make any sound, and the top is the part that's best adapted to do so. The back is usually heavier, so it can't be as responsive, it's on the wrong side to send sound out toward the audience, and it's often up against somebody's pudgy avoirdupois. In a spectrum plot of the output of the guitar out in front when it's driven from the bridge back resonances tend to show up as 'dips' as they steal energy from the top.

This doesn't mean they're unwanted. The only guitar I ever tested that had no back resonances to speak of was an Ovation, and they're not usually seen as the paradigm of tone. Those back resonances can contribute 'color' to the sound. They only get into the act when they're moving, so a back that's heavy (so that they don't move much) and has low damping (so that the resonant bands are narrow) can give a 'colorful' sound without actually costing too much power. Rosewood anyone?

Lighter woods, such as mahogany, can make a stronger contribution in the 'bass reflex' range by responding quickly, and producing low-end 'punch'. There's a reason D-18s are so common in Bluegrass bands. Flamenco guitars use cypress, which is as low in density as a top wood, and also very low in damping, like a rosewood, to get both 'growl' and high-end 'color'.

Keep in mind that all of this is peripheral. 'The sound is in the top'; everything else just colors that. I once made a 'matched; pair of Classical guitars, one with BRW B&S and the other with oak. They were a bit different, and, at the time, I thought I could attribute those differences to the woods used. In retrospect (20+ years), and with a few more 'matched' pairs under my belt, I'm not sure they were any more different from each other than two guitars carefully made from 'the same' wood. The 'Leonardo Project' has shown that blindfolded listeners or players can't reliably pick out the B&S wood type using a wide range of woods. If there's one property that makes the most difference, I'd say it was probably density.

[cliff_the_stiff]

Originally Posted by Alan Carruth

This gets extremely complicated very fast.

In the low range the guitar acts like a 'bass reflex' cabinet. The 'main top' resonance acts like the speaker, and the 'main air' resonance is the Helmholtz resonance of the cabinet. By capturing some of the energy from the back of the 'speaker' you end up with a more efficient system than you would with an 'infinite baffle', where the cabinet was closed and rigid. That said, there are several differences between the guitar and the usual bass reflex system.

One is that the 'free' air (Helmholtz) and 'speaker' (top) resonances are not tuned to the same pitch on a guitar; typically they're a fifth or so apart, with the 'air' pitch lower. Coupling between the two drives them further apart, so that they end up spaced out by about an octave. This 'bass reflex' range accounts for most of the actual power output of the guitar.

Usually the back also gets into the act here, unless it's extremely rigid and heavy. If the back is made of a dense wood, such as a rosewood, that also has low damping, it can act as a 'flywheel', storing energy in this range and pumping air through the sound hole. Thus an 'active back' can actually help increase the output of the guitar, but, it seems, only in this range; normally between G on the low E and the open G string pitch, more or less.

Basically, all of the energy from the strings has to go through the top to make any sound, and the top is the part that's best adapted to do so. The back is usually heavier, so it can't be as responsive, it's on the wrong side to send sound out toward the audience, and it's often up against somebody's pudgy avoirdupois. In a spectrum plot of the output of the guitar out in front when it's driven from the bridge back resonances tend to show up as 'dips' as they steal energy from the top.

This doesn't mean they're unwanted. The only guitar I ever tested that had no back resonances to speak of was an Ovation, and they're not usually seen as the paradigm of tone. Those back resonances can contribute 'color' to the sound. They only get into the act when they're moving, so a back that's heavy (so that they don't move much) and has low damping (so that the resonant bands are narrow) can give a 'colorful' sound without actually costing too much power. Rosewood anyone?

Lighter woods, such as mahogany, can make a stronger contribution in the 'bass reflex' range by responding quickly, and producing low-end 'punch'. There's a reason D-18s are so common in Bluegrass bands. Flamenco guitars use cypress, which is as low in density as a top wood, and also very low in damping, like a rosewood, to get both 'growl' and high-end 'color'.

Keep in mind that all of this is peripheral. 'The sound is in the top'; everything else just colors that. I once made a 'matched; pair of Classical guitars, one with BRW B&S and the other with oak. They were a bit different, and, at the time, I thought I could attribute those differences to the woods used. In retrospect (20+ years), and with a few more 'matched' pairs under my belt, I'm not sure they were any more different from each other than two guitars carefully made from 'the same' wood. The 'Leonardo Project' has shown that blindfolded listeners or players can't reliably pick out the B&S wood type using a wide range of woods. If there's one property that makes the most difference, I'd say it was probably density.

Wow! Tons of information here! Nice answer.
MacPherson guitars has a tone wood guide with general descriptions of sound.

[Bruce Sexauer]

There are two distinctly different approaches to guitar construction, and to make it more complicated, they can be blended. I call them "vintage" (though not all vintage guitars are built this way), and "modern" (again, a general term).

My "vintage" means the entire instrument is live, particularly the back. Most of my work leans in this direction. The back and sides play a more pro-active role in this construction.

My "modern" means the emphasis is on the top, with the back and sides being treated more as a speaker cabinet, being relative rigid in favor of keeping the energy dissipation in the top. The back and side material is de-emphasized in this style as far as tonal influence is concerned. Many contemporary makers lean this way. I believe it explains why Taylors sound more similar to each other despite material choices, as compared to Martin's baseline traditional guitars. . . or mine, for that matter.

Evolved makers in the "vintage" style actually have a lot of control over tonal characteristics or diverse materials through controlling structural mass and compound flexing of the plates. Most modern makers simply go for rigid, though there are an abundance of different approaches.

In other words, it matters hugely who is building the guitar and not only their approach but how great their experience is, not to mention the difficult to define "talent level".

As always the builder matters, choose carefully, and trust them once you have.

[phavriluk]

I asked a custom builder what was important in voicing a guitar (and in his I heard the angels singing). The answer: everything matters. A hundred 'so what' incremental choices can add up to a whole lot. And I think it's easy to find a hundred things on any of my guitars that could be improved next time. I just wish I knew what those were.

[merlin666]

With the back and sides it also depends on the shape and volume (size) of the body, and if there is a flat or arched back. These factors contribute a lot to the volume (decibels) that the guitar can generate, and the subtleties of coloring tone can easily get lost. For example, the plastic ovation bowl may not contribute much to the specific tone color which is dominated by the top design (bracing and soundhole patterns, materials), but it projects that tone with much more might than other designs.

[PTL]

Thanks. Love and appreciate the observations. Keep them coming.

Off the top of my head:

1. Ovation - do ovations really project more with the rigid bowl than traditional acoustics? i've never had the impression that they do.

2. A bit off topic but related to damping, what is Koa like? I always throught that Koa has beautiful overtones - even more than Rosewood but apparently many do not think so, putting Koa between Rosewood and Mahogany. I've had many Koa built from my stash of Koa and everyone has turned out very lively. I have never thought Koa sounded like Mahogany but some do apparently? Curious how its damping compares to Rosewood and Mahogany.

THANKS again for all the comments. I'll read through them again to digest.

Keep them coming.