Tonewoods…and
multi-piece bodies
Tom S
Much is made of how specific woods "sound." There are those who feel
strongly that specific body woods lend certain tonal qualities to a solid-body
electric guitar, and there are those who feel they can describe (predict) these
specific tonal qualities in detail. There are a number of guitar-builder and
guitar-parts websites that even ascribe very detailed tonal aspects to
individual woods, as if these qualities were well-understood and universally
accepted.
Now the first question that should come up when reading these detailed
descriptions is "How is it that a piece of wood can impart a tonal
qualitiy to a solid-body electric guitar?"
The only reasonable theory is that sound waves generated by plucked strings
pass through the body to some degree, are affected in some way by the wood, and
are then transmitted back to the pickups which are mounted to the guitar in any
one of a number of ways.
Since the primary reception of sound waves that will be transmitted to the
guitar's electronics takes place at the the top of the pickup, which is pointed
away from the body and toward the strings, this may seem a bit unlikely to have
much of a noticeable effect. Because in this scenario, the sound wave would
originate above the body, pass into the wood, and then be required to somehow
bounce back to the pickups. Hmmm. But for argument's sake, let's say this is
true.
If it is true that individual wood species can lend specific tonal nuances to a
solid-body electric guitar's sound, then it can only be due to the way sound is
transmitted or propagated through the wood, and the consequent affect of that
on the frequency spectrum generated by the sound wave. In turn, there are only
a few feasible mechanical properties of wood that could account for variances
in how the sound wave is affected, even within a single species. They are:
- The density of the wood, including the size and nature of cell
cavities and pores
- The grain tightness, direction (fiber orientation), and/or grain
pattern of the wood, mostly as it relates to the wood's modulus of
elasticity (tendency to be temporarily deformed; more an aspect of
strength and functionality)
- The current moisture content of the wood
Now if these factors affect
the sound wave moving through the wood, which they would have to do in order to
affect the frequency spectrum, then variances in these factors from one piece
of wood to another should be expected to have varied affects on the sound wave.
No two slabs of wood are the same, nor can they be expected to propagate a
given sound wave in the exact same manner. So what affect can we expect if the
sound wave has to travel through two or more pieces of wood, joined by a
barrier of hardened glue?
It has been shown that sound can be propagated through wood for detecting
defects.1 Yet this has been demonstrated using ultrasonic sound
waves, and is dependent on an entry and exit point of the sound wave. In other
words, there was no "bounce-back" effect or reverberation involved.
As mentioned above, that would be required in a solid-body electric guitar.
In addition, the mechanical properties that could potentially affect a sound
wave can vary quite a bit, even between two pieces of a given species, let
alone between different species. So how can any timbral attributes be reliable
or accurately assigned to any particular species?
"Clear straight-grained wood is used for determining fundamental
mechanical properties; however, because of natural growth characteristics of
trees, wood products vary in specific gravity, may contain cross grain, or may
have knots and localized slope of grain. Natural defects such as pitch pockets
may occur as a result of biological or climatic elements influencing the living
tree. These wood characteristics must be taken into account in assessing actual
properties or estimating the actual performance of wood products."
2
So with variances in grain tightness and pattern, how the wood was sawn, and
any possible inherent defects within a specific piece of wood, how would it be
possible to ascertain how that piece of wood will affect the frequency spectrum
of a sound wave passing through it?
It's possible that the changes in the speed of the sound wave could have an
effect on overtones of a note or other parts of the spectrum. Yet if the sound
is affected by the change in speed at which the wave is transmitted through the
wood, then we have to go back to the same mechanical properties of the
individual piece that would cause that change. We might theorize that this
change could be consistent in a given wood species. However...
"There is no recognized independent effect of species on the speed of
sound. Variability in the speed of sound in wood is directly related to the variability
of modulus of elasticity and density."2
That means every piece of wood has to be judged on its own merits, even two
pieces from the same species. So any affect on the sound due to variances in
the speed of the sound wave would not be attributable to the difference in
species.
In a nutshell, it would seem that any
sonic characteristics attributed to a particular species would be very hard to
define with any degree of accuracy or consistency. Even if it could be done,
those characteristics could at best only be ascribed to a single piece of the
wood because in a multi-piece body, there would be complete disconnects in the
continuity of mechanical properties between the joined pieces.
So in an absolute best-case scenario, any tonal qualities of a specific species
of wood used in a solid-body electric guitar, assuming they could actually be
defined, could only legitimately apply to a one-piece body. Variances in two or
more adjoining pieces, plus the glue, would certainly play havoc with the sound
wave passing through it, rendering any tonal predictions impossible.
So whatcha think? Remember, we haven't even gotten to the possible effects of
finishes (or the lack of effects). 
References:
1. Lumber defect detection by ultrasonics. Forest Products Laboratory,
Forest Service, U.S. Dept. of Agriculture, Madison, WI Research Paper FPL 311,
1978
http://www.fpl.fs.fed.us/documnts/fplrp/fplrp311.pdf
2. Mechanical properties of wood. Green, Winandy, Kretschmann: From
http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf