Tonewood: some general thoughts
A musical instrument built from wood is almost a living thing. It is not a static thing, (affected by use, temperature and humidity), and it is not exactly like any other, (as no two pieces of wood are exactly the same). Its very nature depends upon the wood used in its construction, as each type of wood has different properties.
There are several types of wood needed to build an instrument, because of the nature of the properties required by the different parts of the instrument. The soundboard must generate vibrations; the neck must resist bending; the back and sides will affect the tone; the struts and tone bars will affect the sound; the fingerboard must be hard; and certain woods must be flexible enough to make bindings, linings, and so on.
Tonewood: this is the term used for wood that
will generate the instrument's sound. Typically two different types of
tonewood are used, one for the top, and the other for the back and sides.
The types and combinations of these, together with the shape of the
instrument, are largley but not wholly responsible for its
A word of warning here, as I am sometimes asked to identify exactly what woods are used in a given instrument.... within each species, there is as great a variety between boards as there are trees!! Ebony for example varies from light grey to black. Trying to identify woods by colour alone is almost impossible! Even the suppliers can't do it.... Frank Ford has a an interesting stroy...
The common grading scale for tonewoods is A, AA, AAA, and AAAA (or master grade). This grading scale is used by most retail sellers of tonewoods but is very subjective.
Grade A is clear of knots, swirls, and holes and has fairly straight grain. It may have uneven color, streaks, and wide apart/uneven grain lines. It will probably not be perfectly quartersawn. The piece of wood will also probably have runout. There will be little cross-grain figure.
Grade AAA has even overall color, even and close grain lines, perfectly quartersawn along the whole width of the board, with minimal runout. Grain lines will probably be closer than 12 lines per inch. Cross-grain figure, also called silking or bearclaw will be visible.
Grade AA is somewhere between the two.
Grade AAAA - Master grade has no color variation, very pronounced cross-grain figuring, in addition to being perfectly quartered with minimal runout and close and even grain lines.
(NB 'runout' is where the grain does not run parallel to the length of the board for all its length.)
A general range of thickness of guitar tops is between
0.130" - 0.095". The stiffer a board is, the thinner
it can be and still be structurally adequate.
The stiffness of a wood beam is measured by how far the beam deflects when a certain amount of pressure is applied to it, or how much pressure must be applied to make the beam deflect a certain distance. A formula has been derived that measures the stiffness of a sample (MOE).
Top tonewood, is usually either spruce (in the mandolin
family) or cedar (in guitars) chosen because they promote
vibrations very well. Spruce
is number one on the list of strength-to-weight ratio for all the woods
in the world, and cedar the second. Spruce is a little crisper and more
powerful, whilst cedar is a little mellower.
wide grain top will tend to produce stronger bass response,
because there are fewer stiff grain lines so the top is more flexible.
From the flexibility comes a lower natural resonant frequency and more
easily produced bass notes. A
narrow grain top will tend to have comparatively stronger
treble and more subtle bass, because lots of hard grain lines, is much
stronger and has a higher natural vibrating frequency.
backs and sides of virtually all guitars are made of hardwood. Strange
as it may seem, softwoods just don't bend well and are more difficult to
form into guitar sides.
Sides just don't enter into the vibrations nearly as much as tops and backs. They define the body shape and support the top and back, but the back is very significant. The size and positioning on struts and tonebars is as crucial here as on the top.
Mahogany is the most commonly used hardwood for guitar backs because it's relatively economical, durable, attractive, easy to work and resonant. For the mandolin family it is either maple or rosewood. Mahogany is probably the least dense hardwood, and rosewood one of the most dense.
Click on the wood grain photo for a larger picture, and the hyper-linked name for more informatioon.