Tonewood

Tonewood refers to specific wood varieties that possess tonal properties that make them good choices for use in woodwind or acoustic stringed instruments.
Varieties of tonewood.
As a rough generalization it can be said that stiff-but-light softwoods (i.e. from coniferous trees) are favored for the soundboards or soundboard-like surface that transmits the vibrations of the strings to the ambient air. Hardwoods (i.e. from dicot trees) are favored for the body or framing element of an instrument. Woods used for woodwind instruments include African blackwood, ("Dalbergia melanoxylon"), also known as grenadilla wood, used in modern clarinets and oboes. Bassoons are usually made of maple, especially "Acer platanoides". Wooden flutes, recorders, and baroque and classical period instruments may be made of various hardwoods, such as pear wood ("Pyrus" species), boxwood ("Buxus" species), or ebony ("Diospyros" species).
Mechanical properties of tonewoods.
Some of the mechanical properties of common tonewoods, sorted by density.
Carbon-fiber/Epoxy added for comparison, since it is sometimes used in musical instruments.
Data comes from the Wood Database, except for 𝜈LR, Poisson's ratio, which comes from the Forest Product Laboratory, United States Forest Service, United States Department of Agriculture. The ratio displayed here is for deformation along the radial axis caused by stress along the longitudinal axis.
The shrink volume percent shown here is the amount of shrinkage in all three dimensions as the wood goes from green to oven-dry. This can be used as a relative indicator of how much the dry wood will change as humidity changes, sometimes referred to as the instrument's "stability". However, the stability of tuning is primarily due to the length-wise shrinkage of the neck, which is typically only about 0.1% to 0.2% green to dry. The volume shrinkage is mostly due to the radial and tangential shrinkage. In the case of a neck (quarter-sawn), the radial shrinkage affects the thickness of the neck, and the tangential shrinkage affects the width of the neck. Given the dimensions involved, this shrinkage should be practically unnoticeable. The shrinkage of the length of the neck, as a percent, is quite a bit less, but given the dimension, it is enough to affect the pitch of the strings.
The sound radiation coefficient is defined as:
formula_1
where formula_2 is Young’s Modulus of flexure in Pascals (N/m2, i.e. the number in the table multiplied by 109), and ρ is the density in kg/m3, as in the table.
From this, it can be seen that the loudness of the top of a stringed instrument increases with stiffness, and decreases with density. The loudest wood tops, such as Sitka Spruce, are lightweight and stiff, while maintaining the necessary strength. Denser woods, for example Hard Maple, often used for necks, are stronger but not as loud (R = 6 vs. 12).
When wood is used as the top of an acoustic instrument, it can be described using plate theory and plate vibrations. The flexural rigidity of an isotropic plate is:
codice_1
where formula_2 is Young’s Modulus for the material, formula_5 is the plate thickness, and formula_6 is Poisson’s ratio for the material. Plate rigidity has units of Pascal·m3 (equivalent to N·m), since it refers to the moment per unit length per unit of curvature, and not the total moment. Of course, wood is not isotropic, it's orthotropic, so this equation is at best an approximation.
The value for formula_7 shown in the table was calculated using this formula and a thickness formula_5 of ⅛″=3.175mm.
When wood is used as the neck of an instrument, it can be described using beam theory. Flexural rigidity of a beam (defined as formula_9) varies along the length as a function of x shown in the following equation:
where formula_2 is the Young's modulus for the material, formula_12 is the second moment of area (in m4), formula_13 is the transverse displacement of the beam at x, and formula_14 is the bending moment at "x". Beam flexural rigidity has units of Pascal·m4 (equivalent to N·m²).
The amount of deflection at the end of a cantilevered beam is:
formula_15
where formula_16 is the point load at the end, and formula_17 is the length. So deflection is inversely proportional to formula_9. Given two necks of the same shape and dimensions, formula_12 becomes a constant, and deflection becomes inversely proportional to formula_2—in short, the higher this number for a given wood species, the less a neck will deflect under a given force (i.e. from the strings).
Read more about mechanical properties in "Wood for Guitars."
Selection of tonewoods.
In addition to perceived differences in acoustic properties, a luthier may use a tonewood because of:
Sources.
Many tonewoods come from sustainable sources through specialist dealers. Spruce, for example, is very common, but large pieces with even grain represent a small proportion of total supply and can be expensive. Some tonewoods are particularly hard to find on the open market, and small-scale instrument makers often turn to reclamation, for instance from disused salmon traps in Alaska, various old construction in the U.S Pacific Northwest, from trees that have blown down, or from specially permitted removals in conservation areas where logging is not generally permitted. Mass market instrument manufacturers have started using Asian and African woods, such as Bubinga ("Guibourtia" species) and Wenge ("Millettia laurentii"), as inexpensive alternatives to traditional tonewoods.
The Fiemme Valley, in the Alps of Northern Italy, has long served as a source of high-quality spruce for musical instruments, dating from the violins of Antonio Stradivari to the piano soundboards of the contemporary maker Fazioli.
Preparation.
Tonewood choices vary greatly among different instrument types. Guitar makers generally favor quartersawn wood because it provides added stiffness and dimensional stability. Soft woods, like spruce, may be split rather than sawn into boards so the board surface follows the grain as much as possible, thus limiting run-out.
For most applications, wood must be dried before use, either in air or kilns. Some luthiers prefer further seasoning for several years.
Some guitar manufacturers subject the wood to rarefaction, which mimics the natural aging process of tonewoods. Torrefaction is also used for this purpose, but it often changes the cosmetic properties of the wood. Guitar builders using torrefied sound boards claim improved tone, similar to that of an aged instrument. Softwoods such as Spruce, Cedar, and Redwood, which are commonly used for guitar sound boards, are easier to torrefy than hardwoods, such as Maple. 
On inexpensive guitars, it is increasingly common to use a product called "Roseacer" for the fretboard, which mimics Rosewood, but is actually a thermally-modified Maple.
"Roasted" Maple necks are increasingly popular as manufacturers claim increased stiffness and stability in changing conditions (heat and humidity). However, while engineering tests of the ThermoWood method indicated increased resistance to humidity, they also showed a significant reduction in strength (ultimate breaking point), while stiffness (modulus of elasticity) remained the same or was slightly reduced. Although the reduction in strength can be controlled by reducing the temperature of the process, the manufacturer recommends not using its product for structural purposes. However, it is perhaps possible to compensate for this loss of strength in guitars by using carbon-fiber stiffeners in necks and increased bracing in tops.