Selecting wood for musical instruments

Tonewood Attributes

  • free of structural defects
  • very strong
  • Lightweight
  • Carries sound well

Selection Guidelines

  • no knots, worm holes, fungus, rot, cracks, or pitch pockets
  • quartersawn, straight grain, minimal runout
  • stiff along the grain and across the grain
  • properly dried or seasoned
  • has a ringing sound when tapped

Evaluation Methods

Evaluating Tonewoods

No one evaluation method is sufficient to choose the best tonewood specimen, especially not a scientific one.
Of the three categories of tonewood selection techniques, two depend on experience and personal preferences.
Measurement methods will help to narrow the selection to those pieces which meet the physical structural requirements for a musical instrument.
The following methods are important for all the woods which make an instrument, but the most important part is the top.

Visual Inspection

 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 and is very subjective. Although many of the visual attributes of a piece of tonewood are indicators of structural strength and good taptone.

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, also called compression. It will probably not be perfectly quartersawn or the piece will be perfectly quartered only in part of its width. 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 A and AAA grade.

Grade AAAA or Master grade has no color variation and very pronounced cross-grain figuring in addition to being perfectly quartered with minimal runout and close and even grain lines.

Physical Measurements

 Stiffness along the grain and stiffness across the grain is the main indicator that a builder uses to determine the dimensions of the soundboard and other parts of a musical instrument.
This has traditionally been a learned art taught by a master to an apprentice. A luthier had to learn the "feel" of the wood and plane and scrape the wood to the correct thickness and make the bracing to the correct size for the finished instrument to be strong enough to withstand string tension, yet pliant enough to efficiently translate string energy to sound waves.
There are now more precise ways to measure the strength of tonewood. Precise measurement of tonewood strength can be helpful in deciding the correct thickness and the correct amount of bracing but not without the knowledge of experienced instrument makers correlated to those strength measurements.
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 same thing applies to bracing. The stiffer the brace wood, the smaller the braces can be and still provide adequate structural support. A general range of brace size is not more than 5/16" wide and not more than 3/4" tall.
The thinner soundboard and smaller bracing translates to less mass. Less mass in a soundboard translates to a more responsive and louder instrument.

Note:

Measurement is quantifying something using a standard. Whatever standard is used needs to be used consistently throughout the process. Within the standard, the same units need to be used.
If a metric standard is used, then grams for weight and millimeters or centimeters for length would be appropriate.
If using a non-metric standard, then ounces or drams would be used for weight and inches (64s of an inch or hundredths of an inch) would be used throughout all measurements and calculations.
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).

E = P*l^3/4w*t^3*d
P = the amount of pressure applied
l = the length between supports
w = the width of the wood sample
t = the thickness of the wood sample
d = the distance the sample deflecteded when pressure was applied

An engineer or scientist would limit variability to one of two dimensions; Pressure or deflection. In other words, the wood sample would always be the same dimensions, and either the deflection would always be the same or the pressure would always be the same. This simplifies the collection of data over many tests and helps to reduce error.

MOE Values of some Tonewoods
Species MOE(x10^6 in/lb^2) Weight(lb/ft^3) Top thickness?
redwood 1.34 28  
Western red cedar 1.11 23 .130"
yellow cedar 1.42 31  
englemann spruce 1.30 23  
white spruce 1.43 28  
red spruce 1.61 28 .110"
sitka spruce 1.57 28  
Indian rosewood 1.78 53  
African mahogany 1.31 32  
ebony 1.43 45  
Honduras mahogany 1.42 30  
Brazilian rosewood 1.88 47  
bigleaf maple 1.45 34  
black walnut 1.68 38  

NOTEs:
"Top Thickness?" is a possible safe minimum value.
Both red and white spruce are sometimes called Adirondack, but note the difference in MOE.

Sound Response

 This evaluation method is the most subjective and variable.
Some luthiers will tune a top to some note like F sharp, others will just listen to the top after it had been joined for a musical sound. Still others will sprinkle glitter onto a braced top and vibrate it with a transducer or speaker and make adjustments to braces and top thickness based on the patterns.
There is some value to sound response evaluation as a possible last step validation of the other two methods, or as a way to select tonewood for a certain final sound.
There are other things though, that affect tonality in a finished instrument more than tap tone. Things like the volume of the body, the size and shape of the soundhole(s), the scale length, and the size and composition of the strings to name a few.
Even the species of wood selected for a top probably has more of an effect on the final sound of an instrument than tap tone.
In the end, tap tone methods are at least as variable as musical styles or individual personalities.

Glossary
Quartersawn A method of cutting sections of wood perpendicular to the growth rings of a piece of lumber. Another term for quartersawn is quartered.
Runout Wood that is split with a wedge divides along the weakest part of the wood. When wood is cut by a blade, the wood fibers are torn along the path of the blade. Runout usually occurs in wood cut by a saw blade.
Wood that is split with a wedge will be stronger than that cut by a sawblade and is preferable for tonewood.
The reason is that in split wood, the wood fibers run all the way through the piece. In wood cut by a sawblade, the wood fibers are cut short by the blade and do not run all the way through the piece of wood.
Runout can be detected when planing a piece of wood. Planing against the grain will pull the blade into the wood causing gouges. Visual inspection of the edge of a piece can also show runout where the grain of the wood is not parallel to the edge the whole way down the board. A soundboard with runout will also be noticably less stiff on the end of the board where the wood grain terminates before the end.
Tonewood Wood with the qualities and attributes required for use in musical instruments.
Compression An area where the annular lines change from evenly spaced to significantly farther apart. Compression may occur as a result of a series of warmer than normal winters where the tree has a longer growing season.
Annular lines Also called growth rings or grain, the lines in wood that correspond to one year of growth. For tonewood used for musical instrument tops and soundboards, close annular lines are one of the indicators of strength and good musical qualities. Close growth rings occur in trees that grow slowly. Wood cutters go to great effort to find old trees in areas of poor soil and low moisture with short growing seasons, usually high in the Northern mountains on the lee slopes.

Tonewoods

Sitka Spruce
Adirondack Spruce
 
Split Sitka Spruce
Western Red Cedar

Links to Other Tonewood Information


European Spruce


by Paul Hostetter, luthier