One of the Unit Stress Factor considerations given for wood strength is called the Extreme Fiber Strength, expressed as Fb. This is the amount of force the wood fibers can take when tested in a laboratory controlled atmosphere. We are concerned about the strength as it relates to bending for guitar bracing.
The second Unit Stress Factor is the Modulus of Elasticity. This unit stress is used to calculate the amount of deflection the member will resist.
The Testing Method:
The testing is done for structural lumber as all of these woods are not only used for guitar construction, we also build our homes with them. The difference being in the quality and grading of the wood. The testing agency of the Unit Stress Table that is used for this comparison is the Northeastern Lumber Manufacturer’s Association and the Northern Hardwood and Pine Manufacturers Association.
The Lumber Grading:
The lumber is tested according to the following grades:
Instrument Quality Wood:
The closest category that our guitar brace woods fall into are the Select Structural, even though guitar wood is the best of the best. Guitar wood is usually referred to a “Instrument Quality” and is the highest and most select of the woods available. This is because of our concerns for grain tightness, consistent coloration, minimum run-out and quarter sawn stock.
The Wood Stress Factors:
Although I couldn’t find all of the woods we use in guitar building in the same Unit Stress Table, I did find quite a few of them. The comparisons are shown in the table at the top of this article.
How to Read the Table:
The table is actually very easy to read. These factors are the laboratory results for each species of wood listed and you can relate the units of stress to each other directly. The higher the number the more strength the wood has.
As we discussed before Fb = Extreme Fiber Stress and is the bending that the wood will resist in pounds per square inch (psi). It is measured Perpendicular to the grain of the wood. Ft = the Tension Parallel to the Grain the wood will resist and is measured in pounds per square inch. (psi). E = Modulus of Elasticity and is a stress that resists deflection in the wood and is measured in pound per square inch (psi).
A couple of standouts in this table. Take a look at the Red Spruce and the California Redwood numbers. They are the highest and strongest in the table. That is a surprise, esp. for the Redwood, which is such a lightweight wood. The Red Spruce is also called Adirondack Spruce and was the wood of choice for the pre-war Martin Acoustic Guitars – (not a surprise) and is still the most prized wood used for acoustic guitar tops today.
So you can take this table and discussion for what it is worth – it gives you a bit of guidance into the different strengths possessed by our different tonewoods and possibly can help you make a decision as the to possible stiffness each will possess.