As some of you will recall, TAM 324 teaches about the properties of hardwood vs. softwood. To recap, hardwoods are deciduous and softwoods are coniferous.  Deciduous trees shed their leaves annually, while coniferous trees produce needles and cones. Hardwoods have, on average, less variation in density than softwoods, but are not necessarily denser.  

 

Strength also depends on more than just the hardwood/softwood classification. For example, the modulus of elasticity for walnut (hardwood) is 11,200 MPa, while the modulus for Norway spruce (softwood) is 11,600 MPa. In addition to the presence or lack of needles, difference between the two types of wood is best seen at the cellular level.  

 

With so many varying properties, crafting wooden guitars involves a good deal of mechanics and physics. The sound produced by a guitar is governed by properties of resonance. Resonance is when sound is amplified or extended as a result of reflecting off a particular surface or by causing another object to vibrate at the same frequency.

 

In acoustic guitars, resonance is affected by the type of material used, the body shape and size, and whether the body is solid or hollow. Using one piece or multiple pieces of wood for parts like the soundboard and back also makes a difference. For electric guitars, properties from the build affect sustain, which is the amount of time during which a played note can be heard before becoming inaudible.   

 

In addition to affecting overall resonance, different woods produce different tones. For example, walnut typically has a warmer tone than maple. Alder also has a warm sound but less mid and bass. These properties are most relevant for the soundboard in hollow bodies and the body in solid bodies.

 

The soundboard is the focus for acoustic quality. It needs to be light so that little energy is required to get it to vibrate. Soundboards must also be strong enough to sustain the tension applied by the strings, requiring a fine balance between flexibility and rigidity. The soundboard thickness for cedar (softwood) is suggested 2.8 mm. Hardwoods like spruce have a slightly smaller suggested thickness.  

 

Material for the back and sides needs to be considered not only for resonance but also for its ability to be shaped and bent. Some manufacturers use a laminate like plywood because it’s less expensive than solid wood and can be easier to work with.  In more recent times, some builders have also crafted acoustic guitars partly or entirely from carbon fiber. The evolution thus far of materials suited for guitar building leads me to wonder what will come next.