This post is going to be all about wood and why I like using the analogy of the structure of wood to understand and talk about composite structures. What I want to talk about are the tonal qualities of woods used in musical instruments – specifically the sound boxes, necks, and fingerboards of stringed instruments – primarily violins and guitars. Since these are very similar instruments with very similar sound quality requirements, the woods used for both of these instruments are the same. Of course, the shape of the sound box also has quite a bit to do with the tonal qualities of the instruments, but this blog is about composites and not acoustics, so we are going to stick with the properties of the wood that is used to make these stringed instruments.
To tie this to composites, the tonal quality of the wood is based on the resonant frequencies of the instruments and the woods that they are made out of. And the resonant frequencies of the woods are tied directly to the stiffness properties of the string and glue that make up the wood and the density or weight per cubic inch of the wood. Stiffer, lighter wood has higher resonant frequencies than less stiff or heavier wood.
Spruce is the wood that is the most common for use for front of the sound box for both violins and guitars. This is because Spruce has a higher stiffness to weight ratio than most other woods. This means that for its density which is low for a wood, it is rather stiff as in it resists bending more than a less stiff wood. And it is soft enough that it can be easily worked, and strong and stiff enough that it can be sliced thin for use as what is called the sound board or front of a violin or guitar, while keeping the instrument light enough that the player can hold it up to their chin all the way through a long passage of music.
For the back and neck of both violins and guitars, Maple is the preferred wood. It is harder, denser, and stiffer than Spruce. It also has a much more complex grain pattern than spruce. The cellulose fibers don’t all go one direction, but instead crisscross each other so maple is stiff and strong in all directions. In the violin or guitar neck, this stiffness and strength allow Maple to stand up to the high compression stresses from the strings. This keeps the strings at a constant height from the neck so that the violinist or guitarist can play the thing. For the back it is also resistant to bending in both directions, so it keeps the shape of the violin or guitar constant – no matter how tight the strings are. This makes for a constant pitch coming from the sound box of these stringed instruments. This is, of course, because of the mix of cellulose strings and lignin and pitch glues that makes maple so stiff and strong. And, since the maple is so strong and stiff, it has a tendency to directly transfer the vibrations of the strings to the sound box where the sound box is connected to the neck, imparting a resonant tone to the instrument.
To bring this back to composites, what the craftspeople who make these instruments are doing is selecting a mix of the natural strings and glues afforded them by mother nature. They also choose how those strings and glues are arranged by using a variety of woods. This is because mother nature is such a good engineer that she has created all of these wonderful things for us to use. These craftspeople also choose the way the wood is cut (what directions the strings go) and also the way it is dried (choosing the properties of the glues) to enhance the tonal quality of the instrument. Wood from very mature trees and from the center of the trunk is used for the finest instruments. So, effectively, they are choosing a string, a glue, and a fiber orientation for each piece of the instrument as they cut it to shape and put it together. This is the same thing that a composites designer does when they design a composite structure based on a set of structural requirements.
That’s about enough for my wood series in this blog. In another blog I talk about composites design as systems design and why it is so important to have a systems mindset when you approach using composites. As an example – let’s say you’re a maker and you want to make a bow because you’re also an archer. And you want to make a really good bow that not only fits you but is really accurate. You need to know everything about how a bow shoots an arrow, like the size and stiffness of the bow, what the pull should feel like, how straight does the pull-back on the bow need to be, how thick you want the handle where you hold it, where and how do you nock the arrow when you pull the bow back, etc. All of these things make up the system architecture, system design, and system requirements for your “make a bow” project.
Remember:
When you make the material, you make the part, and
When you make the part, you make the material.
See you next time.
Comments