Q: Is there anyone in the woodworking business that will take on a question? Here goes...rosewood. A lovely smelling wood, moreover, a gigantic gorgeous wood that takes the sound of a guitar to a new level.
And the question is: Several guitar manufacturing companies have fallen victim to the CITES legislation and therefore, stopped using rosewood on their very prominent guitar models, or as another story would have it are now placing the cost of new non-aged rosewood guitars through the roof.
Several companies are using a substitute referred to as high pressure laminate rosewood which by all research indicates that it is particles of rosewood put together under enormous pressure and banded together thus creating the laminate. Is there a difference; my ears need to know!
Also, can anyone take the time and write out or explain the difference between mahogany and rosewood? Especially when used in the production of guitar making.
A: The acoustic, sonic or audio properties of wood are quite complex, but we can provide a lot of understanding if we use an analogy. Consider a pipe in a pipe organ. The length and diameter, as well as the stress, that the pipe is under, and the thickness of the wall of the pipe all affect the sound. The sound is actually related quite strongly to the vibration of the air within the pipe.
So, now let's consider wood. Wood is made of miniature, hollow tubes, called cells, about 3 to 5 millimeters in length and with a diameter of around 1/10 to 1/100 of the length. Sometimes these wood tubes in a species are filled with different amounts of chemicals that give wood its odor, color, taste and the ability to conduct water, plus other properties. These chemicals within the cells maybe be water soluble, alcohol soluble or basically non-soluble. The more chemicals in these cells, the smaller the amount of air space and the greater the variation in diameter. As an example, white oak is used for wine and whiskey barrels because these chemicals totally block the movement of liquids. However, red oak is quite permeable.
Another property is the cell diameter and the thickness of the cell wall. Of course, the cell length is also important. These properties are fundamentally the differences in species, plus differences in growth rate of the tree.
One other key wood acoustic properties is the stress that the cells are under. When we dry wood, it tries to shrink, but is restrained (various reasons) and so, on a small scale has stress within the cell. Most guitar manufacturers heat the wood and use months of heat to cause the wood to relax these stresses.
Because wood shrinks as it dries and swells when it gains moisture, the moisture content does have a small effect on acoustic properties.
Another factor is whether we want to hear a clear set on tones and overtones, of if we want a more mellow sound with a huge variety of overtones. The clearer sound comes when the majority of wood cells are close to the same size. (Pretty complex indeed.). Or maybe we do want these hollow cells filled so that very little vibration of the air inside them will contribute to the sound we hear...the vibration of air within the chamber of the instrument itself will be the key determinant on the sound.
So, now we return to the question about the qualities of different wood species. The truth is that due to many properties that affect the acoustic properties of wood, and maybe even what we in 2018 consider to be good and bad sound, we need to consider, basically by trial and error, how a specific species sounds and how variable that sound is from piece to piece. We also want to be sure that these hollow cells are not filled with adhesive or finish, as we might expect different acoustic properties when filled, that we likely would find inferior...but who knows for sure without trying it? We think we know that open, uniform sized, and stress free cells are best.
Certainly, the uniformity and openness of spruce (a top guitar and violin wood) is noteworthy. Another example is that hickory drumsticks ring pleasantly when struck, but pecan hickory sticks thud. Some drummers like white oak sticks better than other species. The cells in white oak are very well plugged. We think we know that open, uniform sized, and stress free cells are best, and the primary musical species are open and uniform, but ....
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.