Tonewood is a term that people in the guitar making trade use to describe wood with acoustic qualities. The term certainly can apply to the top, back and sides equally, but it is more often used to define the quality of the guitar top plate.
This is mainly due to the fact that most of the tone and volume a guitar possesses, is a result of the type and quality of the top wood. Couple that with the bracing pattern, and the manner with which the guitar was constructed and you have considerable control over the tone production of an acoustic or classical guitar.
The Back Plate and Sides Are Secondary Tone Generators:
The back plate and sides play a subordinate role in that they provide the reflection qualities of the vibrations outward toward the top and soundhole.
I really don’t want to understate the importance of the back and side wood selection either, but this theory was evidenced by the famous paper mache guitar that was constructed by Antonio De Torres, when he was experimenting with the importance of top wood and top bracing.
He made a guitar with the back and sides of paper mache, along with his typical top and bracing with which had been experimenting. It is understood that this guitar had tremendous sound and volume qualities and proved without a doubt the importance that the tonewood and top bracing exhibit.
Additional Factors That Affect Tone:
Since the main vehicle for sound production for any acoustic instrument is the top, the top tonewood plays a very significant role of importance. The tone is enhanced by the shape of the instrument, the volume of air within the body of the instrument, the amount of vibrations that are inherent within the wood, the moisture content of the wood, the type and arrangement of the bracing and the mass of the tonewood.
There are many choices available in fine instrument tonewoods and each has its own quality and color that contributes to the sound of each guitar. As mentioned above, the moisture content in tonewood greatly affects its tone production quality. The dryer the wood, the more tone it is capable of producing, since moisture produces a dampening effect within the wood.
Quality guitar tonewood for top plates typically has a moisture content of around 6%. This is often attained by either air-drying techniques, whereby the wood is kept in heated storage and stacked in such a manner so as to allow adequate air circulation. Tonewood is also kiln dried to achieve the desired moisture content.
Wood that you obtain from the lumber yard will often not meet these moisture content requirements, and needs to be air dried in your shop for many months, or better yet, up to a couple of years.
Continue to Age The Wood After Purchase:
After kiln drying, the wood continues to ‘age’ and dry even more, especially if it is properly stacked and stored in a humidity-controlled environment. This is why you see tonewood that is advertised as aged for 2 years to 10 years or whatever.
If you intend to make quite a few guitars, you would be well advised to purchase quality tonewood that is properly dried, and buy as much of it as you can afford. Then store it in your shop, so you have a great supply of aged tonewood to draw from for many years. I still use wood that I purchased 30 years ago. Now this is aged wood!