How (and why) wood changes over time 7 Years, 5 Months ago
We've had a few threads on this topic in the past. Not too long ago I posted a response by Bob Taylor to a question about how wood changes over time.
Believing that there's more to be said on the topic, Acoustic Guitar magazine had this to say:
A guitar's reaction to humidity, hardening of glue and finishes, and the vibrations produced by playing contributes to its tonal metamorphosis. Changes to the wood itself are also likely to be contributing factors, though this remains controversial among some players. Luthiers and scientists studying wood have a number of theories about what happens as instruments age; unfortunately there has not been much definitive study to absolutely identify the causes of the tonal changes that most agree do happen with age.
Top woods like spruce, cedar, and redwood have resins that harden, as well as volatile organic compounds (VOCs) that evaporate slowly with age–changes that can be accelerated with heat (this is also true for the woods used for backs and sides, but they contribute less to tone than the top woods). The hardening of resins will affect the flexibility of the wood, making it slightly stiffer, while the loss of VOCs will make the wood less dense. These changes tend to favor high-frequency response and how "fast" or sensitive the instrument feels and sounds.
Wood is primarily made up of cellulose fibers in a matrix of lignin and a sugar-like compound called hemicellulose . As wood ages, the hemicellulose migrates out of the wood, regardless of any vibrations it has experienced. As it does so, the wood loses strength and stiffness, but it also loses weight in even greater proportion to the loss of stiffness. The net result is an increase in the stiffness-to-weight ratio. As long as the wood retains enough stiffness and strength to withstand string tension, there will be a net improvement in responsiveness. There are several heat treatments (separate from kiln drying), as well as an enzyme process for removing hemicellulose from wood, that are used to make better-sounding tops. These treatments are usually done before the tops are glued up, so any changes will take place before final dimensioning.
The third change is likely akin to metal fatigue, but in our case, it's wood fatigue. Guitars vibrate in patterns that define areas of little motion called antinodes and other areas of great relative motion called nodes . You can think of these as hinges in the wood that allow certain patterns of vibration at various frequencies. Constant vibration can subtly weaken the hinge lines, allowing for greater motion, and these effects are most noticeable at lower frequencies—the bass region of guitar tone. Processes that apply artificial vibrations to the guitar are designed to accelerate this aging process, and some luthiers and players have noted the changes that can take place when putting guitars near loudspeakers for extended periods of time.
So there are two possible effects that might affect high-frequency response and one that seems to increase bass response, and this tracks with the changes that many musicians and luthiers hear as a guitar ages. "More responsive to touch," "better low end," and "better harmonic response" are all commonly reported effects.