Acoustical Physics 101
Here’s a really simplified explanation of our understanding of the physics behind the acoustic guitar. We welcome your collective editing suggestions.
The guitar body is essentially an enclosure of a typical shape with a hole in it – it’s what the physicists call a “ported enclosure”. The details are very complicated, but the take away is that every shape and soundhole combination will have a characteristic resonant frequency. In the simplest sense, large guitars with lots of air volume have lower resonant frequencies – while small guitars with less air volume have higher ones.
As the strings vibrate they exert varying forces on the bridge, which sends bending waves across the top. This triggers a complex cascade of events that cause the enclosure to resonate. The top, back and sides, are layered around that foundation of acoustical energy, mixing and interacting in unique ways that ultimately define the guitar’s voice.
The soundboard is the largest frequency contributor by far, but certainly not the only one. The back can be an active contributor if it’s constructed lightly enough, and the tonewoods do make a difference in terms of how the back and sides respond to acoustical energy from the top.
We’re learning that the sides don’t contribute much frequency response on their own, but their stiffness and mass seem to have a significant impact on the volume of the guitar. Just today we noticed that as part of their tonal makeover of the 800 series, Taylor has added side bracing … news travels fast!
You can learn more about the hard science behind these principles, in these informative comments by Trevor Gore & Alan Carruth.
The Science of Tonewoods
“If a guitar’s body shape produces the sonic equivalent of a meal, think of tonewoods as the seasoning. The unique acoustic properties of woods help color a body shape’s fundamental sound. The key is to find the woods that match up best with your playing style and intended applications. It might be rosewood’s low-end growl and sizzling trebles, the mid-range overtones of mahogany, the focus and projection of maple, or the warmth of a cedar top for fingerpicking.” – Bob Taylor
Much has been written about selecting and evaluating woods for musical instruments, and sorting it all out is difficult to say the least. Even though it’s 20 years old, this article written by Dana Bourgeois stands out as one of the most concise discussions of the topic we’ve seen.
To summarize Dana’s article, the fundamental characteristic we need to focus on when evaluating tonewoods is the “velocity of sound”, or the ability of the wood to transmit received energy. Lively woods make the best facilitators, while those with a lower velocity of sound (or high internal dampening) are more transparent and actually absorb sonic energy. Red Spruce is a good example of a tonewood with a high velocity of sound, while Gabon Ebony is king of the low velocity dampeners.
In addition to testing for velocity of sound, luthiers also make use of the tapping technique to listen for harmonic content.
“Like a string, a piece of wood is capable of producing a fundamental tone and an array of harmonics. Though the presence and strengths of individual harmonics are distinctly influenced by changes in the geometry and mass of the piece of wood, elements such as clarity of tone, relative harmonic complexity, and high, low, or mid bias can readily be discovered by holding and tapping a piece of wood in a variety of ways.” – Dana Bourgeois
Using these simple principles we can begin to frame a deeper understanding of our tonewoods, and how they are likely to affect the acoustical qualities of our instruments. When we layer that knowledge on top of the underlying physics, we can then start to make reasonable assumptions about how the use of a certain tonewoods will impact the voice of particular guitar.
There are many tonewood reference guides on the Internet, but here are two of our favorites: