Author: Chuck Morrison

Way too much attention has been given to the back and sides in discussions of guitar responsiveness. There are no magic woods that if used for a back and/or sides have any guarantee of tonal quality, volume or responsiveness. For the soundboard you can make a case, but not the back and sides. The reason being that the back is not the primary resonator on a guitar.

But let’s look at the back anyway. There are a few primary functions for the back:

• The back defines the volume of the body, i.e. air cavity. This results in a major resonance known as the body resonance or the air resonance, depending on who you are talking to.
  • The placement and size of the soundhole is also a factor in determining this resonance.
  • The flexibility of the back also contributes to determining this resonance. A stiffer back will push the air resonance up somewhat and a more flexible back will lower it somewhat.
• The back itself has it’s own set of resonances. The main back resonance is the least important of the three main resonances; top, air/body and back. However, if it is in the wrong relationship it can cause problems with comparatively loud or soft notes or even causing a specific note to waver.
• Other than the effect on the resonances, a stiffer back is more likely to act as a reflector. A more flexible back is more likely to act as a resonator. Any back will act as both to some extent, but to what degree depends on the stiffness (and weight).
• Some woods are highly resonant in the sense that they resonate strongly within a confined frequency range. These woods tend to be very dense and heavy woods, and true rosewoods are in this category. But density alone doesn’t assure a highly resonant wood.
  • In “tap” tests, the unassembled back is tapped and analyzed by how clear the resulting sound is. When viewed on a spectrogram, this is a narrow, sharp peak for the highly resonant pieces.
  • The assumption has always been that the highly resonant back woods are superior, but guitars built with them can be noticeably erratic in the response, volume and sustain of individual notes.
• Other woods are much less resonant, and will resonate more evenly across a broader spectrum of frequencies. Woods like maple are in this category. Lower density does not guarantee this kind of  response characteristic.
  • This actually makes for a more balanced guitar, with fewer hot notes and fewer dead notes overall. It’s still possible to have these problems if the overall frequencies are in conflict.

I categorize back/side woods according to density, response type and appearance. Appearance being unimportant to sound.

The stiffest and heaviest brace on a guitar is the bridge. Changing the mass and/or the design of this important element will have discernible effects on the sound of the strings.

• A relatively heavy bridge will cause the attack (response) to be slow when compared to a relatively light bridge.
• A relatively heavy bridge will also reduce the overall volume as it takes more energy to move a heavier object than a light one. Thus a heavier bridge makes for a less responsive guitar.
• However the law of inertia implies that it will also take longer for the heavier bridge to stop vibrating (An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.) So the heavier bridge augments sustain. How much that matters with the weights involved here is debatable.

Taking all this into account, my approach is to go for maximum attack and volume by using relatively light weight woods for bridges. My target weight is 20 grams for the bridge, not including the saddle. Using a lighter wood like sapele, I can get a bridge as light as 14 grams, which includes a tie block overlay of a denser wood like ebony or gidgee. I also save a few grams by making the saddle from a dense wood like african blackwood or snakewood. The end result, when used in conjunction with properly built composite soundboard with diamond bracing is a very loud and responsive guitar. No one has complained of lacking sustain on these instruments.

The road to building exceptionally responsive classical guitars is littered with failed attempts and discarded theories. As I experiment a lot I’ve had my fair share of both. But scattered among my failures are diamonds that have pointed the way to designs, materials and processes that consistently result in instruments deemed extraordinary by experienced professionals.

The bracing pattern outlined above is the basis for the most recent, and most highly acclaimed series of guitars I’ve built. I refer to it as the Spyder pattern. I seldom build two identical instruments, but all the Spyder braced guitars have this common conceptual beginning. This design is extremely flexible and seemingly minor variations, like the size of the center diamond, can have very discernible effects on the sound of the instrument. 

When paired with a composite soundboard, and properly constructed, this pattern is a great platform for a highly responsive instrument.