Building a highly responsive guitar: back and sides

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.