FAQ #7: Flat Backs and Arch Tops

September 22, 2012

Q: Recently I bought your books & DVD and I found one sentence particularly interesting: you mentioned that if a guitar with a normal flat back had an arched top, its dynamics would be unique. Can you please reveal from your experiences in which direction the sound will change, compared to that of a normal flat/domed top?

A: It’s an interesting question, and to my knowledge no one has yet made a guitar like this. Mario Beauregard of Quebec, on the other hand, has been making something truly new: nylon string guitars with arched backs and flat tops.

The arching of a plate stiffens it: it improves its stiffness-to-weight ratio. And, acoustically, it raises the plate’s pitch: its vibrational behaviors are shifted toward high-frequency signal — such as the violin has. A small highly domed plate is not likely to have a good monopole — that is, a good low end. Also, the greater the arch, the shorter the sustain is likely to be.

Cellos have a low end, and they have violin-like arched plates — but they are huge compared to a guitar. So part of what we are discussing is the SIZE of the plate, in addition to its doming vs. flatness. But it would be difficult to play a cello-sized guitar.

There’s another factor too: what wood the arched plate is made of. Traditionally, all arched-plated instruments (violins, violas, cellos, standing basses, and jazz guitars) have used spruces and maples — spruces for the tops, maples for the backs. Maple does not have much sustain compared with some of the woods used in guitars, especially Brazilian rosewood (although, in my experience of the maples, Eastern rock maple has the most, Western broadleaf maple has the least, and European maple — which is a sycamore — has some). Therefore, if we’re talking about a guitar with an arched spruce top and a flat maple back, it would likely have a sound characterized by a quick attack and a quick decay: bright, brisk, zingy, sharp, and not much sustain.

Sustain is not a factor in arched-plate and bowed instruments. They don’t need natural sustain: they will make sound as long as the player continues to scrape his bow over the strings. In the guitar on the other hand, because it is a plucked rather than a bowed instrument, the sound stops as soon as the strings do — just as happens with the banjo, lute, koto, ukulele, mandolin, dulcimer, harp, or harpsichord. [NOTE: the harp and the harpsichord are both excited by plucking action; the piano is excited by hammering action.]

It’s not likely that these traditions had such acoustic considerations behind them. The science of acoustics didn’t yet exist, and early European makers would of course have used the woods available to them — in this case the European alpine spruces and maples. They were a long way from having access to imported exotics from the New World. Also, in those days, the cost of labor was cheap and the cost of materials was high, so a thick plate of an imported exotic wood (that you’d carve down into an arched surface, and in the process wasting much of the wood) would have been quite expensive, compared to a thin plate of the same wood such as would eventually be used on guitars.