A decision to have a guitar re-voiced is often weighed against the comment I have often heard: “if you don’t like your guitar, sell it and buy another one”.
This is, of course, what most players do, owning one guitar after another in search of the elusive perfect guitar. Implicit in this is the assumption that you can actually find another guitar that will be better than the one you now own after it has been re-voiced by Parabolic Brace Works!
Most people who call me say something along the lines of: “It just seems as though my guitar should be able to make more sound”. And they’re right.
The braces on every guitar I’ve seen, including a number of hand built ones, have been way too heavy to balance the string tension acting on them in most places. The exception to this is where the top braces are scalloped, and sometimes the scallops leave them too weak, allowing the bridge to rotate, bellying too much below the bridge.
Most factories and luthiers focus on how much to scallop braces to allow the bridge to be looser in flexing and vibrating the top. But they ignore a simple rule of engineering we all learned in childhood on a teeter-totter: The farther you sit from the center fulcrum, the less weight it takes to balance the person on the other side.
Acoustic steel string guitars’ top bracing includes an X brace, whose cross point can be seen looking down into the sound hole, where the two, angled braces on each side of it meet. There are also two diagonal belly braces, or tone bars, below the bridge, and two short “finger braces” on the outside of each of the X below the soundhole. Imagine the X as a stick figure, with arms and legs. The “arms” run upwards on each side of the soundhole, and the “legs” run down to the bottom.
The bridge sits atop the legs, and there is a hardwood (maple or rosewood) bridge plate, underneath between the legs to help support the string tension and provide a hard surface for the string ball ends to pull against.
On most, if not all guitars, the height of the top’s braces is the same for all of them, except where they cujrve down near each end. It stands to reason that the tension along the legs decreases as you move further away from the bridge area towards the ends. However, the height remains constant, which means that at some point, the brace becomes stronger than needed to balance string tension.
At this point, I ask you to make a leap of faith, and accept my observation that if a brace is too heavy, i.e., stronger than needed, it will dampen the surface of the soundboard to which it is glued. This is true for all of the top’s braces, except for the area near the bridge.
While scalloping a guitar’s braces loosens the bridge area to allow more vibration, it does not deal with this problem. The proceedure involves making a circular scoops, maybe six inches long, out of both legs of the X brace under the bridge area and below, leaving a peak about two to three inches in from each end. Also, these scallops are shaped on the two tone bars across the top below the bridge, leaving them to look like the Golden Gate Bridge. Six peaks, in all, with each one of them creating a dead spot about an inch wide and several inches long where the are located where the soundboard cannot vibrate. In addition, most luthiers and factories make the finger or side braces peaked, as well, adding four more dead spots. Ten peaks, totaling about 30 square inches of the soundboard!
That adds up to a lot of expensive, prime real estate that simply cannot make sound!
One theory of how a guitar top works calls these peaks nodes, which are defined in the dictionary as places where there is no vibratrion. Oh. And there are “anti-nodes”, where there is vibration. It is true that the sound can be shaped by selectively dampening certain parts of the vibrational spectrum, and variations on the placement of these peaks and the length/depth of the scallops contribute to the particular sound of one guitar brand compared with another.
However, only by shaping braces so that they are balanced with the force of string tension will the entire top be able vibrate, producing its full sound potential.