I am happy to hear that I am on the right track.
I have also been plodding along on a writing desk and recently finished a guitar that I think sounds even nicer than QC #2, built on the form I made to construct resonators in. The shape was originally inspired by the late 1920s Gibson L-0/1 shape and so it is only fitting to build a steel string acoustic with it.
One of the big question about this guitar was how to ensure the top would resist bellying, that deformation of the top where the bridge wants to rotate along its horizontal axis flexing the wood between it and the sound hole down and bellying out the wood behind it. On a more traditionally shaped X braced guitar the wings (the edges to the treble and bass side) sit over the legs of the X brace and help to keep the bridge from rotating, but the shape of this guitar makes that difficult while maintaining the bridge near the center of the lower bout. You could do it if you wanted to anchor the front legs of the X into the front of the upper bout and make some sort of A-brace/X-brace hybrid, but I want to keep my experiments on the small side so I can trace my steps and better learn about guitar tops and sound production.
|The top two are of the Roundbout soundboard and the bottom is QC#2 (Martin 0 style).|
It got me thinking about what exactly I am trying to do when I brace a guitar. This seems like an unimaginably simple question, but it is certain that no one has definitely answered how to brace a guitar because of both how complex sound production is and how subjective are our opinions on the sound of a guitar.
When I make a guitar I know my personal preference is for something that is responsive with a fast attack. I want some sustain to add to the fullness of the sound, but I also don't want it to get muddy with fast finger picking. I want it to project and retain its quality of sound through a large dynamic range. Most factory guitars are heavy and overbuilt because wood strength and stiffness varies a great deal and an overbuilt guitar ensures the weakest brace/top combo won't self destruct without requiring much if any variation between guitars. This makes the most obvious step to making a more responsive guitar lightening braces, but as you take away mass you can also lose things, like sustain. There is less mass to perpetuate the energy from the string. The best analogy I can think of is the flywheel of a car, if you lighten it up the engine will rev up faster, but a heavy flywheel will help keep the engine rotating once it is up to speed. If you take mass away in the wrong places it might not even make the top more responsive and leaving mass in the wrong places could hinder the ability of the top to vibrate.
I started thinking about this while building this particular guitar because I knew I would have to add mass to help ensure the bridge didn't dramatically belly the top. most discussions of improving the tone of a guitar focus on removing mass so even thinking about adding more seems wrong. The more I thought about it while working up to the top bracing, and the more I think about it now (being encouraged of course by the fact that the guitar has been strung up and I am very happy with the sound) the less worried I am about it.
A top vibrates in specific patterns, or modes, the positions of which are influenced by the bracing of the guitar. As I understand it which mode is active on the top is determined by the frequency, and similar to the frequency of a wave as you would think of it affecting a string. The top is a fixed space that will vibrate as one node, monopole vibration, at a given frequency the same way a string on a guitar will vibrate at a given frequency. Because the top is a fixed space, unlike the string which we fret to make smaller, producing higher notes, but not ever vibrating with multiple poles, the top must reach a doubling of frequency to enter into dipole vibration (a distinct mode, where two nodes, the bass and treble sides of the sound board vibrate separately, but not independently) and upward with an increasing number of nodes vibrating separately, but not independently across the top.
These nodes need to be flexible around the edges to allow them to vibrate freely, and once they are started vibrating they are aided in perpetuating that vibration by the weight they carry in the interior of the node. If you think about monopole vibration on a guitar it is essentially the entire lower bout. The edges run along the rims and the center is precisely where I was worried about adding mass in the form of a larger bridge plate. That extra mass could make it harder for the strings to get the top moving and it probably slows down the speed they get it moving, the attack (like a heavy flywheel in a car), but the extra weight is small and not in any area affecting the tops ability to move. Even in higher modes, like dipole, where the edges of nodes cross the bridge plate its restriction on vibration is bound to be far less than the heavy, cross grained, bridge glued on the the face of the top. I also feel like it might aid things in helping to transmit the force of the string across the top. Without the Bridge plate the bridge of this guitar would be surrounded by thin, unbraced top. How much energy would be lost flexing this small section in order to drive the rest of the top?
This instrument gave me a lot of interesting things to explore as I build in the future, and I could go on and on about the things I thought about while bracing it, but I am not going to. Here is what those thoughts translated into a guitar sound like. As always sorry my playing is a bit rough. Thanks for reading.
You can see more pictures of Queen City Guitars # 4 by clinking on the link on the top right of your computer screen or by clicking the scroll down menu under "Guitars" at the bottom of the page on your mobile device. Thanks again.