Guitar Making Tip No. 113 is about the internal braces. The guitar braces are a balance of form and function. An instrument is one of the few projects where you have to worry about more than beauty and stability. Here is how the braces balance both structure and sound.
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The Braces are a Balance of Strength and Weakness
Strong and weak are a contradiction in most cases, but not with an instrument. The guitar works by the energy from the strings pulling the soundboard up and down. This is similar to a speaker cone moving up and down.
Bracing the top too much will give it incredible strength. It will last a very long time, and it will never deform under the string tension. However, that’s about all it will do well.
If the top is over braced, it chokes the ability of the guitar to make sound. It will last longer, but it will never really live. This is where making the top weak as well as strong is very important, and here is what I mean by weak…
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The Only Reason a Top Vibrates is Weakness
When the top is weaker than the tension of the strings, it vibrates. When the top is much much weaker, it vibrates far too much, and it can also deform. However, when the top is just weak enough to vibrate well and not deform over time, that is the sweet spot in guitar making.
You want there to be vibration in the top, and you want a top that can be driven by the strings to produce good sound. Making the top light, and bracing it properly ensures that the top is is as strong as it needs to be, but not so strong that it overpowers the ability to vibrate.
If you are a new builder, follow the directions in the book or plans that you are working with, and be sure to thin the braces like they say. This will help you get a guitar that is not over braced, and not under braced. Most books/plans are a right down the middle approach.
How to Make the Top Vibrate Well Without Compromising Strength
You can do a few things to make the top vibrate well without reducing the strength of the instrument very much. These have the effect of loosening up certain areas, and lightening the board itself. Both of these things make a more responsive top that can still carry the weight of the string tension.
First, make sure to carve all of your braces into a parabolic shape. Removing wood from the sides of the braces and creating this shape reduces the weight to the minimum. It also does almost nothing to the strength of the brace. In this way, you create a lighter top without losing much if any strength.
Also, you can widen the spread of the X brace so that it is more open at the head and tail areas, and less open hear the waste areas. This reduces the bracing in line with the tension, and essentially opens up the belly more.
This does weaken the structure a bit, but small changes can be made without risking the strength very much. You can also adjust your lower face braces to help balance the area if needed.
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