Frequently Asked Questions
About Acoustic Guitar

How Does a Guitars Construction Affect Its Sound?

Bracing
Bracing adds strength to the top without (hopefully) killing too much of the top's vibration.   A set of medium gauge steel strings on a normal dreadnought scale length (25.4") guitar exerts about 185 lb of tension.  This would splinter a thin wood top if it weren't braced.  A top thick enough to hold this much tension without bracing would be very quiet and tinny-sounding.  Another important function of the braces is to efficiently propagate the vibrations through a large area of the top.  Bracing also plays a major role in determining the tone of a guitar.  A picture is worth 1,000 words:

Scalloped Bracing
In scalloped braces, wood is selectively removed from certain areas of the braces to weaken the top enough to allow it to vibrate freely without weakening it so much as to make it structurally unsound.   Scalloped braces typically have a longitudinal cross-section reminiscent of a suspension bridge.

All current Martin steel-string guitars that have scalloped bracing have the following stamp on the inside: "USE MEDIUM GAUGE, OR LIGHTER, STRINGS ONLY." 

X-bracing
Martin steel-string guitars (and most of the multitude of guitars that are copies of them) have X-bracing.  This means that the two main braces under the top run in an "X" from the upper bouts to the lower bouts.  The "X" crosses somewhere between the soundhole and the bridge (about which more below).  There are several auxiliary braces other than the main X-braces.

High-X bracing
On most X-braced steel string guitars, the "X" crosses about 1.5 - 2" below the soundhole. On guitars with "high-X" bracing, the "X" crosses about 1" below the soundhole.   The effect of this is that the bridge rests less directly on the main X-braces, and can thus transfer more of its vibration to the top.  This is also called "advanced X-bracing" and "pre-war" bracing.

One of the features that make the pre-war Martin steel-string guitars so desirable (and sound so good) is their scalloped, high-X bracing.   Supposedly the reason that Martin stopped using this type of bracing in the '40's is that so many people back then used heavy-gauge strings, which will quickly damage a guitar with such light bracing, and which led to many warranty repairs.

Some builders currently offer models with high-X bracing. These include (but are no doubt not limited to) Martin (D-16H, HD-28 Custom 15, and some "Guitar-of-the-Month" models) and Collings (dreadnoughts).  High-X bracing is usually scalloped.

Why is split wood better than sawn wood for making guitar tops?

Answer provided by Jim Grainger, from Custom Fretted Instruments in Sparta TN.

Actually, tops are resawn out of split billets, so they are split & then sawn. 

The short answer is because it helps reduce runout.    More specifically, many spruce trees tend to grow in a spiral, so if the log is simply sawn straight down the middle, as most sawmills saw lumber, the saw won't follow the spiral, and the wood sawn from the log will not follow the direction the tree grew.   By splitting out "cants" or "billets" from the log, using wedges, the splits will follow the grain, and veneers cut from the cants will run in the same direction that the tree grew, so the grain runs straight through the top, instead of going through it at an angle.  You can tell if a top has runout, because it will reflect light differently from side to side, & will look lighter on one side than the other...until you turn it over.  That's the light reflecting off the end grain on one side of the top & off the flat grain on the other.  All this isn't necessarily bad, as I've played some outstanding guitars that had tops with runout, & many tops have grain that changes direction within the length of the top, but tops with severe runout are usually relatively weak, because of the short grain that runs at an angle through them, and often tend to be problematic & tricky to repair.