Frequently Asked Questions
About Acoustic Guitar
How Does a Guitars Construction
Affect Its Sound?
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:
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."
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.
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
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
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