Guitar bracing performs two wildly different functions:
strengthen the top of the guitar while allowing it to
sufficiently vibrate to produce a warm and resonant
tone. In a standard scale guitar with medium guage strings,
the guitar's top withstands approximately 185 lbs of
constant tension. A thin top without bracing would buckle
or warp in very little time. A top thick enough to withstand
the pressure could not sufficiently vibrate and would
result in a thin tone with little volume. Bracing a
thin top then finds the best of both worlds.
Bracing plays a major role in determining the tone
of a guitar as well. Although there are many theories
about tone production as related to bracing, there is
little disagreement on its importance. A luthier makes
purposeful decisions about the placement, pattern, and
physical shape of bracing because, in many ways, these
decisions affect the final tone of the instrument as
much as the actual tone wood. For instance, Taylor Guitars'
distinctive voice is heavily influenced by their bracing
patterns. Likewise, vintage Martin guitars are prized
for the shape of the braces and how they affect tone.
A final word on bracing in general:
Too much bracing = a dead, muffled tone
Too little bracing = poor structural integrity as well
as an airy tone with no definition
X bracing, forward shifted, scalloped... all
on a Colling Guitar in production in 2003.
The bracing pattern found in most steel-string dreadnoughts
is the "X" pattern. Originally developed by
C.F. Martin in the 1850's, this pattern features the
two main braces running in an "X" from the
upper bouts to the lower bouts. The "X" crosses
somewhere between the sound hole and the bridge. There
are several auxiliary braces other than the main X-braces.
This pattern provides the strength and well-balanced
tonal palette that most builders find attractive.
Pre-war era Martins have a bracing pattern that many
enthusiasts believe to be the best. In truth, Martin
did two things differently. First, braces were scalloped
in that era. That is, wood was 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. Scalloping
opens up the mids and increases volume. Second, Martin
used a forward shift of their X bracing. On most X-braced
steel string guitars, the "X" crosses approximately
2" below the soundhole. On pre-war Martins, the
"X" crosses about 1" below the soundhole.
The result is that the bridge rests less directly atop
the main X-braces and transfers more of its vibration
to the top.
In the 1940s, Martin moved the X bracing away from
the soundhole to its current position and quit scalloping
braces. According to Martin, they did this due to the
preponderance of players using heavy -gauge strings
to boost their volume. The light bracing pattern coupled
with heavy strings resulted in a high damage rate. Because
pre-war Martins are becoming so rare and expensive,
Martins along with many other builders are now producing
guitars with scalloped and forward-shifted bracing.
All builders recommend using light-guage strings on
these instruments though.