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Back to Home-Page Acoustic Guitar Tops
You can own a guitar for decades, appreciate
the attention to detail involved in its exterior aesthetics, and never
realize that the majority of thought, experimentation and effort are
expended on the interior of the instrument. On inspection of the sound
chamber you will be surprised by the maze of braces that are so critical to
the sound of your guitar.
When a string is plucked, it sets
into motion a complex series of energy transfers that ends with the sound
waves received by your ears. The small mass of the string itself though,
does not move enough air to be musically useful. This is where the top and
its bracing come into play.
Woods like spruce, cedar, and even fir have
traditionally been used as top or "tone" woods largely because of their good
"strength to weight ratio". They are strong enough to withstand the constant
torque of the bridge but sensitive enough to respond to the subtle nuances
of musical dynamics. The un-braced top though, is not strong enough to
withstand the pull of the string's vibrations. Through the years, as the
guitar evolved into a larger bodied instrument with longer scale lengths and
added tension, more sophisticated bracing has appeared, to counteract the
compression of the strings and shape the timbre of the instrument.
There are essentially two kinds of
bracing, namely struts and tone bars. Struts are the reinforcing members
between the sound hole and neck whose purpose is primarily to maintain the
structural integrity of the instrument. While tone bars also strengthen the
top, their importance lies in the fact that their positioning and dimensions
have a profound effect on the sonic properties of the guitar.
Some instruments have what is known as
"scalloped" bracing, meaning tone bars that are dimensioned to a lower
profile in the area under the bridge. This effectively "loosens" the top
allowing different modes of vibration and a distinct tone desired by some.
"Scalloping" should not be confused with "voicing", a technique where the
top is thicknessed and struts shaved to achieve a particular resonant
frequency or tap tone. For instance if a component (i.e. the top) is tuned
to a certain note, when the corresponding note is played on the strings, it
will be reinforced. Practically speaking, this results in a loud attack
transient, followed often too quickly by the overtone series. Musically, we
perceive this tone as "thin" or "harsh" because of a lack of the
fundamental.
With experience, a luthier can
control the resonant frequencies of the top, air, and back of the guitar and
ultimately achieve a more even response from the instrument. Researchers
have been tracking resonant frequencies of guitars using frequency
generators and Brush chart recorders for years. Recently, laser
interferography has also been employed to determine the complex modes of
vibration of top plates in an effort to further understand and improve
instrument making. By lightly touching the top of your guitar while singing
into the sound hole (sweeping your voice slowly from low E up as high as
possible) you can feel the frequencies at which the top "excites",
indicating the resonant frequencies of your own guitar!!! !