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 is 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 comes 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 unbraced 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 soundhole 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!!!