Acoustic Soundboard: Flattop Fajitas, Curry, and Cordon Bleu
Like a chef, a guitar builder strives to create a palate-pleasing balance of ingredients.
Our kindergarten-age son recently asked what makes guitars sound different from each other. In an instant, I felt the history of guitars flash through my mind as I pondered the lowest common-denominator answer.
I love to celebrate the diversity of guitars. Big guitars, parlor guitars, ladder-brace, or X-brace: The hours spent studying and obsessively analyzing the minutiae of instruments by the guitar makers and players of the world have reached the clouds. Of course, none of that matters to a 5-year-old. So here it is: The two real factors are the design of the guitar and the materials the guitar is made from. There are other distinctions that push guitars in different directions, but in basic terms, there is the recipe and there are the ingredients.
In its essential form, a guitar is what people in lab coats call a coupled resonator. Strings vibrate. Those strings are attached to another object that is set in motion by the vibrating strings. Every guitar falls into this camp. With purely acoustic instruments, we can narrow in on the top as a significant point of contact. The initial distinction between types of guitar is whether the strings are anchored to the top and pull on it, or they are anchored elsewhere and merely push on the top: flattop or archtop, respectively.
The next distinction is how the string vibration is amplified to an audible level. For us to hear the sounds played, the sound waves must be loud enough to reach our ears and set our eardrums in motion. This requires displacement of air. And because strings don’t displace much air, we either rely on pickups driving a speaker or on an amplifying chamber (a.k.a. the guitar’s body). Subtleties in the design elements of this amplifier, or air pump, are what give rise to the vast trove of makes, models, and construction styles available.
Let’s consider sound generated by a string. If we pretend the string isn’t affected by what it’s attached to, we find it produces not only the note we played but a series of mathematically related sub-vibrations called harmonics. These harmonics tend to be weaker the higher they are in pitch.
The particulars of a guitar’s design will be more receptive to some of these harmonics more than others. Quite simply, this selective reception alone will make a guitar of one design sound different than another. A layer of complexity is introduced when we understand that rather than vibration originating strictly from the string and transferring to the guitar, the vibrating guitar also influences how the string attached to the guitar moves. Effectively, once a player hits the string, he or she sets up a feedback cycle.
The selective reception of vibration that gives an instrument a unique sonic fingerprint can be altered in different ways. The exact size and shape of the guitar, the shape and placement of structural parts like braces, the glues that adhere them to each other, the finish that covers the guitar, and the relationship between parts all contribute to an instrument’s exact personality. Inasmuch as there are practically infinite possibilities, there can be a nearly infinite number of guitar sounds based on the guitar’s design alone.
Switching from the recipe aspect of the guitar to the ingredients introduces more options. Each material used in an instrument—most often wood—has its own tendency to accept or reject certain frequencies. Makers select specific materials for various components of a design based on how well their properties match the function of the part. For an acoustic guitar top, for example, makers will usually select a wood that is both lightweight and strong. This enables it to be set in motion easily, yet withstand the pulling of tight strings. A hard and dense wood makes a poor top, as it tends to absorb most of the energy produced by the strings before vibrating enough to produce volume.
One way to look at the relationship between the design and materials of a guitar is to draw a comparison to food. Start with a basic ingredient such as chicken. The recipe used to prepare it results in radically different outcomes. In India, it might be transformed into a curry. In Mexico, it might be turned into fajitas. In France, perhaps cordon bleu. The construction of these dishes is very different, with naturally different outcomes. So it is with guitars. Guitars sound different simply because the people who make them design them in different ways. The builders make some big choices early on, and allow their sensibilities to further guide their construction, leaving the door wide open for musicians to discover unlimited variety.