I learned to play the piano and the French horn as a kid rather than the guitar. That’s a shame, because a guitar is far more useful around a campfire. It’s also (need I say it?) scientifically interesting.
The guitar appears to have originated in Spain. Early guitars, from around the 14th century, had three double pairs of strings, plus a single string (the highest-pitched one). By the late 17th century a fifth pair of strings had been added below the other four, and in the 18th century the double pairs became single strings and a sixth string was added above the other five, giving us today’s “six-string.”
Whenever a tightened guitar string is plucked, it vibrates at a specific frequency (pitch). Thicker strings vibrate more slowly than thin strings, so the thickest string has the lowest pitch. The pitch can be adjusted by adjusting the tension on the string, using the tuning pegs: the tighter the string, the higher the frequency at which it will vibrate.
The other way pitch can be adjusted is by adjusting the length of the strings. On a guitar, this is done by pressing your fingers down on the strings. The shorter you make the vibrating part of the string, the higher the frequency at which it vibrates.
The plucking of a string stretched tautly between, say, two fence posts might produce a pleasing sound, but it would also be a very soft sound, because sound is transmitted to our ears by vibrations in the air, and the string is so thin it simply can’t disturb enough air to make much of an impression. That’s where the guitar’s body comes in.
The front of the guitar’s body is a large, thin piece of springy wood (spruce is a common choice) about 2.5 millimetres thick. Attached to the soundboard is a piece of wood called the bridge, to which one end of the strings are anchored; attached to the bridge is a thin, hard piece called the saddle, on which the strings rest. When the strings are plucked, their vibration causes the saddle to vibrate; the saddle passes the vibration on to the bridge, which in turn causes the soundboard (and, indeed, the whole guitar body) to vibrate.
In general, the higher-pitched notes are produced by the interaction between the string, the bridge, and the soundboard, while the lower-pitched notes are produced much more by the movement of the air inside the guitar’s body, which vibrates a little like the air in a bottle when you blow across the top. This effect, called the Helmholtz resonance, occurs because air is springy: when you compress it, its pressure rises. The vibration of the soundboard compresses the air inside the body; the increased pressure drives some air out of the sound hole; its momentum carries it past the soundhole, which lowers the pressure inside the body, which in turn results in the air that was just driven out being sucked back in. The result is a constant vibration of the air inside the guitar rather like somebody bouncing up and down on the end of a bungee cord–and a nice rich low sound.
Every guitar sounds a little bit different, because so many different aspects of its construction affect the vibrations that are produced, from the precise shape of the body to the woods used, to the size of the sound hole, to the string material.
For environmental and cost reasons there have been some efforts to replace wood with materials like fiberglass, carbon fiber and various plastics, but the results haven’t been very promising, acoustically or aesthetically.
The exception, of course, is the electric guitar, where synthetic materials have almost completely taken over (although the earliest electrics, from the late 1920s, had regular hollow wooden bodies). Electric guitars don’t have to be made of wood because the vibration of their strings is amplified electronically. Mounted in the guitars body beneath the strings is a magnetic pickup–in its simplest form, a bar magnet wrapped with fine wire. The steel strings vibrating in the magnetic field generate a matching electrical current in the wire wrapped around the magnet. This current can then be amplified and used to drive speakers. It can also be altered in any number of ways, which is why an electric guitar often sounds nothing at all like an acoustic one.
After 500 years, the guitar may have changed, and the music played on it certainly has, but one thing hasn’t: musicians with enormous creativity and talent continue to gravitate to the instrument, and to surprise and thrill us with the sounds they produce.
