How Does a Guitar Make Sound? Strings, Bodies and Pickups

A plucked string vibrates at a frequency set by three things: its length, its tension and its weight per unit length. Shorter, tighter and lighter strings vibrate faster and sound higher, which is why the six strings of a guitar are of different thicknesses, tuned to different tensions, and why pressing a string against a fret shortens its vibrating length and raises the note. The frets are spaced so that each one raises the pitch by a semitone, which means each is closer to the next by a fixed ratio, the twelfth root of two, and the twelfth fret sits exactly halfway along the string, an octave up. A string does not vibrate at one frequency only; it vibrates at its fundamental and, at the same time, at two, three, four and more times that frequency, the harmonics, and the mix of them is what makes a guitar sound unlike a violin playing the same note. Plucking near the bridge excites more of the high harmonics and gives a brighter tone; plucking over the neck gives a rounder one.

In an acoustic guitar the string passes over a bridge glued to the soundboard, the thin spruce or cedar top of the body, and the string's vibration rocks the bridge and sets the whole top moving like a drumskin. The top is large enough to push a great deal of air, and the air inside the body resonates as well, at a frequency set by the size of the sound hole, which reinforces the bass. Almost all the sound comes from the top and the air, very little from the back and sides, and the bracing glued under the top, a pattern of struts in an X on most steel-string guitars and a fan on classical ones, is what lets a plate two or three millimetres thick take the pull of the strings, around 80 kilograms on a steel-string, and still vibrate freely. The luthier's craft is in tuning the top: too stiff and the guitar is quiet and thin, too flexible and it bellies up and dies. What decides the sound:

• •The strings: steel for volume and brightness, nylon for the softer classical voice, wound with bronze or nickel on the lower strings to add weight without stiffness
• •The top: spruce or cedar, and how it is braced
• •The body's size and shape, which set the air resonance and the bass
• •The bridge and saddle, which pass the vibration to the top
• •The player: where and how hard the string is plucked, and with what

An electric guitar can have a solid body because it does not use the body to make sound. Under the strings sit pickups, each a magnet wound with thousands of turns of fine wire; the steel string above the magnet is magnetised, and when it vibrates it disturbs the magnetic field through the coil, which by Faraday's law induces a small alternating voltage at the string's frequency. That signal, a few hundredths of a volt, goes down a cable to an amplifier, which makes it large enough to drive a loudspeaker. Because the pickup senses only the string, the body's job is to hold the strings still and sustain them, and a solid plank does that better than a hollow box, which is why the first practical electrics of the 1940s and 1950s, by Leo Fender and Les Paul, were slabs of ash and mahogany. Where the pickup sits changes the tone, since a string's harmonics have different strengths at different points along it, and the switch on an electric guitar chooses between the warm neck pickup and the sharp bridge one.

The electric guitar's voice is as much the amplifier's as the instrument's. A valve amplifier driven harder than it can cleanly handle clips the top of the signal, adding harmonics that were not there and sustaining the note as the string decays, and that distortion, discovered by accident and by guitarists slashing their speaker cones in the 1950s and 1960s, became the sound of rock. Pedals between guitar and amplifier bend the signal further: delay, reverb, wah, chorus, and fuzz. Feedback, in which the speaker's sound sets the string vibrating again through the pickup in an endless loop, is a fault in any other instrument and a technique in this one. The acoustic guitar makes sound; the electric guitar makes a signal, and the signal can be anything.

Plucked instruments with necks and frets go back to the Hittites and the lute of the Islamic world, and the guitar in something like its present form, with six single strings, is Spanish, from the late eighteenth century; Antonio de Torres in the 1850s set the classical guitar's size and fan bracing, and the American steel-string with its X bracing was developed by the Martin company in the 1840s to 1850s. The electric guitar answered the problem of being heard in a dance band, with a pickup fitted to a hollow guitar in 1931, the solid body by 1950, and it was the instrument of the popular music of the second half of the twentieth century. About two and a half million guitars are sold each year in the United States alone, and the physics in every one of them is a string that cannot be heard until something moves the air or the electrons on its behalf.

A guitar string's pitch is set by its length, tension and weight, its tone by the mix of harmonics, and it is nearly silent alone; an acoustic guitar makes it audible by passing the vibration through the bridge to a thin braced soundboard that moves air, with the body's air cavity reinforcing the bass, while an electric guitar turns the vibration into a voltage through magnetic pickups and leaves the amplifier to make the sound, which is why it can have a solid body and why distortion and feedback became its voice.