Electric guitars

by Chris Woodford. Last updated: December 9, 2011.

If you had to write a soundtrack for the 20th century, electric guitars would almost certainly be playing the tune. No other instrument defines the angry rebelliousness of the modern age quite like it. Who could forget Chuck Berry, Jimi Hendrix, the Rolling Stones, or Nirvana—some of the greatest exponents of guitar-driven rock? But if you think playing an electric guitar is all about attitude and dexterity, think again: it's actually about the science of electromagnetism. Let's take a closer look a how these amazing instruments turn electricity into sound.

Photo: Playing an electric guitar. Note the Seymour Duncan pickups underneath the strings. Photo by courtesy of Gabriel Pollard, published on Flickr under a Creative Commons Licence.

What's the difference between acoustic and electric guitars?

An ordinary (acoustic) guitar makes sound entirely by vibration. When you pluck a string, it vibrates back and forth, making the air move around it. Waves of sound energy travel into the hollow body of the guitar, making that resonate (vibrate in sympathy) too and amplifying the sound (making it considerably louder).

If you've ever seen an electric guitar, you'll have noticed that most of them have solid bodies (made out of wood or occasionally plastic). Although resonance still plays a part, solid-body electric guitars actually generate their sound through an entirely different process from acoustic guitars. In fact, even though acoustic and electric guitars look similar, and you play them in a broadly similar way, they are quite different instruments.

Electromagnetism in electric guitars

Electric guitars are powered by electromagnetism—and electromagnetic induction to be precise. That might not sound familiar, but you've probably used it if you've ever ridden a bicycle at night with a dynamo-powered light. A dynamo is a simple electricity generator with two basic parts: a rotating coil of wire that spins around inside a hollow, curved magnet. As the coil spins, it cuts through the magnet's field. This makes electricity flow through the coil. Two electrical connections from the coil are wired up to a lamp and the electricity generated makes the lamp light up.

To cut to the chase, we can say that a changing magnetic field generates or "induces" electricity. It's also true that a changing electric field generates magnetism. If you feed electricity through a coil of wire, you generate a magnetic field around it. That's how you can make a magnet controlled by electricity—better known as an electromagnet. Electricity and magnetism are really two different aspects of a single phenomenon: electromagnetism.

What does all this have to do with guitars? Crudely speaking, the metal strings of an electric guitar are a bit like dynamos. Under the strings, there are two electricity-generating devices called pickups. Each one consists of one or more magnets with thousands of coils of very thin wire wrapped around them. The magnets generate a magnetic field all around them that passes up through the strings. When you pluck a string, it moves back and forth through the magnetic field. The moving metal string "cuts" through the magnetic field and causes a very small electric current to flow through the coil. The coil is wired up to an electrical circuit connected to an amplifier that boosts the small electric current and sends it on to a loudspeaker, making the familiar electric guitar sound. Usually, the amplifier and loudspeaker are built into a single unit called an "amp."

Photo: Electromagnetism at work. Photo by Michael Worner courtesy of US Navy.

Improving the sound

It's a basic rule of physics (called Faraday's law) that a changing magnetic field produces electricity. So a guitar string will produce electricity only for as long as the magnetic field is changing—in other words, for only as long as the metal string is moving. Once the string stops vibrating, the sound stops. In that respect, an electric guitar is just like an acoustic one.

Unfortunately, a simple pickup with a single coil of wire is just as good at picking up stray electrical energy from power supplies and other interference, so it generates a certain amount of unwanted, background noise. Some guitars solve this problem using what are known as humbucking pickups. These have two coils of wire, arranged so they capture double the signal from the moving guitar strings to produce a richer sound. Each coil is wired up so any stray "hum" it captures from nearby electrical equipment is canceled out by the other coil. Most guitars have two or more pickups, which create a variety of different effects. Typically, there's one pickup under the bridge of the guitar (where the strings are supported) and another one slightly higher up at the bottom of the "neck" (the part of the guitar that sticks out of the main body).

The way a pickup is constructed can make a dramatic difference to the sound the guitar produces. Magnets made from different materials, pickups with more coils of wire, different shapes of pickups, and different thicknesses of wire used in the coil—these are just some of the factors that will alter the sound.

Guitar effects

The pickup coils are wired to the amplifier through an electrical circuit. The circuit usually also contains volume and tone controls, which allow the basic sound to be adjusted by turning knobs on the guitar body. A guitar with two pickups will have four knobs on its body: one to adjust the volume and the tone of the sound from each pickup. More complex circuits can be added to change the sound of an electric guitar in all kinds of interesting ways.

Further reading

On this website

• Amplifiers
• Pianos
• Sound
• Synthesizers

Books

General

• The Electric Guitar: A History of an American Icon by A. J. Millard. JHU Press, 2004. Charts the popular history of iconic guitar makers and players through the 20th century.
• The Electric Guitar by Paul Trynka. Virgin, 2002. An alternative look at guitar history and culture.

Technical

• Electric Guitar Construction by Tom Hirst. Hal Leonard Corporation, 2002. How to build your own guitar from scratch. Also explains in detail all the different kinds of pickups and how they work.
• Getting a Bigger Sound for Your Musical Instrument by Bart Hopkin. See Sharp, 2003. Explains in detail how to improve guitar sound with better microphones amplifiers, and pickups.

Articles

• Mechanical Engineers Rock Out On Guitars They Construct Themselves: Science Daily, January 1, 2009. How engineering students are getting a deeper appreciation of rock music by building their own guitars.