Electric guitars
Last updated: July 4, 2007.
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: A typical electric guitar.
This one has two pickups.
Acoustic and electric
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 thin, solid bodies often made out of plastic.
Resonance isn't at work here: electric guitars produce their sound
through an entirely different process. In fact, even though acoustic
and electric guitars look similar, and you play them in a broadly
similar way, they are quite different instruments. Electric guitars
actually have more in common with bicycles than with acoustic guitars!
Dynamo power
Bicycles? Let me explain. Your bicycle probably has a dynamo light
on it that flickers when you go along. 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 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.
Generating sound
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."
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 cancelled 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.
Go shop
