by Chris Woodford. Last updated: September 9, 2014.
Sound—energy we can hear—travels only so
far before it soaks away into the world around us. Until electrical microphones were
invented in the late 19th century, there was no satisfactory way to
send sounds to other places. You could shout, but that carried your
words only a little further. You couldn't shout in New York City and
make yourself heard in London. And you couldn't speak in 1715 and
have someone listen to what you said a hundred years later! Remarkably, such things
are possible today: by converting sound energy into electricity and
information we can store, microphones make it possible to send the
sounds of our voices, our music, and the noises in our world to other
places and other times. How do microphones work? Let's take a closer look!
Photo: Left: A high-quality professional microphone, typical of the ones used by radio DJs, turns incoming sound into outgoing electricity. Photo by Gary Ward courtesy of US Navy.
Right: Loudspeakers do the opposite, converting incoming electrical energy into outgoing sound.
Microphones are loudspeakers in reverse
Microphones look very different from loudspeakers so most people never realize how similar
they are. If you've read our article on loudspeakers,
you'll already know how microphones work—because they're literally loudspeakers working
In a loudspeaker, electricity flows
into a coil of metal wire wrapped
around (or in front of) a permanent magnet. The changing
pattern of electricity in the coil creates a magnetic field all around it that
pushes against the field the permanent magnet creates. This makes the
coil move. The coil is attached to a big flat disc called a diaphragm
or cone so, as the coil moves, the diaphragm moves too. The moving
diaphragm pushes air back and forth into the room and creates sound
waves we can hear.
In a microphone, there are almost identical parts but they work in exactly
the reverse way.
Types of microphones
Photo: Left: A typical BBC-Marconi radio broadcast microphone from about the mid-1930s. Right: A simple, modern headset microphone.
All microphones turn sound energy into electrical energy, but
there are various different kinds that work in slightly different
Dynamic microphones are just ordinary microphones that
use diaphragms, magnets, and coils. Condenser microphones
a slightly different way by using a diaphragm to move the metal
plates of a capacitor (an electric-charge storing device) and
generate a current that way. Most microphones are omnidirectional,
which means they pick up sound equally well from any direction. If
you're recording something like a TV news reporter in a noisy
environment, or a rare bird tweeting in a distant hedgerow, you're
better off using a unidirectional microphone that picks up
one specific direction. Microphones described as cardioid and
hypercardioid pick up sounds in a kind of "heart-shaped" (that's
what cardioid means) pattern, gathering more sound from one direction
than another. As their name suggests, you can target shotgun
microphones so they pick up sounds from a very specific location
because they are highly directional. Wireless microphones
use radio transmitters to send their signals to and from an amplifier or
other audio equipment (that's why they're often called "radio mics").
Intercoms are used as baby monitors and in those desktop gadgets that allow bosses
to speak to their secretaries (or vice versa). The most basic kind of intercom has two
handsets in different rooms connected together by a length of copper
cable stretching between them. Each handset contains a loudspeaker—and a couple of push buttons. The loudspeaker functions as either a microphone (absorbing sound) or a loudspeaker (giving out
sound) depending on which person wants to talk.
Let's suppose Annie (the boss) and Bob (her secretary) are in neighboring rooms. Bob
wants to alert Annie that it's time for a meeting so he presses the
intercom call button. Annie's intercom beeps so she presses her
"talk" button. The loudspeaker on her handset now functions like
a microphone. She talks into it and the sound energy produced by her
voice is converted into a fluctuating electric current that travels
down the wire to Bob's intercom. When the current flows into Bob's
loudspeaker, it gets converted back into sound waves and Bob hears
Annie's voice. When Annie's done with talking, it's Bob's turn. He
presses his "talk" button and now the intercoms reverse their
functions. Bob's loudspeaker now works like a microphone, capturing
his voice and turning it into an electric current that flows back
down the cable to Annie's office. Annie's handset is now functioning
as a loudspeaker and reproduces the sound of Bob's voice.
Photo: Left: In a basic intercom, like this military one, the same device works as both the loudspeaker and the microphone. There's a PTT ("push to talk") button on the side that you press when you want to speak that turns the device into a microphone. If the button isn't pressed, it works as a loudspeaker. The sound quality isn't too good, though, and that's one of the big drawbacks of simple intercoms: one device can't do a great job as both a microphone and a loudspeaker—there's some compromise involved. Photo by Brien Aho courtesy of US Navy.
Photo: Right: Emergency telephones on trains, in elevators, and in public places are usually simple intercoms. There's a single loudspeaker/microphone with a button to press to attract someone's attention. When the button is pressed, the intercom functions as a microphone and transmits your voice. When you release the button, the intercom switches to a loudspeaker so you can hear what the person at the other end has to say in response. An intercom like this is much harder to break or vandalize than an ordinary telephone handset, so it's particularly suitable for use in public places.
From a scientific viewpoint, these simple intercoms are the most interesting: they
teach us that loudspeakers and microphones are opposites. From a user's viewpoint, there are other kinds of intercoms you might prefer to use. Some have both microphones and loudspeakers
in each handset so two people can talk simultaneously. Wireless intercoms are more like
walkie-talkies (short-range radio sets) and have no awkward cables
to tangle up or get in the way. Still others plug into household
electricity outlets and send their voice signals round the household
wiring instead of using a wire cable of their own. (That means they
operate a little bit like broadband over powerlines or BPL.)
Photo: A wireless baby monitor intercom. There are two separate units that communicate via your home powerlines and normally you'd use them in different rooms, but I've put them alongside just to take the photo. The "baby" unit on the left contains the microphone (and a nightlight). The "parent" unit on the right contains the loudspeaker and also lights up when sounds are heard. Because sounds are transmitted directly from one unit to the other, there's no need for any cumbersome wiring between the two.