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: Above: This Samson Meteorite is a typical studio-grade professional microphone that hooks up to your computer for podcasting or other high-quality audio recording. Technically, it's a condenser microphone and picks up sound in a cardioid (heart-shaped) polar pattern. The protective metal grille is meant to reduce wind and pop sounds.
Microphones like this turn incoming sound into outgoing electricity.
Below: Loudspeakers, like this, work the opposite way to microphones, converting incoming electrical energy into outgoing sound.
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 reverse!
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: A typical BBC-Marconi ribbon microphone used for radio broadcasts from about the mid-1930s. (I photographed this in the Think Tank science museum in Birmingham, England.)
All microphones turn sound energy into electrical energy, but
there are various different kinds that work in slightly different
ways.
Dynamic microphones are just ordinary microphones that
use diaphragms, magnets, and coils—"loudspeakers in reverse," as
we've already seen. Condenser microphones
work a slightly different way, capturing sound with the moving metal plates of a
capacitor (an electric-charge storing device).
Ribbon microphones pick up sound with a strip of metal ribbon that vibrates
between the poles of a magnet.
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
sound from 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").
Photo: The Blue Yeti is a popular podcasting microphone. It works using the condenser (capacitor) principle and can be set to pick up sound in four different ways: cardioid, omnidirectional, bidirectional, and stereo. Photo by Taylor Phifer courtesy of US Air Force and DVIDS.
How microphones work
How does a microphone turn sound energy into electrical energy? Like this:
Dynamic microphones
When you speak, sound waves created by your voice carry energy toward the microphone. Remember that sound we can hear is energy carried by vibrations in the air.
Inside the microphone, the diaphragm (much smaller than you'd find in a loudspeaker
and usually made of very thin plastic) moves back and forth when the sound waves hit it.
The coil, attached to the diaphragm, moves back and forth as well.
The permanent magnet produces a magnetic field that cuts through the coil. As the coil moves back and forth through the magnetic field, an electric current flows through it.
The electric current flows out from the microphone to an amplifier or sound recording device.
Hey presto, you've converted your original sound into electricity! By using this current to drive sound recording equipment, you can effectively store the sound forever more. Or you could amplify (boost the size of) the current and then feed it into a loudspeaker, turning the electricity back into much louder sound. That's how PA (personal address) systems, electric guitar amplifiers, and rock concert amplifiers work.
Condenser microphones
If you remember that "condenser" is an old-fashioned word for a capacitor, you can probably guess how a condenser microphone works: much like a capacitor!
Incoming sound waves carry energy into the front of the microphone.
The sound waves hit a flexible diaphragm, which is actually one of the plates (charge-storing metal elements) of a capacitor.
As the diaphragm vibrates, it moves closer to or further from a second, fixed backplate.
The changing distance between the two plates changes their capacitance (charge-storing ability). The microphone converts this into an electric current that corresponds exactly with the original, changing sound waves.
Ribbon microphones
Ribbon microphones (like the BBC one in the picture up above) use a thin vibrating ribbon of lightweight metal that moves between the poles of a magnet instead of a diaphragm and coil. Old-fashioned broadcasting microphones from the early days of radio worked like this—and some modern microphones still do:
Artwork: How a ribbon microphone works. A pair of crimped ribbons of aluminum foil (blue) are stretched between the pole pieces (green) above a permanent magnet (orange) and move back and forth as sound waves hit them, causing an electric current to flow in the cables (brown) attached to their ends. The yoke (yellow) closes and concentrates the magnetic field. Artwork from US Patent 2,113,219: Microphone by Harry F. Olson and Frank Massa, RCA, April 5, 1938, courtesy of US Patent and Trademark Office (with colors added for clarity).
Sponsored links
How do intercoms work?
Intercoms are used as baby monitors and in those desktop gadgets that allow bosses
to speak to their secretaries (or vice versa). The simplest 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.
In the ship's intercom shown below, the same device works as both the loudspeaker and the microphone. There's a PTT ("push to talk") button 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: A simple intercom being used onboard a ship. Photo by Carlos M. Vazquez II courtesy of US Navy and
DVIDS.
Photo: 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.
Wireless intercoms
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.
Make your own microphone!
Photo: Using an ordinary earbud as a microphone.
Don't have a microphone? What we've just learned about intercoms suggests you can make your own very simply—just by plugging a pair of earbud headphones into your microphone socket and talking into them!
This neat little trick should work with audio equipment, but it won't
necessarily work on your computer. That's because your earbuds are wired into a stereo jack plug, while your microphone socket
will be wired for mono input (and your computer's internal sound card will most likely be mono too). But give it a go and see. You may need to adjust your sound card properties in your control panel or sound settings. If you're lucky, you'll find one of the earbuds works as your microphone while the other doesn't do anything (because of the mismatch between the stereo plug and the mono socket). I plugged my earbuds straight into my computer's mic socket and got a perfectly reasonable audio input into Skype running on Windows, but I couldn't get sound playing through Windows Sound Recorder. If you want to record stereo sound on your computer with a microphone, your best bet is to use an external sound card (such as a Griffin iMic).
More microphone activities
These great websites go into more detail about making your own mic:
The Stealth Mic: Bill Byrne explains how to turn a pair of earbuds into a simple but effective binaural microphone.
Build Your Own Microphone Pop Screen: This one involves building a simple screen out of pantyhose (tights) to improve the quality of microphone-based sound recordings.
The Ear and the Microphone by Joseph D. Ciparick, The Science Teacher, Vol. 55, No. 9 (December 1988), pp. 46–47. A simple classroom demonstration of how microphones turn vibrations into electric currents... using little more than a beaker of salt water.
Lloyd Microphone Classics: A super website charting the history and development of microphones, including a gallery of popular microphones from the USA, UK, Japan, Germany, and other countries. Now offline, sadly, but here is the archived version from the Wayback Machine.
Books
Please note that these are highly technical guides, not well suited to beginners.
Handbook for Sound Engineers by Glen Ballou (editor). Gulf Professional Publishing, 2015. A huge (1700-page!) reference for professional sound engineers covering the theory of acoustics, electronic components used in audio systems, and practically every aspect of sound recording, both indoors and outdoors. Heavy emphasis on theory.
The Microphone Book by John Eargle. Focal Press, 2012. A detailed review of how microphones work, the various different types, and applications (mostly focusing on broadcasting and studio use).
Professional Microphone Techniques by David Miles Huber and Philip Williams. Mix Books, 1999. A practical guide to placing and using microphones for recording a wide range of different musical instruments.
Graphene-Based Microphone Could Let You Hear Like a Bat by Dexter Johnson. IEEE Spectrum, July 7, 2015. Sonic scientists at the University of California, Berkeley develop an ultrasound microphone that uses graphene in place of the traditional diaphragm.
Birth of the Microphone: How Sound Became Signal by Matthew Shechmeister. Wired, January 11, 2011. A fascinating look at the early history of microphones, from Emile Berliner's carbon-button of 1877 to instruments like the RCA 77, popular in the 1940s.
Fly Lends an Ear to Microphone Design by
Peter Weiss, Science News, Vol. 160, No. 23, December 8, 2001, p. 359. How the tiny design of a fly's coupled eardrums inspired a new approach to microphone design.
More technical/scholarly
Choosing the right microphone by Jim Edwards, The Choral Journal, Vol 21 Number 3, November 1980. A more detailed technical introduction to microphones from a specialist at Electro-Voice.
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