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Microphones

by Chris Woodford. Last updated: December 16, 2013.

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 of Sony bookshelf Loudspeakers.

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 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.

How microphones work

How does a microphone turn sound energy into electrical energy? Like this:

Simple artwork showing how microphones work

  1. 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.
  2. 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.
  3. The coil, attached to the diaphragm, moves back and forth as well.
  4. 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.
  5. 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.

BBC broadcast microphone from the 1930s

Types of microphones

Small headset computer microphone next to someone's mouth

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 ways.

Dynamic microphones are just ordinary microphones that use diaphragms, magnets, and coils. Condenser microphones work 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 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").

Intercoms

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.

US Navy sailor talking into an intercom.

Elevator emergency telephone.

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.

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.)

Make your own microphone!

Voiceprint for the word rhino spoken at normal speed

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:

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Text copyright © Chris Woodford 2008, 2011. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Microphones. Retrieved from http://www.explainthatstuff.com/microphones.html. [Accessed (Insert date here)]

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