Microphones

Last updated: August 23, 2009.

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 in 1750. 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: A high-quality, professional microphone typical of the ones used by radio DJs. Photo by Gary Ward courtesy of US Navy.

Microphones are loudspeakers in reverse

If you've read our article on loudspeakers, you'll already know how microphones work—because they're literally loudspeakers working in reverse. Indeed, you can actually take a loudspeaker and wire it into an electrical circuit so it works as a microphone if you speak into it. Intercoms (electrical gadgets that allow you to speak to someone in the next room) often have a combined loudspeaker/microphone. It works as a microphone when you press a button to speak into it and as a loudspeaker when the person next door pushes the button on their intercom instead. It's exactly the same piece of equipment working in two different ways. How's that possible?

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 reverse. Sound waves created by your voice push against a diaphragm, making a coil move near to a magnet. This makes an electric current flow through the coil into an electrical circuit. 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.

Types of microphones

Photo: Right: A typical BBC-Marconi radio broadcast microphone from about the mid-1930s. Left: 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").