Baby monitors and intercoms

Last updated: November 18, 2009.

Is that the baby crying? How can you tell? Many parents invest in a low-cost baby monitor so they can listen to their child from a safe distance. Baby monitors are modern versions of intercoms: devices that secretaries use to talk to their bosses in the next office. How do they work? Let's take a closer look!

Photo: Domestic bliss—the baby's asleep! A simple intercom helps keep your baby safe and sound, but it also illustrates some interesting science. Photo by courtesy of LabGP & SigOther, published on Flickr under a Creative Commons Licence.

Microphones and loudspeakers are the same thing

Until you've used an old-fashioned intercom, you might not realize that a microphone can work as a loudspeaker and vice-versa—but it's absolutely true!

• In a microphone, there's a small piece of plastic called a diaphragm attached to a coil of wire placed near a magnet. When you speak near a microphone, you make the diaphragm vibrate, which moves the coil through the magnetic field that the magnet creates all around it. This makes electricity flow in the coil and, hey presto, you've converted your original sound into an electric current.
• In a loudspeaker, exactly the opposite process happens. You feed an electric current into a coil of wire, it generates a magnetic field around itself that pushes it away from a nearby magnet. The coil is attached to a paper or plastic cone, which moves too. As the cone moves, it pushes against the air creating sound waves you can hear.

Microphones look very different from loudspeakers so most people never realize how very similar they are. But you really can wire up a loudspeaker so it works like a microphone. Talk into the speaker cone and it will actually record the sound of your voice! Getting sound out of a microphone is somewhat harder because the cone is usually very small but, in theory, it's possible too.

Photo: You'd never guess to look at them, but loudspeakers (left) and microphones (right) are exact opposites of one another.

How intercoms work

What does all this have to do with intercoms? 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 hear's 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: 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.

Other kinds of intercoms

From a scientific viewpoint, these simple intercoms are the most interesting: they teach us that loudspeakers and microphones are opposites of one another. 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 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: In a basic intercom, the same device works as both the loudspeaker and the microphone. This one's a military intercom. Photo by Brien Aho courtesy of US Navy.