The first telephone was invented in the 1870s. It used an electromagnetic transducer to convert vibrations in the air into a varying electrical signal - an early microphone. The signal travelled along wires to a second transducer that converted the electrical signal back into vibrations in the air - and early loudspeaker.
Although designs have improved drastically, many modern day microphones and speakers operate in essentially the same way as the original designs.
Since then we have seen the development of:
All of these inventions were based on the idea that a single, varying, electrical signal could be used to represent sound. Here is a graph of a typical sound signal:
To understand how this works, lets look at a crude and simple loudspeaker:
The loudspeaker consists of a thin cardboard cone. The front edge of the cone is fixed to its box, but since cardboard has a bit of give, the back of the cone can move a little.
There is a coil of wire, wound round the back part of the cone. There is also a magnet nearby (also fixed to the loudspeaker box).
What happens if you apply a voltage to the coil? Well the coil becomes an electromagnet. When electricity flows through a coiled of wire, it creates a magnetic field. This is the effect used to pick scrapped cars up in a breaker's yard, but here in miniature it works inside a loudspeaker.
Since there is a fixed magnet nearby, when the current flows the coil will be attracted to the magnet (or repelled, depending in the direction of the current). This will move the cardboard cone a little. The greater the voltage, the more current will flow, so the greater the movement will be.
If you apply a voltage that oscillates between positive and negative values, the cone will be pushed and pulled backwards and forwards, vibrating the surrounding air in sympathy with the voltage - and creating a sound!
What about a microphone? Well, oddly enough, a simple loudspeaker can also act as a simple microphone. The electromagnetic effect works in reverse. If you move a coil of wire that is close to a magnet, it will generate an electric current (that is how a dynamo works). So if you speak into microphone, the cardboard moves, the coil moves close to the magnet, and generates a signal like the one shown above.
Of course, these are not ideal designs! A set of expensive speakers will use a more sophisticated material than cardboard, and a microphone will have different design constraints to a loudspeaker.
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