How Sound is Recorded & Played

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

What is a sound? How do you record it and play it back? Ever wondered about the inner workings of a microphone or speaker? Learn all the basics, and then test your knowledge with a quiz.

What is Sound?

Sound is a type of wave that is made up of vibrations in the air or another material, which can be detected by the human ear. Humans use sounds to communicate through speech patterns, and to create aesthetic art in the form of music. Because of the human love of music, we need to find ways to record the sounds we make, and play them back at will. Today we're going to talk about how that is done.

How a Sound is Recorded & Played

Sounds, like all recorded information, can be sent or stored as analogue (sometimes spelled 'analog') or digital signals. Since analogue is the simplest and first way that we recorded and played back sounds, we'll talk about that first.

Using magnets, analogue technology takes sound waves and turns them into electrical pulses. When a metal conductor (like an electrical wire) is moved inside the magnetic field of a magnet, or a magnet is moved near a wire, an electric current is produced in that wire.

When you speak into a microphone, the motion of the air particles created by your voice causes a diaphragm (a thin piece of material, often plastic) to move up and down inside the microphone. This diaphragm is connected to a magnet, and that magnet moves up and down in time with your voice. By doing this, electrical signals are created in the microphone's wire that matches the pattern of sound you created.

Microphone
Microphone

We then have a choice in how we use that electrical signal. We could send the signal straight to a speaker, and play the sound right away. A speaker is just a microphone in reverse. When we run the electrical signal through a wire inside a magnetic field, it causes the magnet to move in the exact same pattern. This pushes a diaphragm in a speaker up and down in that pattern.

The only difference is that the diaphragm in the speaker is much larger than the one in the microphone, and it therefore produces sound. It's set up inside a box that amplifies the sound to make it clearer and easier to hear.

Speakers are Microphones in Reverse
Speakers are Microphones in Reverse

In fact, if you've ever plugged a microphone into a headphone socket by accident, you might have noticed that a sound came out of the microphone. They work the same way except that a microphone is an extremely quiet speaker that isn't much use.

But what if we don't want to play the sound right away? Well, then we need to record it. Traditionally, recording also used magnets, but instead of driving a diaphragm up and down inside a speaker, we have to drive a magnet to move up and down inside a tape recording device.

Magnetic tape is a special kind of tape that be used to record sound, and is coated in a magnetic material. When a magnet moves near it, the motion causes different amounts of magnetization in the tape, which stores the recorded sound. This idea formed the basis of cassette and VHS tapes.

Cassettes Contain Magnetic Tape
Cassettes Contain Magnetic Tape

Digital Recording

These days we mostly use CDs and MP3 players. So how do those work?

Well, the recording part isn't all that different. We still produce an electrical signal in a wire in the same way as microphones. The difference is what we do with that signal afterwards. Using complex electronics, digital technology turns an electrical signal into a digital (or computerized) one made of lots of ones and zeros, which we call binary code.

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