How do
microphones
pick up sound?
and never wondered how sound gets into a wire
Can't play sound? The illustrated version is right below.
Read the transcript
The full narration of the video.
Sound is just shaking air. So how does it get into a wire? The answer hides in a strange fact. Open up a singer's mic and a loudspeaker, and you find almost the same thing. First, sound itself. When you speak, the air at your lips gets squeezed, then let go, layer after layer, rippling outward.
When those squeezes reach a microphone, the first thing they hit is a very thin sheet. Air pushes in, the sheet bends in. Air eases off, it springs back. It shakes exactly as fast as you talk. But shaking isn't enough. The shaking has to become electricity.
Think of a bike light's little dynamo. The wheel spins it, the lamp lights up. Pedal faster, it glows brighter. A microphone hides the same trick. A tiny coil of copper wire is glued behind the sheet, and the coil sits around a magnet. When the sheet shakes, the coil slides back and forth by the magnet, and a tiny current appears in the wire.
Sheet in, the current flows one way. Sheet out, it turns around. However the sound shakes, the current shakes the same. This is called a dynamic microphone, the kind you usually hold at karaoke. It needs no power. The sound itself pushes out the electricity. Now run it backwards. Send current into the coil, and the magnet pushes it around. The sheet shakes and pushes out sound. That's a loudspeaker.
So a mic and a speaker share one set of sheet, coil and magnet. One turns shaking into electricity, the other turns electricity back into shaking. What about your phone? Behind that tiny hole at the bottom, it uses a different trick. Inside are two thin plates, extremely close together, with a gap thinner than a strand of hair.
Both plates carry a charge. Sound pushes the front one a bit closer, then pulls it a bit away, and the charge stored between them changes with it. A tiny chip next to it amplifies that change. This is a condenser microphone. The big mics in recording studios work the same way. Whichever kind, a microphone can't understand a single word. It just shakes along with the air, faithfully.
Audio folks, help us out: use a speaker as a mic, and what does the recording sound like?
In one sentence
A mic has a very thin sheet that shakes along with the air. The part behind it turns that shaking into electricity that rises and falls the same way.
Sound is just air being squeezed and let go. A mic doesn't need to understand what you said. It only has to shake along, faithfully.
What you
might expect
a speaker puts it out.
Two different things.
almost the same parts.
Like a bike light's
little dynamo
Wheel spins the dynamo = sound moves the sheet. Faster pedalling, brighter lamp = louder sound, stronger current. Where it breaks down: a dynamo spins round and round, while the mic coil only shakes back and forth. A mic also makes far less electricity, so it has to be amplified.
Run it backwards:
a speaker
Send current into the coil and the magnet pushes it around. The sheet shakes and pushes out sound. That's the speaker in your stereo.
The mic you hold at karaoke is usually this kind, called a dynamic mic. It needs no power. The sound itself pushes out the electricity.
Your phone uses
a different trick
This is a condenser mic. The big studio mics work the same way. It needs power to work; a dynamic mic doesn't.
One thing
it doesn't do
a single word Whatever the kind, a mic just shakes along with the air. Recognising words is the job of the computer and software behind it.
share one set of
sheet, coil and magnet.
Quick check
Why does a karaoke dynamic mic need no power?
A coil glued behind the sheet sits around a magnet. When sound moves it, the coil makes its own current, so no power is needed.
