What makes a speaker a microphone and does this mean that a microphone works as a speaker?
In: Technology
Speakers DO in fact work as microphones. And the othe way around. Just as electric motors can be spun by hand and they’ll output electricity at the other end (working as a dynamo).
It all has to do with electromagnetism. If you have a magnet and a coil of wire around it, if you put electric current through the wire then the wire itself becomes magnetic according to the current you put through it (this is where the electromagnetism comes in) and the two magnets start to move relative to each other, since now we’re dealing with magnetic polarity. Since the sound signal comes as a wave and not a constant electric signal, these two magnets are going to start vibrating in relation to the sound wave (which comes as current and changes the magnetic strength and polarity). That vibration of the two elements moves the speaker’s membrane and air is moved. That movement of air is what we hear. This is what sound is.
BUT! Electromagnetism also works in reverse. If you mechanically move those two elements (the magnet and the wire), this will actually produce current. Current that would otherwise be coming from the other direction. But this time, you’re the one creating the air waves that move the membrane and you reverse the process.
And the same goes for electric motors, except the magnets and the type of current (which in this case is direct current and constant) are set up in such a way that the movement they create provides rotation rather than vibration.
A speaker works by moving a membrane to vibrate the air. To do that there’s a permanent magnet attached to the membrane which gets moved by an electromagnet, essentially a coil of wire. The strength of the movement is defined by the voltage applied representing the sound.
A microphone works by a membrane being moved by air vibrations. This moves an attached permanent magnet. The movement induces a voltage in a nearby coil, converting the sound into voltage.
When I was learning how to do audio engineering it was a pretty common party trick to wire up a microphone as a speaker. Singers generally do not expect microphones to sing back at them.
The principles that govern the conversion of air pressure to voltage work just fine in either direction, although the qualities which make for a good loudspeaker are not necessarily the same qualities which make a good microphone.
A microphone and a speaker are what’s called a “transducer”, which is something that converts energy from one form to another. Many transducers can work both ways.
In the case of a microphone the transducer is converting vibrations in the air (sound energy) to electrical signals in a wire (electrical energy). It works by focusing the sound waves to a smaller area using a cone, then using the focused sound waves to move a magnet inside a wire. A speaker is the exact same principle except it goes from a wire, to a magnet, to a cone.
The speakers inside headphones are actually not that dissimilar to a dynamic microphone. You can plug a microphone into a headphone socket and put your ear next to the end and hear music. You can also plug some headphones into a microphone input and shout into the earpiece and it will be picked up.
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