What are the constructional details of a speaker?
If you observe the constructional details of a speaker you will see that it contains one coil suspended between two permanent magnets. The terminals of the coil are connected to high power audio output of an amplifier, so that current flows through this coil and an alternating magnetic field is produced around it.
This magnetic field interacts with the magnetic field of permanent magnets and so the suspended coil starts vibrating to-and-fro.
Since the cone or diaphragm of the speaker is also connected to this coil it also vibrates and sets up sound waves in air.
What is Faraday’s law of electro-magnetic induction?
There are two laws of electro-magnetic induction. They are as follows.
First law: When current flows through a conductor, proportional magnetic field is produced around the conductor.
The strength of magnetic field thus produced is directly proportional to the magnitude of current flowing through the conductor.
Also, if the current is AC current, then the magnetic field is also changing i.e. alternating magnetic field.
Second law: When a conductor is placed in changing magnetic field, proportional e.m.f. and current are produced in the conductor.
The amount of e.m.f. thus produced is directly proportional to the rate of change of magnetic field near the conductor and it also depends on the number of turns of the coil placed in the changing magnetic field.
Similarly the magnitude of current produced in the conductor is directly proportional to strength of magnetic field and the cross sectional area of the conductor.
This is also connected with Lenz’s law. Read more about it.
So how is the Faraday’s law can hold to use the speaker as a microphone?
Well, its so simple now. As explained in the first answer, the coil inside the speaker is suspended within two permanent magnets and also connected to the diaphragm.
So if the two terminals of the coil are connected across the INPUT SECTION of an amplifier and sound if you speak in front of the speaker, then the sound waves will produce vibrations in the cone of speaker. So the coil attached to the cone will also vibrate.
Thus, the Faraday’s second law will hold good here. Since the coil is in magnetic field, it vibrations will proportionally produce an e.m.f. in it.
Now if this e.m.f. is connected to the amplifier, then the amplifier will amplify these signals and will produce your voice as a loud sound, through, of course, another speaker connected across the output section of the amplifier…!
What about the quality factor of using speaker as a microphone?
Well, this is rather a tricky question.
If you observe the cone of speaker, it is rather thick. And the entire assembly of cone, together with the coil and the spider (see the constructional diagram of speaker), is rather “heavy” to vibrate equally to the variations in sound waves, if the intensity of sound is low.
Now it is needless to say that we generally speak politely in front of a microphone. If you are using the speaker as a microphone, you will have to speak somewhat “heavily” in front of it, so that the entire assembly of the diaphragm and the coil itself will vibrate correctly as per the frequency and intensity of your voice or a song.
So in fact, this issue is far more than experimental. You will have to go with trial and error basis.
So in a nutshell, I will say that you can use speaker as a microphone, but with limited reproductive quality…!
Yes, this is true. The quality of the sound converted into electrical signals with the help of speaker, used as microphone, will be definitely less.
If you use a mid-range speaker, then the chances are that the medium sonic range will be converted into electrical signals, FLATLY.
If you use a tweeter, then its response will be more for upper sonic range and it will definitely convert high frequency sound waves into electrical signals. This is the case because the cone of a tweeter is very thin.
And lastly if you use a woofer, the its needless to say that its entire assembly is very heavy. So its responses will be quite low. It may convert very heavy sound waves into electrical signals, but of course, flatly.
Suggested videos to understand the fundamentals of speaker mechanism…
Enjoy with the experimental facts…! Just try it. It really works…