Back in the physics section of the music library. (Why isn't there such a thing, BTW?) We all listen to music in many ways. Live is nearly always the best way, of course, but we also listen to recorded music. To hear recorded music, we need speakers. The last time you went to see a band at a bar or at a fair, or even just listened to a band set up outside of some guy's house, there was probably some sort of sound amplification system being used.
The sound waves are picked up from the instrument (or voice) via a microphone, which is, technically, a sort of reverse speaker that converts the sound wave into an electrical signal. This signal can then be sent through amplifiers that literally makes the signal's size big enough to run through a speaker. Inside of speakers are electromagnets, which are essentially metal coils that create a magnetic field when electricity is run through them. This results in something that is a lot like a normal magnet except that by reversing the direction of the electrical current, it is possible to flip the poles of the electromagnet. Why is this important?
The electromagnet is placed in front of a normal (permanent) magnet. Because the normal magnet's poles do not change, the electromagnet is alternately attracted and repelled by the normal magnet (think back to 4th grade science class where you learned about poles of magnets repelling and attracting each other. Like poles repel, unlike poles attract.) The electromagnetic coil is attached to a paper or plastic cone that is allowed to move. When the signal is sent through this system, the coil reconverts the signal back into sound waves that are amplified by the cone. The pulses of electricity lead to a rapid switching of poles, moving the cone. On very large speakers, known as woofers, used for low frequencies (remember that low frequencies have large sound waves), you can actually see the cone move if you turn up the volume (amplitude). Very cool!
Smaller cones are used for high frequencies. These are known as tweeters, and though they may sometimes vary a bit in materials used, they operate in much the same way. Quality, high-end speakers will have a combination of small, medium, and large cones, capable of transmitting high, middle, and low frequencies in order to achieve the best fidelity (most like the originally recorded impulse).
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