The concept behind a speaker is
electromagnetism. Electricity is run through two
coils, placed end-on-end to one another. This
spiraling of electric current turns the coils
that the current passes through into magnets.
These magnets generate an attractive or repulsive
force upon each-other depending on the relative
directions of the current within them. Now, by
rapidly adjusting the current, you can alter the
amount of magnetic field, and thus the force, that
the two coils exert upon each other. This means
that you can rapidly move one coil, the "speaker",
in and out of the boom box. As it does so, it
displaces the air (if it is being moved out) or
creates a vacuum (if it is being drawn in). This
movement causes pressure waves - sound - to be
formed in the air.
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