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They don't necessarily.
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Those that operate using the piezo-electric effect use crystals, but no magnets.
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The so-called "dynamic" earphones use a system based of the "motor effect" in which the audio signal current flows through a small coil of thin wire in each earpiece, thus creating a varying magnetic field.
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Those magnetic fields interact with the magnetic fields of the permanent magnets in such a way as to create a force that acts on the coils, moving them and the diaphragm to which they are attached, thus converting the energy delivered by the signal currents into sound waves.
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The fundamental equation governing the process is:
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..... f = B I ℓ ..... where
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f is the force in Newtons;
B is the magnetic flux density due to the permanent magnets in Tesla;
I is the current in the coil in Amperes;
ℓ is the effective length of the coil in metres.
Best wishes - Majikthise. .
I didn't know they did. I guess maybe to keep sound vibrations in your ear so nobody else can hear outside of the ear phones. I'm guessing, so try doing some research:)
