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Headphones can be used as a microphone
If you connect headphones to the microphone input and can be used as a microphone. Simplified design headphone and microphone is the same: the membrane is connected to a coil of wire in a magnetic field of a permanent magnet. In headphones in normal use the current supplied to the coil is converted into vibrations of the diaphragm and the microphone - the contrary.
via factroom.ru