You can also use a microphone as a speaker.
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VinsCool
ciro64
My mind is gonna explode
VinsCool
The principle is that reversing the polarity will do the reverse effect, it's actually very simple.
SylverReZ
Because both headphones and microphones have a diaphragm consisting of a coil wrapped around. When you speak through it, your voice makes it vibrate to generate current from the opposite end. Using your headphones as a microphone, however, won't sound as great, though.