I have seen that inside headphones there is a magnet with a coil of thin wire around it. There must be longitudinal waves coming out of it that is why we can listen to audio. There must be pressure waves created as sound is nothing but pressure wave. So, my question is how is the pressure actually created there in the first place and how are the variations in pressure created? There is the electric current that flows through the wires and the electric and magnetic fields due to the current and the magnet. So, how do they "exert" pressure on air?
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asked Feb 2, 2016 at 14:49
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1$\begingroup$ The membrane (usually made from some plastic or some form of paper or cardboard) driven by the linear motor realized by the coil and the magnet. $\endgroup$– Sebastian RieseCommented Feb 2, 2016 at 14:54
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$\begingroup$ @SebastianRiese but who puts pressure on the membrane to make it vibrate? Could you write a detailed answer with diagrams? $\endgroup$– Tea is lifeCommented Feb 2, 2016 at 15:16
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$\begingroup$ I beg to protest a bit. :-) It is right, but just for the electrodynamic case. Beside that we have piezoelectric etc. Please rephrase the question, if you just care for an electrodynamic loudspeaker. $\endgroup$– Victor PiraCommented Feb 2, 2016 at 20:40
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1 Answer
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A permanent magnet has a fixed north/south polarity - in this example, lets say north is facing up and south is facing down. This magnet has a membrane of some kind attached to its north face.
An electromagnet beneath the permanent magnet can switch the direction of its north/south polarities by changing the direction of the electric current running through it.
When the electromagnet's south is pointing up, it pushes away the permanent magnet's south pole and therefore the membrane, creating an area of pressure in the air.
When the electromagnet's north is pointing up, it attracts the permanent magnet's south pole and therefore the membrane, allowing for the next pressure wave to be made.
