A change in the magnetic field through a coil can induce a potential difference across it which drives a current. The voltage and current changes in a transformer can be calculated, as power transfer is constant.
The motor effect is used inside headphones, which contain small loudspeakers. In these devices, variations in an electric current cause variations in the magnetic field produced by an electromagnet. This causes a cone to move, which creates pressure variations in the air and forms sound waves.
Alternating current supplied to the loudspeaker creates sound waves in the following way:
A current in the coil creates a magnetic field around it.
The magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards.
The current changes direction.
The direction of the magnetic field around the coil reverses.
The force on the cone now pulls it back in.
Repeatedly alternating the current direction makes the cone vibrate in and out.
The cone vibrations cause pressure variations in the air, which are sound waves.
To make a loudspeaker cone vibrate correctly, the electric current must vary in the same way as the desired sound.