Microphones and the generator effect

1. A moving-coil microphone has a thin flexible diaphragm fixed to a light coil of wire, and the coil sits in the field of a permanent magnet.
2. A sound wave arriving at the diaphragm is a series of compressions, where the air pressure is above normal, and rarefactions, where it is below normal.
3. The changing air pressure pushes the diaphragm inwards during a compression and lets it spring outwards during a rarefaction, so the diaphragm vibrates at the frequency of the sound.
4. The coil is attached to the diaphragm, so the coil vibrates with it and moves back and forth inside the magnetic field.
5. The moving coil changes the magnetic field passing through itself, so a potential difference is induced across the coil by the generator effect.
6. The coil moves one way during a compression and the opposite way during a rarefaction, so the induced potential difference reverses each half cycle and the output is an alternating signal.
7. A louder sound carries a larger pressure variation, which pushes the diaphragm further and faster, so the coil moves faster and the amplitude of the induced potential difference is larger.
8. A higher-pitched sound vibrates the diaphragm more times each second, so the frequency of the electrical signal is higher.
9. The signal therefore has the same frequency as the sound wave and an amplitude that follows its loudness, which is what makes it an accurate electrical copy of the sound.

Loudspeakers and the motor effect

1. A loudspeaker has a coil wound around one pole of a permanent magnet, and the coil is fixed to a stiff paper or plastic cone.
2. An alternating current from an amplifier is passed through the coil, so the coil becomes a current-carrying conductor sitting in a magnetic field.
3. A current-carrying conductor in a magnetic field experiences a force, so the coil is pushed along the axis of the magnet.
4. The direction of the force reverses whenever the direction of the current reverses, so the coil is pushed outwards and then pulled inwards, once for each cycle of the current.
5. The cone moves with the coil and pushes on the air in front of it, creating a compression as it moves out and a rarefaction as it moves back, and these travel outwards as a sound wave.
6. A larger current amplitude produces a larger force, a larger cone movement and therefore a louder sound.
7. A higher current frequency makes the cone vibrate more times each second, giving a higher-pitched sound of the same frequency as the current.
8. Fleming's left-hand rule gives the direction of the force on the coil at any instant from the directions of the current and the field.

Comparing the two devices

1. Effect used: a microphone works by the generator effect, in which movement induces a potential difference, and a loudspeaker works by the motor effect, in which a current produces a force.
2. Direction of energy transfer: a microphone transfers energy from a sound wave to an electrical circuit, and a loudspeaker transfers energy from an electrical circuit to a sound wave.
3. Input and output: the sound wave is the input to a microphone and the output from a loudspeaker.
4. Shared parts: both devices contain a coil, a permanent magnet and a movable surface, so the two are close to being the same device operated in opposite directions.
5. Signal matching: in both devices the frequency of the electrical signal equals the frequency of the sound, and the amplitude of the signal corresponds to the loudness.
6. A small loudspeaker will in fact act as a rough microphone if sound is directed at its cone, because the moving coil then induces a potential difference.