13.4P - Microphones, loudspeakers and headphones

From sound pressure to current

A sound wave in air contains travelling pressure variations. In a moving-coil microphone, these variations act on a thin diaphragm, producing a changing force that makes the diaphragm vibrate backwards and forwards.

A small coil is attached to the diaphragm and sits in the radial gap field of a permanent magnet. The diaphragm and coil move along the coil axis, perpendicular to the diaphragm, so the wire moves across the field. The magnet stays fixed. As the diaphragm vibrates, the coil moves relative to the magnetic field. The field through the coil changes, so a varying potential difference is induced across the coil. When the microphone is connected into a complete circuit, this drives a varying current: the microphone's electrical signal.

The conversion chain is:

sound-pressure variations -> diaphragm vibration -> coil motion in a magnetic field -> varying induced potential difference -> varying current in a complete circuit

The coil must move relative to the field. A stationary coil beside a stationary permanent magnet does not produce the changing microphone signal. The sound wave also does not act directly on electrons in the connecting wire; the diaphragm and coil provide the mechanical stage between sound and electrical output.

For a simple sound, faster pressure variations make the diaphragm and coil vibrate more frequently, so the electrical signal varies more frequently. Larger pressure variations usually produce larger diaphragm motion and a larger signal. A real microphone has limits, so this is a useful operating model rather than a claim of perfect conversion at every frequency and amplitude.

Reversing the effect for sound output

A moving-coil loudspeaker uses the same core parts in the reverse direction. A varying current from an amplifier enters a coil in the field of a permanent magnet. The current produces a magnetic field around the coil. Interaction between the coil's field and the permanent magnet's field exerts a force on the coil.

The field in the circular gap is radial. Opposite sides of the coil have opposite current directions and opposite local field directions, so the forces act in the same axial direction: they move the coil bodily rather than turn it. When the current reverses, the magnetic field around the coil reverses, so the driving force reverses. The attached cone or diaphragm is driven backwards and forwards along the coil axis. A reversing force drives an oscillation; force and instantaneous velocity need not point the same way throughout a vibration. It pushes and pulls the nearby air, producing pressure variations that travel away as a sound wave.