Microphones and loudspeakers

​​In a nutshell

Microphones use the generator effect to transfer sound waves into electrical signals. Loudspeakers use the motor effect to convert electrical signals into sound waves.

Microphones

A microphone uses the generator effect to transfer sound waves into electrical signals. The generator effect occurs when a coil of wire moves in a magnetic field which generates a current. 

1. Permanent magnet (south pole) 2. Coil wrapped around iron core (north pole) 3. Potential difference produced 4. Diaphragm  5. Sound waves vibrating diaphragm

Sound waves are made up of high pressure areas (compressions) and low pressure areas (rarefactions). A sound wave's compressions cause the diaphragm to move inwards. The sound wave's rarefaction cause the diaphragm to move outwards. 

The movement of the diaphragm causes the coil of wire attached to also move. This produces the generator effect, which generates an alternating current through the coil. The variations in pressure of the sound wave transfer to the oscillations of the electrical signal.

The frequency of the signal depends on the pitch of the sound. The amplitude of the signal depends on the volume of the sound. 

Loudspeakers

A loudspeaker uses the motor effect to convert electrical signals into sound waves. The motor effect occurs when a current-carrying conductor (e.g. a coil of wire with a current flowing through) is placed in a magnetic field.

1. Permanent Magnet (south pole) 2. Coil wrapped around iron core (north pole) 3. Applied varying potential difference 4. Cone 5. Sound waves produced

A varying potential difference is applied to the ends of the coil which produces an alternating current through it. The coil is wrapped around an iron core. It is placed between the poles of a permanent magnet and attached to the cone.

The alternating current through the coil creates a varying magnetic field around it. This interacts with the magnetic field of the permanent magnet which causes a varying force to be exerted on the current-carrying coil. As the coil is attached to the cone, any movement of the coil will also cause the cone to move.

The movement of the cone causes the air around it to vibrate. The variations of pressure (compressions and rarefactions) due to the varying force produces a sound wave.

The pitch of the sound produced depends on the frequency of the vibrations. The volume of the sound produced depends on the amplitude of the vibrations. The frequency and amplitude are the same as the electrical signal's.