7.4 Microphones and speakers — Learn

Prerequisites: Section 7.2, especially the motor effect and the force on a current-carrying conductor; Section 7.3, especially the generator effect; Section 4.1, especially sound as a pressure wave.

By the end of this section you can: explain how a moving-coil microphone converts the pressure variations in a sound wave into variations in current; and explain how a moving-coil loudspeaker and headphones do the reverse.

1. Two effects, two devices

A sound wave is a pressure wave. The air pushes on a surface, and the pressure rises and falls as the wave passes. A microphone turns those pressure variations into variations in an electric current. A loudspeaker does the opposite: it turns variations in a current into pressure variations in the air.

The two devices use the two electromagnetic effects from this chapter.

• A microphone uses the generator effect. A change in a magnetic field induces a potential difference, and so drives a current.
• A loudspeaker uses the motor effect. A current in a magnetic field produces a force.

2. The moving-coil microphone

A moving-coil microphone has a diaphragm, a small coil of wire, and a magnet.

1. A sound wave arrives at the microphone. The pressure variations push the diaphragm in and out.
2. The diaphragm is attached to the coil, so the coil moves backwards and forwards in the magnetic field.
3. The coil is moving relative to the magnetic field, so a potential difference is induced across the ends of the coil. This is the generator effect.
4. The induced potential difference drives a current in the circuit. The current rises and falls as the coil moves faster or slower.

The result is a varying current that is a copy of the sound wave. The microphone has converted sound into an electrical signal.

Worked example 1. Explain how a moving-coil microphone works.

A model answer is:

• Sound waves make the diaphragm, and with it the coil, move backwards and forwards.
• The coil moves relative to the magnetic field, so a potential difference is induced across the coil. This is the generator effect.
• The induced potential difference drives a current that varies as the coil moves. The microphone has converted sound into a varying current.

3. The moving-coil loudspeaker

A moving-coil loudspeaker has a magnet, a coil of wire, and a cone. The coil is attached to the cone, and it sits in the magnetic field of the magnet.

1. A varying current from the amplifier flows through the coil.
2. The coil is in a magnetic field, so the current experiences a force. This is the motor effect.
3. As the current changes, the size and direction of the force change. The force moves the coil, and the coil pushes the cone.
4. The moving cone pushes the air, so it makes pressure variations. These pressure variations are a sound wave.

The current has been converted into sound. Headphones work in the same way, with a small diaphragm in place of the cone.

The figure shows the coil and the cone of a loudspeaker. You can change the current in the coil.

Predict first. Do not move the slider yet. Write down your answers.

1. When the current is positive, which way does the cone move?
2. What happens to the cone when the current is reversed?

Test. Set the current to 2.0 A and watch the cone. Now set the current to −2.0 A and watch it again. Then set the current to 0 A.

Explain. The loudspeaker uses the motor effect, not the generator effect. Why does a changing current make the cone vibrate, rather than move once and stay still? Write your answer in two or three sentences.

Worked example 2. A loudspeaker coil has 0.050 m of wire inside the magnetic field of the magnet. The flux density is 0.20 T, and the current in the coil is 0.50 A. Calculate the force on the coil.

The force changes size and direction as the current changes, so the cone vibrates.

Watch out. A microphone uses the generator effect, and a loudspeaker uses the motor effect. It is a common mistake to swap them. Check by asking which way the energy is going. A microphone starts with movement and makes a current, so it is the generator effect. A loudspeaker starts with a current and makes movement, so it is the motor effect.

4. Comparing the two devices

Both devices turn one kind of variation into another. The microphone converts sound into an electrical signal. The loudspeaker converts the electrical signal back into sound.

Progress check

Return to the chapter problem. The scrapyard office has a microphone and a loudspeaker. Explain how the microphone turns the yard manager's voice into a varying current, and how the loudspeaker turns that varying current back into sound.

You should now be able to

• explain how a moving-coil microphone uses the generator effect to convert sound into a varying current;
• explain how a moving-coil loudspeaker and headphones use the motor effect to convert a varying current into sound;
• compare a microphone and a loudspeaker, and state which effect each one uses;
• use $F = BIl$ to calculate the force on the coil of a loudspeaker.