7.5 Loudspeakers and microphones

Using electromagnetism to handle sound

A loudspeaker and a microphone are two devices that do opposite jobs. Both have a coil of wire and a permanent magnet, but they use two different effects: the motor effect in the loudspeaker and electromagnetic induction in the microphone.

The loudspeaker

A loudspeaker is made of:

• a permanent magnet, one pole of which sits inside a coil;
• a coil of wire, which is free to move and is attached to a paper cone;
• the cone, which pushes on the air.

How it works

1. The amplifier sends an alternating current (the electrical signal for the music or speech) to the coil.
2. The coil is in the magnetic field of the permanent magnet, so a current in it means the coil experiences a force (the motor effect).
3. Because the current is alternating, the direction of the force keeps changing, so the coil and the cone move backwards and forwards.
4. The moving cone makes the air vibrate, producing compressions and rarefactions: a sound wave.

• The frequency of the sound is the same as the frequency of the a.c. A 440 Hz current makes the cone vibrate 440 times per second, and produces a sound of 440 Hz.
• A bigger current gives a bigger force, so the cone moves further, and the sound is louder (the amplitude is larger).

Worked example. An a.c. signal of frequency 440 Hz is sent to a loudspeaker. The speed of sound in air is 330 m/s.

• The cone vibrates at 440 Hz.
• The wavelength of the sound is \lambda = \frac{v}{f} = \frac{330}{440} = 0.75 \text{ m}.

Different speakers for different sounds

A real sound system usually has more than one loudspeaker. A large woofer produces low frequencies, because a big cone moves a lot of air with each slow vibration. A small tweeter produces high frequencies, because a small light cone can vibrate very quickly. A circuit sends the right range of frequencies to each. This shows that the design of a speaker has to suit the frequencies it makes. A good loudspeaker also needs a cone that is stiff but light, so it follows the signal closely and does not add any distortion of its own.

The moving-coil microphone

A moving-coil microphone works in the opposite way:

1. Sound waves hit the diaphragm and make it vibrate, at the frequency of the sound.
2. The diaphragm is attached to a coil that is in the field of a permanent magnet. The coil moves backwards and forwards with the diaphragm.
3. As the coil moves, it cuts the magnetic field lines, so a potential difference is induced across the coil (electromagnetic induction).
4. Because the coil moves one way and then the other, the induced p.d. is alternating, with the same frequency as the sound wave.
5. A louder sound makes the diaphragm move further and faster, so a larger potential difference is induced.

The tiny signal from a microphone is then amplified, and can be recorded, or sent to a loudspeaker.

Comparing the two

Because the two are so similar, a small loudspeaker can be used as a microphone, as in some intercom systems: sound moves the cone, which moves the coil, and this induces a signal.