Make Electronics

Experiment 27: Loudspeaker Destruction

244 Chapter 5

theory

Sound,  electricity,    and sound

Time now to establish a clear idea of how sound is transformed into electricity

and back into sound again.

Suppose someone bangs a gong with a stick. The flat metal face of the gong

vibrates in and out, creating sound waves. A sound wave is a peak of higher air

pressure, followed by a trough of lower air pressure.

The wavelength of the sound is the distance (usually ranging from meters to

millimeters) between one peak of pressure and the next peak.

The frequency of the sound is the number of waves per second, usually ex-

pressed as hertz.

Suppose we put a very sensitive little membrane of thin plastic in the path of

the pressure waves. The plastic will flutter in response to the waves, like a leaf

fluttering in the wind. Suppose we attach a tiny coil of very thin wire to the

back of the membrane so that it moves with the membrane, and let’s posi-

tion a stationary magnet inside the coil of wire. This configuration is like a tiny,

ultra-sensitive loudspeaker, except that instead of electricity producing sound,

it is configured so that sound produces electricity. Sound pressure waves make

the membrane move to and fro along the axis of the magnet, and the magnetic

field creates a fluctuating voltage in the wire.

This is known as a moving-coil microphone. There are other ways to build a

microphone, but this is the configuration that is easiest to understand. Of

course, the voltage that it generates is very small, but we can amplify it using

a transistor, or a series of transistors. Then we can feed the output through the

coil around the neck of a loudspeaker, and the loudspeaker will recreate the

pressure waves in the air. Figures 5-29 through 5-32 illustrate this sequence.

PRESSURE

TIME

Waves

of air

pressure

traveling

at the

speed of

sound

Figure 5-29. Step 1 in the process of converting sound to electricity, and back again.

When the hammer hits the gong, the face of the gong vibrates, creating pressure

waves that travel through the air.