Make Electronics

What Next? 245

Experiment 27: Loudspeaker Destruction

theory

Sound, electricity, and sound (continued)

Somewhere along the way, we may want to record the sound and then replay
it. But the principle remains the same. The hard part is designing the micro-
phone, the amplifier, and the loudspeaker so that they reproduce the waveforms
accurately at each step. It’s a significant challenge, which is why accurate sound
reproduction can be elusive.

Time now to think about what happens inside the wire when it generates a
magnetic field. Obviously, some of the power in the wire is being transformed
into magnetic force. But just what exactly is going on?

VOLTAGE

TIME

MOVING-COIL

MICROPHONE

Figure 5-30. Step 2: the pressure waves penetrate the perforated shell of a micro-
phone and cause a diaphragm to vibrate in sympathy. The diaphragm has a coil
attached to it. When the coil vibrates to and fro, a magnet at its center induces
alternating current.

VOLTAGE

AMPLIFIER

Figure 5-31. Step 3: the tiny signals from the microphone pass through an amplifier,
which enlarges their amplitude while retaining their frequency and the shape of
their waveform.

VOLTAGE LOUDSPEAKER

Waves

of air

pressure

traveling

at the

speed of

sound

Figure 5-32. Step 4: the amplified electrical signal is passed through a coil around
the neck of a loudspeaker cone. The magnetic field induced by the current causes
the cone to vibrate, reproducing the original sound.