Speakers: How They Make Sound (Reverse Microphones)

If you understand a Microbial, you understand a Speaker. If you understand a Speaker, you understand a Robot Motor. They are all Electromagnets.

The Voice Coil

Inside a speaker is a coil of copper wire wrapping around a magnet. The coil is glued to a paper cone.

1. Electricity In: We send an electrical pulse into the wire.
2. Magnetism: The wire becomes a magnet.
3. Push/Pull: It repels against the permanent magnet. The cone shoots forward.
4. Sound: The moving cone smacks the air molecules. A sound wave is born.

Making Motors Sing

In robotics, our motors are also coils of wire inside magnets. Usually, we spin them to drive. But if we pulse the electricity back and forth really fast (Stalling the motor intentionally), the motor shaft vibrates.

• Vibrate at 440 Hz -> It plays an “A” Note.
• Vibrate at 261 Hz -> It plays a “Middle C.”

Old robotics teams used to write code to make their robots “sing” the Star Wars theme song using nothing but their drivetrain motors. It’s inefficient, but it proves the physics works!