A sound science activity from Science Buddies

Key Concepts Physics Sound Magnetism Electricity

Introduction Do you like to listen to music? Have you ever wondered how a TV, computer or phone turns music into sound that your ears can hear? In this project you will build your own speaker from household materials and find out how speakers convert electrical signals into sound.

Background Sounds, such as songs or the audio track on a movie, can be stored as an electronic file. The data in the file shows how the loudness and pitch of the sound changes over time. This information can be sent electronically through a wire (or in the case of a Wi-Fi signal, through the air using radio waves). This process moves the information from one place to another in digital form—but it does not produce a sound.

To make sound from an electrical signal we need another piece of the puzzle: electromagnetism. When an electrical current flows through a wire, it produces a magnetic field around the wire. The magnetic field around a single, straight piece of wire is fairly weak. Wrapping a bunch of wire into a tight coil, however, can make the magnetic field much stronger. So when we send the changing electrical signal from an audio file through a wire coil we get a changing magnetic field that corresponds to the original sound.

This changing magnetic field can push and pull on the magnetic field of a nearby magnet (called the permanent magnet). When magnets push and pull on each other they can create motion. You've noticed this if you have ever snapped two magnets together or used one magnet to push another magnet away. When one of the magnets (either the electromagnet or the permanent magnet) is attached to a thin membrane the rapidly changing magnetic field makes the membrane vibrate. The vibrating membrane bumps into nearby air molecules causing them to vibrate as well. This vibration travels through the air as a sound wave. Eventually it reaches your ears, and you hear a sound.

Normally speakers are covered in a case or built into an electronic device so you can't see inside them. In this project you will build your own speakers from scratch so you can see how they work!

Observations and Results When you held your speaker up to your ear and held the magnet near the coil you should have been able to hear very faint music. If you moved the magnet away, the music would disappear. This happens because magnetic forces are very strong close to the magnet but quickly get weaker farther away from the magnet.

Unlike a regular speaker your speaker was probably not loud enough for you to hear it from across the room. Regular speakers usually have a separate power supply (they plug into a wall outlet or a USB port or have an internal battery) and an amplifier, which makes the sound much louder. Your speaker functions more like wired headphones, which don't have an external power supply. You can hear headphones when you put the earbuds directly into your ear but not from across the room.

More to Explore What is a Magnet? from Physics4Kids Make Sprinkles Dance, from Scientific American How Loud Can Paper Speakers Get? from Science Buddies Making Sound Waves, from Scientific American Talk through a String Telephone, from Scientific American STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Ben Finio is a senior staff scientist at Science Buddies and a lecturer at the Cornell University Sibley School of Mechanical and Aerospace Engineering. Follow him on Twitter @BenFinio.

Saima May Sidik and Nature magazine

Christina Maslach and Michael P. Leiter

Chelsea Harvey and E&E News

Elizabeth Gibney and Nature magazine

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