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.
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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!
Materials
Electronic device (phone, tablet, computer and so on) with a headphone jack and the ability to play music
3.5-millimeter stereo cable (a typical "headphone" plug) that can be cut and modified
Neodymium magnet (also called a "rare earth" magnet) that is approximately 0.5 inch in diameter and 0.5 inch long; this can be purchased at a hardware store or online. (These can be dangerous if accidentally swallowed, so make sure to keep them away from little kids.)
At least 6 feet of 30-gauge magnet wire (also called enameled wire), which also can be purchased at a hardware store or online. Make sure the wire is insulated and not bare copper
At least one paper or plastic cup
Clear tape
Scissors
Fine grit sandpaper
Adult helper
Wire strippers (optional)
Preparation
Carefully cut your 3.5-mm audio (headphone) cable in half. Ask an adult to help you strip off about two inches of the outer insulation from the cut end. You can do this using wire strippers or by scraping the insulation off with scissors.
There should be three smaller wires inside the cable. Usually there will be one bare copper wire (this is the "ground" wire) and two other insulated wires: one red and one white (these are the left and right audio signals for a stereo system).
Strip the insulation off about two inches of one of the audio wires (it does not matter which).
Procedure
Make a coil of wire by wrapping the magnet wire around your finger about 50 times (making sure not to wrap it so tightly that you cut off the circulation!). Leave at least 6 inches of wire loose at both ends of the coil.
Tape the coil flat to the outside bottom of your cup. Make sure the coil does not unravel.
Use fine grit sandpaper to strip the insulation off about 2 inches of each end of the wire.
Tightly twist each end of your coil wire to one of the stripped wires from the 3.5-mm audio cable. The wires need to be in good electrical contact with each other, so they cannot be loose.
Wrap each twisted wire connection in tape. Completely cover all the exposed wire where you stripped off insulation. This will help prevent short circuits.
Plug the other end of the 3.5-mm cable into your electronic device's headphone jack. Start playing a song.
Hold the cup up to your ear with one hand.
Hold the neodymium magnet directly below the coil at the outside bottom of the cup so it is almost touching. Can you hear the song playing?
Try slowly moving the magnet closer to or farther away from the coil. How does the loudness of the music change?
Troubleshooting: If you do not hear anything, double check to make sure your wires are tightly twisted together and not loose. Make sure the volume on your electronic device is turned up all the way.
Extra: Try using a bigger magnet (or multiple magnets stacked end-to-end) or wrapping a new coil with more turns of wire. Can you make your speaker louder?
Extra: Strip both audio wires in the 3.5-mm cable. Build and connect a second speaker (connect one end of the coil wire to the ground cable and the other end to the new audio wire—so both of your speakers will be connected to the ground wire). Can you build "headphones" so you can wear both speakers on your ears?
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
