Mike Isley
Product Developer

Your students enjoy listening to their favorite playlists on MP3 players and cell phones. With this engaging inquiry activity, now they can understand how their headphones or speakers create the sounds they love. They will make speakers from ordinary household materials and, in the process, discover the science of how speakers create sound.

Background

Audio speakers consist of a thin coil of copper wire (voice coil) that surrounds a stationary permanent magnet. The top of the voice coil is attached to the center of a thin cone-shaped paper diaphragm. The coil and diaphragm are both free to move. As the audio electrical signal comes from a radio, MP3 player, or cell phone, it enters the voice coil. The electron flow through the coil produces an electromagnet with a north and a south pole. Unlike a permanent magnet, an electromagnet can switch its poles by reversing the flow of electrons. The amplifier in the music device constantly switches the direction of flow for electrons, which causes the magnetic poles of the voice coil to oscillate back and forth from north to south. This results in a push-pull effect as the permanent poles of the surrounding magnet attract and then repel the switching poles of the electromagnetic voice coil. This attraction and repulsion of the voice coil causes the connected diaphragm to vibrate up and down, creating sound waves for voice and music. See the cross section of a speaker (Fig. 1) below¹.

Figure 1  Cross section of a speaker.

Materials

Per group

• Earphone Plug (3-1/2 mm, 1/8")
• 2 Alligator Clips
• 2 Pieces of #22 Insulated Hook-Up Wire (90-cm pieces)
• 1 Piece of Bare #28 Enameled Magnet Wire (6 m)
• Aluminum Pie Pan
• 6 Round Magnets (about 1" diameter)
• Small Piece of Sandpaper
• Phillips-Head Screwdriver

Teacher preparation

1. Divide your class into 10 laboratory groups of 2 to 4 students.
2. Cut twenty 90-cm pieces of #22 hook-up wire and strip 2 cm of insulation from both ends.
3. Distribute materials for each group.
4. Obtain several AM/FM radios or portable stereos (boom boxes), which have powerful amplifiers that drive their speakers. Small devices, such as MP3 players or cell phones, will not work because their audio signals are too weak. Make sure these radios have standard headphone jacks (3-1/2 mm, 1/8 in).
5. Prior to the lab, build a pie pan speaker to familiarize yourself with the procedure. Test the speaker by plugging it into the headphone jack of each radio or boom box. Increase the volume control until the sound is audible through the pie pan.

Student procedure

1. Remove the housing of an earphone plug by turning it counterclockwise.
2. Insert the bare ends of two 90-cm hook-up wires through the housing and insert the end of each wire through the hole of each post and twist tightly against the post to make a good contact (see Fig. 2).
3. Add a piece of paper as a spacer between the bare wires connected to the post to keep the posts and bare wires separated. If they touch, they will not transmit a signal.

Figure 2  Earphone plug posts with connected wire.

4. Slide the housing over both wires and turn clockwise to tighten.
5. Take the other bare ends of the 2 hook-up wires and wrap them around the screw of each alligator clip; tighten with a Phillips-head screwdriver (see Fig. 3). This will be the speaker wire connecting the earphone jack of the radio to the homemade speaker.  

Figure 3  Alligator clips attached to other ends of the wires.

6. Wrap the 6-m piece of #28 bare magnet wire around a cylinder that is the same size or slightly larger than the round magnets. Make a tight, narrow coil, leaving about 25 cm on each end to wrap around the left and right sides of the coil to keep the coil intact (see Fig. 4). Leave 12 to 15 cm free on each end to attach the alligator clips from the speaker wire you assembled. This coil is your voice coil.

Figure 4  Coil with wrapped sides.

7. Sand the enamel finish from each end (3 to 5 cm) of the wire coil down to the bare copper wire to allow good contact with the alligator clips.
8. Stack 3 circular magnets on a level surface and slide the wire coil down onto the first magnet until the coil is even with the top of the magnet (see Fig. 5).

Figure 5  Wire coil on magnets.

9. Center an aluminum pie pan on top of the magnet-wire assembly (see Fig. 6).
10. If the pan is unstable, you can place a magnet in the center of the pan to secure the magnet-wire assembly underneath; however, the sound is louder without the magnet on top.

Figure 6  Pan and magnet/wire assembly (top).

11. Plug the earphone plug into the radio headphone jack and connect the 2 alligator clips to each end of the voice coil wires.
12. Turn on the radio, select a station, and turn the volume all the way up. If there is no sound, make sure the alligator clips are not touching and are making good contact with the sanded ends of the voice coil wires. If there is still no sound, check the bare wires in the earphone plug to make sure they are still separated. 
    

Extension activities

1. Place a few sand or salt crystals halfway between the center and the outer edge of the pan and then turn on the radio. Describe what you see and tell why it happens.
2. Add more magnets beneath the pan (up to 6 or more). How does this affect the sound?
3. Try different types of speaker materials such as metal pans, cans, plastic deli trays, thin plastic or paper plates and cups, and aluminum foil, and note how this affects the sound.
4. Devise an experiment to see if speaker diameter makes a difference in pitches (low, mid-range, or high).

Footnote

¹http://www.simplyspeakers.com/speaker-repair-how-to-tips-tricks-faq.html (accessed March 12, 2013).