Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the magnet wire into 50-cm lengths and the speaker wire into 30-cm lengths. Cut enough wire for each group of students.
2. Cut the sandpaper into 2" x 3" strips.
3. Insert two AAA batteries into the radio.

Attaching the “Ear Bud” Headphones (Optional)

1. Obtain the “earbud” headphones (one is included with the radio).
2. With scissors, cut off both earbuds near the ends of the leads.
3. Use wire cutters or wire strippers to strip about 3 cm of the plastic sheath off the ends of the clipped wires. Several strands of wire should be present under the plastic sheath.
4. Obtain the alligator clips.
5. Thread the wire strands from one headphone wire lead into the hole at the end of the alligator clip.
6. Tightly wrap the loose wires around the end of the alligator clip to bind them. Use tape to secure the wire and alligator clip, if necessary. Alternatively, use a soldering iron and solder to secure the wire to the alligator clip.
7. Repeat steps 5 and 6 for the second wire and alligator clip.

 

Safety Precautions

Handle wire cutters carefully. Be cautious of the ends of the wire as they may be sharp. Students should wear safety glasses. Remind students to wash hands thoroughly with soap and water before leaving the laboratory.

Disposal

The materials may be saved and stored for future use. Unwrap the wire, if desired, to allow a new class to build the speakers. The cups may be thrown into the normal trash.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The materials can be used by other classes but some steps will no longer be necessary (e.g., sanding, cutting, poking a hole, wire stripping)
• Caution: Use only inexpensive radios for this experiment. Do not plug these speakers into any valuable radio/tuner device. Commercial speakers are designed to electrically match the requirements of an amplifier’s output. These handmade, “unmatched” speakers may cause an amplifier to burn out.
• Create “stereo” speakers by connecting two alligator clips to each headphone lead from the “earbud” headphone. Each lead has multiple wire strands for the leading currents and one solid wire for the returning current. Gently sand the enamel (red and blue) off the end of the single wire. These wires are thin and fragile, so handle them carefully. Connect one alligator clip to the multiple strands and a second alligator clip to the single “returning” wire (see Figure 8). It might be helpful to solder the wires to the alligator clips for this assembly. The bare copper wire must make solid contact with the alligator clip. Then, connect these alligator clips to one speaker. Repeat for the other wire lead and now two groups can listen at the same time and compare the quality of their speakers. Important note: These are fragile wires and connections—make sure students handle these “stereo” headphone jacks carefully. Two speakers must be connected to the “stereo” headphone jack in order to complete the circuit. This setup will not work with only one speaker.
  {12529_Hints_Figure_8}

Sample Data

Observations

Describe the volume and quality of the sound emitted by the speaker. Compare to the sound quality obtained by different speakers.

The sound had a lot of static, but it may have been the result of poor reception by the radio. The sound was soft and somewhat muffled but by holding the speaker close to the ear, the words and music were distinctly heard.

Most speakers had about the same sound quality and loudness. Speakers with fewer coils tended to have softer sound.

Describe modifications or adjustments that were made to try to improve the sound quality.

Student answers will vary, but may include rewinding the coil, using fewer coil loops, sanding the enamel again, using more magnets, trying different cups, etc.

Answers to Questions

1. Write a step-by-step explanation for how the speaker converts electrical signals into sound waves.

   See Background information for an appropriate discussion.

2. What was the purpose of placing a hole at the bottom of one of the cups used to make the speaker?

   A hole was needed to allow air to move between the outside cup and the inner cup. Without the hole, the air between the cups would be compressed or become partially evacuated as the inside cone vibrated. The hole equilibrated the air surrounding the inside cone.

3. Predict how each of the following variables would affect the volume or quality of sound from this simple radio speaker.

a. Increasing the number of coils in the wire. — Increase loudness and quality
b. Increasing the diameter of the coil of wire. — Decrease loudness and sound quality
c. Using weaker magnets. — Decrease loudness
d. Using larger cups. — Increase loudness, deeper tones (lower frequencies) will be clearer.

Introduction

Build an audio speaker and discover how it converts electrical signals into sound.

Concepts

• Electromagnetism
• Energy conversion
• Sound waves

Background

An important property of a moving charged particle, such as an electron, is that it produces a magnetic field. So when many electrons travel in a conducting wire to produce an electric current, these moving electrons produce a magnetic field that surrounds the wire.

When current travels around a wire loop, the magnetic field produced by the current is the strongest at the center of the loop. The direction of the magnetic field produced by a looping current is perpendicular to the face of the loop (see Figure 1). (Use the “right-hand rule” to determine the direction of the magnetic field produced by a current-carrying loop—curl your fingers on your right hand in the direction of the current flow in the loop. Your thumb will point in the direction of the “north end” of the magnetic field.) When many current-carrying loops are grouped together in a line, such as with a spring or coil of wire, the current travels in the same direction in all of the loops and the magnetic fields produced by all of the loops add together. Any magnet located near the magnetized coil will either be pulled toward the coil or pushed away, depending on the direction of the magnetic fields. The strength of the magnetic field produced by the coil can be varied by changing the amount of current that travels through it.

In a sound system, a tuner decodes various types of electrical signals (i.e., an FM or AM radio signal, or a digital signal from a CD player) and converts the signals into oscillating electric current that is proportional to the frequency and amplitude of sound waves. The signals get an electrical boost in the amplifier which then sends the electrical signal to the wire coils in the speakers. The rapidly changing current running through the coil varies the magnetic field of the coil. An external permanent magnet positioned in the middle of the wire coil is pushed and pulled by the oscillating magnetic field, thereby converting the original electrical energy into mechanical energy. Attaching a paper cone to the oscillating magnet will result in oscillating air pressure waves. If the air pressure waves are oscillating at the proper audible frequency (20–20000 Hz), and the amplitude is large enough (3–75 decibels, depending on the frequency), then the oscillations will be heard as sound.

Experiment Overview

The purpose of this activity is to build a simple speaker using wire, magnets and cups. The speaker will be tested using a radio.

Materials

Safety Precautions

Handle wire cutters carefully. Be cautious of the ends of the wire as they may be sharp. Wear safety glasses. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

Building the Speaker

1. Obtain the magnet wire and sandpaper.
2. Sand off approximately 3 cm of the red enamel at both ends of the magnet wire to expose the shiny copper wire.
3. Starting approximately 10 cm from one end of the wire, wind the wire around a pencil or pen. Keep the coils tight.
4. Continue wrapping the wire until approximately 10 cm of wire remains at the other end.
5. Carefully remove the coil from the pencil.
6. To secure the coil, wrap one end of the wire through the loop and around the coil two or three times to form a binding loop as shown in Figure 2.
   {12529_Procedure_Figure_2}
7. Repeat step 6 for the other end of the wire.
8. Obtain one of the plastic cups, tape, and scissors.
9. Using scissors, carefully poke a hole with a diameter about the size of a pencil through the bottom center of the plastic cup.
10. Securely tape the coil to the bottom of the cup on the outside. Make sure the center of the coil lines up with the hole in the cup.
11. Obtain the second plastic cup and two neodymium magnets.
12. Put one magnet inside the cup at the bottom, and place the second magnet on the outside bottom of the cup. The two magnets will “stick” together through the plastic cup and hold each other in place. Slide the pair of magnets so they are in the center of the bottom of the cup (see Figure 3).
    {12529_Procedure_Figure_3}
13. Place the cup with the magnets into the cup with the coil on the outside. The speaker is now assembled.

Connecting the Headphone-Jack Plug and Leads

1. Obtain speaker wire, alligator clips, headphone–jack plug, scissors and wire cutters/strippers.
2. Using scissors, carefully cut down the middle of the speaker wire, between the two sheathed wires and then separate them to make a Y shape (see Figure 4).
   {12529_Procedure_Figure_4}
3. Use wire cutters or wire strippers to remove about 3 cm of the plastic sheath off the two split ends (see Figure 4). Several strands of wire will be exposed under the plastic sheath. Use care not to cut the fine wires during the wire-stripping process.
4. Repeat steps 2 and 3 for the other end of the speaker wire.
5. Obtain two alligator clips.
6. Thread the fine wire strands from one wire end into the hole at the end of the alligator clip.
7. Tightly wrap the loose wires around the end of the alligator clip. Use tape to bind the wire and alligator clip, if necessary. Alternatively, use a soldering iron and solder to bind the wire to the alligator clip.
8. Repeat steps 6 and 7 using the separated wire adjacent to the wire that was just attached to an alligator clip.
9. Obtain the headphone-jack plug.
10. Unscrew the plastic covering off the metal plug. This will expose the binding posts for the speaker wire (see Figure 5).
    {12529_Procedure_Figure_5}
11. Carefully bend the two binding posts outward to allow room to thread the wire. (see Figure 6).
    {12529_Procedure_Figure_6}
12. Thread the wire strands from one of the free ends of the speaker wire into the hole in one of the binding posts.
13. Tightly wrap the loose wires around the binding post. Make sure extraneous wire strands do not make contact with other metal parts of the headphone-jack plug. Use scissors to cut off any extra wire, if necessary.
14. Repeat steps 12 and 13 for the other wire end and binding post. The completed headphone–jack plug and leads should look like Figure 7.
    {12529_Procedure_Figure_7}

Testing the Speaker

1. Attach one of the alligator clips to one of the sanded wires extending from the coil at the bottom of the speaker.
2. Attach the second alligator clip to the second extended wire.
3. Plug the headphone-jack plug into the output port on the radio.
4. Adjust the radio volume to its maximum setting.
5. Turn the tuning knob until noise/music can be heard from the speaker. Hold the speaker near your ear, but do not touch the speaker to your ear which will prevent the speaker cone from vibrating. If no radio stations tune in on the radio (due to interference), listen for “white noise” to determine the functionality of the speaker. Best reception is usually obtained near a window with the radio held high and upright.
6. Adjust or modify the speaker design (e.g., position of the magnets, number of coils on the diameter of the coil) to obtain the best audible sound, if needed. Record any trial modifications or changes on the worksheet.
7. Compare the sound emitted by speakers built by different groups.
8. Consult your instructor for appropriate storage and disposal procedures.

Student Worksheet PDF

12529_Student1.pdf