Lesson Title: Paper Plate Speakers
Grade Level: 10-12
Students will explore the physics and construction of a dynamic loudspeaker. After they have completed the lesson they will be able to: (1) Describe the mechanics of sound list several properties of sound waves; (2) Describe the magnetic field generated by a wire and solenoid; (3) Explain the design and construction of a dynamic speaker.
Prescribed Learning Outcomes:
Taken from the Applications of Physics 11 and 12 Integrated Resource Package 2007
Physics 12 D1 (Page 61)
Materials and Resources:
Without revealing the topic of the presentation show the students the tuning fork and ask them to describe how it produces sound. Reinforce that sound is caused by vibration. Repeat the process with the simple electromagnet to arrive at the conclusion that electricity can be used to create a magnetic field. Hypothesise a device that it powered by electricity and produces sound. Motivate the students to guess what such a device would look like by alluding to how common this device is. Identify the device as a speaker and explain how what seems like a complex device is actually incredibly simple. Begin the presentation; covering the outline for the lesson.
Physics of Sound (5 minutes):
Define sound as a mechanical wave of pressure travelling through a medium. Reinforce that sound is created by vibrating objects and is detected by our ears to be interpreted by our brain. Ask the students to think about different sounds, how they are different and why. Describe the following fundamental properties of sound. Amplitude is the height of the wave and affects how loud the sound is. Frequency is how fast the wave oscillates and affects the sounds pitch. Frequency is measured in Hertz (Hz) which is the number of vibrations per second. The range of human hearing is 20Hz to 20kHz.
Physics of Electromagnetism (5 minutes):
Referring to the earlier demonstration, explain that a moving current generated a magnetic field. Explain that the direction of the magnetic field can be determined using the right hand rule. Show that the magnetic field flows perpendicular to the direction of current. Emphasise that reversing the direction of the current flow, the magnetic field is also reversed. Describe the concept of a solenoid and how the magnetic fields become aligned. This field is strongest in the center of the solenoid.
Construction of a Speaker (15 minutes):
Quickly review the two important concepts covered: sound is caused by vibration, and magnetic fields can be generated by electricity. Tell the students that we can construct a speaker using these two concepts.
Start by showing the students a simple pre-constructed solenoid. Ask them what will happen if the solenoid is connected to a current. Will it produce sound? Get them to explain why no sound was produced. Explain that there was sound produced because there was nothing for the solenoids magnetic field to push against.
Place a magnet on the table with the solenoid over top; now there is an existing magnetic field. Again, ask the students what they think will happen and to explain the results once the solenoid jumps off of the magnet. Explain that the small paper tube won’t displace much air.
Place a paper place on top of the solenoid. Now there is a large surface that can create larger amounts of pressure. However, the entire setup is pretty flimsy. Use this as motivation to bring out the pre-constructed speaker. Explain that it is an identical setup just with everything secured into place. Point out all the familiar elements and explain the new features. Repeat the line of questions. This time, large amounts of air pressure were created. Why was no sound produced? Refer to the slide about the range of human hearing. Connecting and disconnecting the battery rapidly produces a vibration of around 2Hz which is much too slow. We need a way to switch the current on and off at a much higher rate.
Bring out the function generator and explain that it is capable of switching current on and off at very high rates. Connect the speaker to the generator and ask, once again, if we will finally be able to produce sound. Start with a very low frequency to replicate the results with the battery. Slowly increase the frequency until a tone can be heard. Ask the students if they think we’ve created a “real” speaker and what they think will happen if it’s connected to an MP3 player. Show that the speaker is indeed fully functional.
Further Investigation (10+ minutes):
Show that the speaker can be made even simpler than the paper plate model. Explain the flat coil speaker and ask the students if they think the new model will function as good as the other, if at all. Place the speaker next to the magnet and show that it too is a functioning speaker. Bring back the solenoid, magnet and paper plate, showing that it was actually a functioning speaker. Remove the paper plate and that the solenoid itself is also functional.
Divide the students into small groups and distribute the different speakers between them. Have them explore the differences and functionality of the models using their own MP3 players or other devices.