Concepts

Resonance, standing waves in 2D

Overview

A large speaker is driven by a function generator and an audio amplifier. When the speaker excites a wine glass at resonance, the glass oscillates and may break. A strobe light makes the oscillations easier to see.

Details

Equipment

• [1] Wine glass
• [1] Paper riders
• [1] Foam isolator
• [1] Plexiglas shield
• [1] Speaker
• [1] Steel brick
• [1] Audio amplifier
• [1] Speaker wire
• [1] Function generator
• [1] BNC cable
• [1] BNC to RCA adapter
• [1] Oscilloscope
• [1] Microphone with holder
• [1] 3.5 mm to BNC adapter
• [1] Lab stand
• [1] 90-degree clamp
• [1] Lab jack with sidewalls
• [1] Strobe light
• [1] Video camera
• Power bar

Safety Equipment

• [1] Earmuffs (for the instructor)
• Earplugs (for the class)

Classroom Assembly

1. Mount the speaker on the lab jack, with the steel brick as a backstop.
2. Connect the speaker to the amplifier.
3. Put the wine glass on the foam isolator and adjust the speaker height so that the middle of the speaker is level with the rim of the glass.
   
4. Mount the microphone on the lab stand using the 90-degree clamp. Make sure it's close to the rim of the glass.
5. Connect the microphone to the oscilloscope using the 3.5 mm to BNC adapter.
6. Put the Plexiglas shield around the speaker and wine glass.
7. Set up the strobe light on top of the shield.
8. Connect the function generator to the audio amplifier using the BNC cable and the BNC to RCA adapter.
9. Set up the video camera for a good view of the rim of the glass.
10. Connect the video camera to the room projector.
11. Plug everything in.
12. Test the setup and find a good strobe frequency for viewing the oscillations.

Important Notes

• Strobe lights can cause seizures in some people. Make sure no one in the class is sensitive to them.
• Loud noises can cause hearing loss. Everyone should wear hearing protection or cover one's ears.
• Do not go over the rated power for the amplifier. Otherwise, it will destroy the outputs.

Script

1. Hand out earplugs to the class and wait for them to wear them.
2. Make sure no one in the class is sensitive to strobe lights.
3. Put on your earmuffs.
4. Put paper riders on the wine glass rim.
5. Turn on the function generator, the amplifier, the oscilloscope, and the video camera. Increase the function generator signal to 100 mV RMS and increase the amplifier gain.
6. Scan the function generator frequency for the resonance of the wine glass. Use the microphone signal on the oscilloscope as a guide to getting close to resonance. The riders should start shaking.
7. Turn on the strobe light and scan the strobe light frequency for the best visibility of the oscillations.
8. Try to find resonance. If you're lucky, the glass will break.
9. Turn everything off.
10. Take off your earmuffs.

Additional Resources