Sound Wave
Light, Sound and Waves

Practical Activity for 14-16

In this demonstration students are introduced to a mechanical model of how sound displays on an oscilloscope.

## Learning outcome

Students can describe what an oscilloscope shows when displaying a sound wave and determine the time period from the trace.

## Equipment

• Metre rule
• Rubber band
• Clamp stand
• Felt tip pen
• Sticky tape
• A few sheet of A3 squared or graph paper (eg made by sticking two A4 sheets together)
• Stopwatch (optional)

## Preparation

Build and test your slink-o-scope before the lesson. For instructions, watch the video above.

## Procedure

1. Ask for a volunteer. They will be in your assistant in charge of holding the paper under the pen.
2. Holding one end of the slinky in place move the other end back and forth to generate longitudinal waves. The pen will move on the paper.
3. Show the resulting graph to the class - they should see that that it is close to a straight-line.
4. Place a new sheet of paper under the pen.
5. Ask for another volunteer. They will be in charge of timing (they can use a stopwatch or count - eg “one thousand, two thousand..”).
6. Send longitudinal waves down the slinky again. Ask the timing volunteer to shout “start” so that your assistant can start moving the paper at a steady speed in a straight line towards the clamp stand.
7. Ask the assistant to stop when they near the end of the paper and to shout “stop”.
8. Display the resulting trace to the class – they should see that it is close to a sine shaped curve.

## Discussion prompts

• What labels should I use for the graph axes?
• What does the distance between two peaks show?

## Teaching notes

Students may think that the distance between two peaks represents the wavelength. Encourage them to think about what caused the motion of the pen across the paper. In the vertical direction it was driven by the motion of a slinky coil, in the horizontal your assistant pulled the paper at steady speed. It’s a displacement-time graph. The distance between two peaks represents the time period T.

Discuss how to find T by averaging over a number of peaks. For example, the graph in the video above took 6s to plot and has 12 peaks. So T = 6/12 = 0.5 s.

Introduce an oscilloscope as the electronic equivalent of a slink-o-scope.

 device detector display slink-o-scope metre-rule displacement-time graph oscilloscope microphone voltage-time graph

## Extension

Model increasing an oscilloscope’s time-base setting by increasing the speed of the paper. Model increasing its vertical sensitivity by increasing the distance between pen and pivot.

This experiment was safety-checked in March 2020.