Sound Wave
Light, Sound and Waves

Practical Activity for 14-16 In this demonstration students see how a longitudinal wave can generate a sine-shaped curve. You can use it to introduce sound displays on an oscilloscope.

#### 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 & safety

In this activity there is no need for precise timing. The student in charge of timing can use a stopwatch or count (eg “one thousand, two thousand..”) to estimate how long it took to plot the graph.

#### Procedure

1. Ask for two volunteers, one to be your assistant that holds the paper under the pen and the second to be in charge of timing.
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. 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.
6. Ask the assistant to stop when they near the end of the paper and to shout “stop”.
7. Display the resulting trace to the class – they should see a curve with a shape close to a sine wave.

#### Discussion prompts

• What labels should I use for the axes?
• What does the distance between two peaks show?
• How can I work out the frequency?

#### Teaching notes

To identify labels for axes encourage them to think about what caused the motion of the pen across the paper. The movement in the vertical axis is driven by the slinky coil. In the horizontal direction the paper was pushed at a steady speed. It’s a displacement-time graph and so the distance between two peaks on the graph represents the time period T.

Show how to estimate the frequency f by finding the average for T over a number of oscillations. For example, the graph in the video above took 6s to plot and has 12 peaks. So T = 6/12 = 0.5 s and f = 1/0.5 = 2 Hz.

Introduce oscilloscopes as electronic equivalents of slink-o-scopes. Explain that switching on the oscilloscope’s time-base is comparable to asking the assistant to push the paper, connecting a microphone is similar to slotting the meter rule between the slinky coils and an incoming sound wave is like a longitudinal wave on a slinky. Provide an example of a sound trace and explain that the time period can be found by counting the number of squares between two peaks and multiplying it by the time base setting in seconds per division (s/div). #### Learning outcome

Students determine the frequency of a sound wave from an oscilloscope trace.

This experiment was safety-checked in March 2020.