Through the medium

What the Activity is for

Finding vibrations in the medium.

The aim of this activity is to look for the vibrations between the source and detector. What happens between source and detector is not directly accessible with the senses, so now you are leading a hunt for some clues. You are trying to get children to think about the mechanism that allows them to hear. That is why we look at source and detector first.

What to Prepare

• a battery powered buzzer in a container that can be evacuated, perhaps with a flashing LED wired into the circuit
• a suitable vacuum pump

Safety note: Only a bell jar sold for use at such reduced pressure should be used. It should stand on a metal or thick glass base and be checked for chips or cracks before each use.

What Happens During this Activity

You might start by saying that now you are searching for the same to and fro motion seen in the source and detector in the medium. You can only look for clues, because the vibrations are very small and there are lots of them produced each second.

One place to start is by finding out how to cut the chain between source and detector. Place the buzzer and LED in the jar and pump the air from inside the jar. You continue to see the LED and the buzzer, but you can no longer hear anything. Emphasise that sound needs a medium to travel through – something that can move to and fro at every point.

Here's a good teaching question:

Teacher: Can sound be swept away?

Imagine the scene. You are standing on a cliff top high above the beach on a storm-tossed day. You call to your friend below, but the wind sweeps your words away.

So what is happening here? The answer is that the medium through which your words are travelling (the air) is moving wholesale as gusts of wind. You are trying to transmit a sound to the beach, but the medium is gusting along the cliff top. Your voice is swept away with the wind.