Knock Knock - and - Tick Tock

We’ve seen how sound can travel through the air to your ears, but have you heard? It can also travel through solid objects… Here are two experiments for the price of one to demonstrate.

You will need:

* a table
* a metre stick
* a big ticking clock


What to do:

1. Knock on the table in front of you. Now rap your fingers on it, and try gently tapping on it with your nails as gently as you can. Try and describe the sounds and how loud they are.

2. Now put your ear to the table and knock again. Tap again, and do your scratchy-tapping thing again.

3. How does the sound sound now? Is it louder or softer?

4. Now for the TICK TOCK part of the activity. Stand a metre away from your ticking clock. How loud does it sound?

5. Now carefully place one end of the metre stick on the clock, and the other close to your ear. Can you hear the ticking? Again, is it louder, or softer than before?


What's going on?

In each case, the sounds appear louder when they travel through the surface of the table, or through the metre stick, straight to your ear. This is because if sound travels every effectively through a solid substance, as both reflection and absorption of the wave are reduced or eliminated. This means the amplitude of the sound wave is maintained much better on its way to your ear than if it had travelled through the air = you hear a louder noise.

Sounds also travel faster in solids than in air. This is because the molecules in a solid are more tightly packed and bound together than in liquids, and those in liquids are more tightly packed than in gases. Vibrating effects are more easily passed on from one molecule to the next when they are in close proximity.

Want to find out how sounds travel through liquid? Well, get in the bath and have a try for yourself!