Sophie Duncan follows the path of sound waves

14th October 2005 at 01:00
There are lots of ways to help pupils explore the noisy world of sound.

Take a shallow dish and cover it with clingfilm. Place a few grains of rice on top of the clingfilm. Make a loud noise by hitting two cymbals together and watch the rice grains. You should find that they jump up and down.

The clashing cymbals cause air molecules to vibrate. The vibration travels through the air as a sound wave, and when it gets to the clingfilm it causes the clingfilm to vibrate, moving the rice.

Attach two pieces of string, about 35cm long, to a metal coathanger, one to each corner so that it hangs downwards, with the hook pointing towards the floor.

Gently wrap the end of one string around the index finger of your right hand, and the other around the index finger of your left hand. Gently swing the coat hanger so that it hits a table, or ask someone to hit it lightly with a metal spoon.

Repeat the experiment, but now place your index fingers in your ears, taking care not to push anything into your ear canal. You should find that the sound is a lot louder the second time. This is because sound waves travel better through solids than through gases.

To find out how well different materials reflect sound, take two cardboard tubes. Place them in a triangle shape, but instead of the two tubes joining at the top of the triangle, leave a gap of about 5cm. Place a watch at the opening of one tube and listen at the end of the other. You should find that you can't hear the watch ticking.

Place a flat piece of cardboard upright at the top of the triangle, about 5cm from the end of the tubes. Listen to the sound of the watch again.

Cover the cardboard with foam and see what effect that has on the sound you can hear. Experiment with different materials to see which reflects the sound best.

Sound waves are reflected when they hit a surface. Hard, flat surfaces tend to reflect sound really well, whereas bumpy, soft surfaces don't.

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