Rebecca Julian, of West End School, asks :-
Jessie Gibb, of Ardgowan School, asked:-
Why can you hear the sea when you put a sea shell over your ear?
Rod Lambert, a physicist, and John Podd a Psychologist, at Massey University responded.
We think what happens is this. When you put the shell to your ear, you stop most of the sound from around you from getting into your ear. The world around you seems quieter.
When that happens, your ear adjusts itself to become more sensitive to sound. You can check this by scratching your head with your ears plugged. The sound of your scratching gets to your ears by the vibration travelling through the bones in your head and seems louder when you block your ears. When your ears are at their most sensitive, the "sound" of blood flowing through your ears and the "sound" of the tiny pressure fluctuations that occur naturally in the air are just below the "threshold of hearing". That is, they are only just quiet enough not to be heard. The shell makes these sounds louder in the same way that the hollow body of a guitar or a violin makes the sound of the string louder. It forms a reverberation chamber that makes the sound of the blood in your ears and the air molecules loud enough to be heard by your more sensitive ear.
You can do some experiments with reverberation yourself by using, for example, a tube from a vacuum cleaner. Hold the end of one tube to your ear and listen. Then try again with two tubes joined together. What do you notice? Your local Science Centre quite likely has tubes of different lengths set up for people to experiment with.