What causes the sound you hear when you hold a seashell up to your ear?

I'm not sure, but my guess is it's just the air moving around.
The reason you can't hear it without the shell, is because it has more room to move around.

It's also important to note that when you "hear the ocean" in a seashell, you're usually standing on the beach right next to it. :D

Either way, you can get the same effect by cupping your hands around your ear(s). I'm pretty sure it just has to do with the sound waves hitting your ears differently than normal.

	Kenbo -  6965 pts -  (3 years ago)
	I believe you can still hear the ocean even if your not close to it.

It's the amplification of sound through vibration. There's always something around making noise, even if you don't pay any notice to it. These sound waves enter the shell and then project themselves outward again. Like mentioned above, it happens when cupping your hands or using a glass.

Some used to argue that it could be the echoing of the blood vessels in your ears pumping blood, but this has been widely criticized and considered untrue because of certain unvariable factors (ie. working out, which increases bloodflow, should cause the sound to intensify, which it does not.)

	who -  1144 pts -  (2 years ago)
	Shells probably make the best resonators as their formation is based on the golden ratio!

well what really cause it is the air moving all around you and when you put it agaist your ear in lets just say in your living room you can not hear it because the air is not blowing all around you so mostly it is only cause by when you are out side of somewhere