Why can you hear the sea in shells?

I would like to no what the sound is when you hold your ear inside a shell. When I was little I was told it was the seas. But now I would like to know what it really is.

Why can you hear the sea in shells?
Sea shells that you can hear the sea in collect any nearby sounds, like waves splashing onto a beach, and focus it. When you hold the shell to your ear you're just hearing the same sounds you'd hear without the shell, only they're more concentrated by the hollow inside the shell. You can get the same thing by holding a small cup or glass to your ear while you're at the beach.
Reply:It's really your own blood rushing through your head.
Reply:Some people have suggested that the sound you hear from the seashell is the echoing of your blood rushing through the blood vessels of your ear. That is not the case. If that were true, then the sound would intensify after exercising, since your blood races faster after exercising. However, the sound is the same even after exercising.



Others say that the whooshing sound inside the shell is generated by air flowing through the shell - air flowing through the shell and out creates a noise. You'll notice that the sound is louder when you lift the shell slightly away from your ear than it is when the shell is right against your head. However, this theory doesn't hold true in a soundproof room. In a soundproof room, there is still air, but when you hold the seashell to your ear, there's no sound.



The most likely explanation for the wave-like noise is ambient noise from around you. The seashell that you are holding just slightly above your ear captures this noise, which resonates inside the shell. The size and shape of the shell therefore has some effect on the sound you hear. Different shells sound different because different shells accentuate different frequencies. You don't even need the seashell to hear the noise. You can produce the same "ocean" sound using an empty cup or even by cupping your hand over your ear. Go ahead and try it and vary the distance at which you place the cup near your ear. The level of the sound will vary depending on the angle and distance the cup is from your ear.