The New Leader Of The Opposition Party/
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No best answer has yet been selected by earthling. Once a best answer has been selected, it will be shown here.
For more on marking an answer as the "Best Answer", please visit our FAQ.According to HowStuffWorks:
"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." [Complete article here.]
By "capture", I imagine the author means that the some of the sound waves that usually bounce off your head and escape are instead reflected back towards the ear, probably interfering with other sound waves in the process and adding to the "white noise" effect.
I think tony1941 is correct that what we hear when it's quiet is just a distorted echo of any ambient sound (it must bounce around within the ear). In a totally silent room, perhaps we would be able to hear the sound of our own heartbeat, although whether this sound would be heard via vibrations in the air or directly from within the body, I can't imagine.