Sea Shell Sea Sounds?

This answer is from Matthew Mason, lecturer in physiology, development and neuroscience at the University of Cambridge.

The answer involves the shell acting as something called a helmholtz resonator.

Now, a simple form of helmholtz resonator would be something like an empty wine bottle. You have a contained volume of air, which is connected, to the outside world through the neck of the bottle. We're familiar with the idea that if you blow across the neck of a wine bottle you hear a certain tone, and different sizes and shapes of wine bottle will produce different sounds.

If you hold a shell to your ear, the shell is exposed to background noise from the environment around you. There's always noise wherever you are in the world and even your ear can produce background noise itself through the blood passing through the blood vessels within the ear.

As a Helmholtz resonator, what the shell is doing is it's selectively amplifying some of the frequencies in that background noise, relative to the others.

The larger the shell, the lower the frequencies you would tend to hear, so it would sound deeper with more bass. It could be any kind of solid, hollow body, for example, a coffee cup-you can try this yourself if you tip out all of the coffee first.

Hold it next to your ear, but leave a little gap between the cup and your ear in order to hear something. If you try this and rattle your chair or wander around a little bit, what you might find is that some of the things that you do sound particularly loud to you in that ear, and those are the frequencies that are being particularly exaggerated by the helmholtz resonator.

As to how long it would last, because the shell itself is not making the noise, the noise is coming from the background environment around you, it would carry on forever, or at least until your hearing went at those particular frequencies.

So the noise we're hearing is an amplified background noise that can be amplified by any partially enclosed vessel. There has to be a gap for the sound waves to enter, and once inside they bounce around the walls of the container, some reinforcing each other, before they're picked up by your ear. There's also the possibility that your ear actually generates the noise from blood vessel flow and that this is also amplified by the shell.