HOW COME? Hearing the "ocean" -- in a seashell

How can we hear the sound of the ocean in a big seashell? asks a readerEven if it's too cold to go to the beach, you can always hear the ocean. Just pick up an empty coffee mug, and hold it to your ear, tilted slightly away. Ah, there it is -- the gentle, echoing roar of the waves.

That's right, you don't need a big conch shell to hear the sea. In fact, many objects around the house can function as your ocean-listening portal. Even a coffee-stained mug.

As lovely as the idea is, it's not really the ocean you're hearing when you put your ear to big seashell (or, of course, to the nearest coffee cup). So what, exactly, are you hearing? Some will tell you it's the sound of your own blood, rushing through the vessels in your ear. The seashell, pressed to your head, simply amplifies the sound. The hard walls of the shell then bounce it back to your eardrum. And you feel relaxed, as if you've actually dug your toes into sun-warmed sand.

But scientists note that the "ocean" sound in a shell doesn't get any louder when your blood is really rushing in your ear -- say, just after hard exercise. In fact, they suggest listening to a shell in a soundproof booth. While your own blood continues to pulse through your ear, the sound of the sea abruptly vanishes.

So where does the roar come from? Even a quiet (regular) room is filled with sounds, many so faint that we can't hear them. A big seashell amplifies these sounds, bouncing them back to us in a rush of noise that sounds a lot like ocean waves.

The trick is resonance. If you've ever swung on a swing, you'll have noticed that if you pump your legs in just the right rhythm, the swing will go higher and faster. Why? When your rhythm exactly matches the natural rhythm (frequency) of the swing, its own back-and-forth motion is amplified. Thanks to resonance, you get the fun of a wilder ride.

Resonance can also shatter glass, when a musical note matching the glass' own natural frequency makes it vibrate violently.

Which brings us back to shells. The chambers and twisting, turning walls of a large conch shell will resonate to many different natural frequencies. So as sounds from a room enter the shell, some set up extra-strong vibrations here and there. The sounds that match the shell's own frequencies resonate, becoming louder.

Bouncing off the shell's hard, echo-chamber walls, the strengthened sounds combine, emerging as a kind of roar that rises and falls.

Presto: Instant ocean.