If you hold up a seashell to your ear, you will hear the sea, no matter how far inland you currently are. In fact, it is not the case. So, what's actually going on here?
One popular explanation is that you are listening to your own blood coursing through you. Popular as this blood theory is, it doesn't hold water. "Press your ear to a shell and listen, then run around on the beach for a few minutes to increase the blood flow all through your body, and again listen to your magic shell," Kruszelnicki wrote. "You'll find that the loudness of the ‘sound of the sea is still the same'."
If we truly were hearing the sound of our blood rushing through our bodies, that wouldn't be the case: exercising makes your blood pressure rise, which would thus increase the supposed sounds being "reflected" by the shell. The fact that we don't hear a difference before and after exercise, therefore, makes quite an evident statement.
There's another idea that the "sea" you can hear in a shell is actually air-air flowing through the shell and out again, which creates the noise. "In a soundproof room, you won't hear anything from a shell," confirmed Andrew King, director of the University of Oxford's Centre for Integrative Neuroscience. "Background noise must be present." That's the biggest clue as to what's really going on here: the sounds we hear "inside" seashells are not coming from inside our bodies, but rather around them.
"You are hearing surrounding or background noise that has been increased in amplitude (振幅) by the physical characteristic of the seashell," King explained, "the specific sounds we hear within a shell depend on the exact shape of itself-the hard, curved surfaces inside the shell cause the sound waves that enter to bounce around, increasing some frequencies while reducing others."
Seashells may be the most poetic of ways to experience this resonance (共振), but they're definitely not the only method - pretty much any convex (凹面的) surface will do.
