The ocean-wave sound a conch shell makes when held to the ear comes from a combination of the air in the shell and the coming and going of the noises of the outside environment. The air in a shell resonates at a characteristic frequency. When outside noises excite the air, a sound is heard.
"The coming and going of these resonant frequencies gives the listener the illusion of hearing the ocean waves waves come and go," says Jearl Walker in "The Flying Circus of Physics" (John Wiley & Sons). The outside sounds may be actual ocean waves, but a breath of air will have the same effect.
- C. Claiborne Ray