Have you ever held a seashell to your ear and listened? You've probably been told that you were hearing the ocean, or, from more scientifically minded folks, that it was just the sound of the blood pulsing through your ears. In fact, the truth is closer to the first explanation than the second. If you were to listen to a seashell, then run around for a few minutes before listening again, your heart would beat faster but there would be no difference in the sound you heard in the seashell. That means the sound you hear isn't just your blood.
Here's what's really going on: a shell is what's known as a resonant cavity, like that of a glass bottle or the body of an acoustic guitar. The same way that a plucked string makes the body of the guitar vibrate, the sound waves around the shell make it vibrate ever so slightly. Therefore, if you're near the ocean, you'll hear the actual vibration of ocean sounds, but if you're in the park, you'll hear the ghostly vibrations of the birds and the breeze. The same vibration affects regularly shaped cavities like water glasses, too, but nobody hears the ocean in a water glass—just in a seashell. That's because a water glass generally has one frequency that makes it vibrate the most, whereas the seashell's irregular shape makes it vibrate at many different frequencies. That's why you hear a "whoosh" like an ocean wave instead of a single soft tone. Learn more about the science of sound with the videos below.