Vacuum or rolling ocean simply just background noises

Why do seashells sound like the ocean when you hold them next to your ear? - Connor, age 6
If you hold a seashell, a coffee mug, or even your cupped hands over your ear, you can hear a sound like ocean surf.
The sound doesn't come from the shell or your ears. It's actually just plain old background noise that's been changed by the shell.
Noise goes inside the shell and bounces around again and again, losing very little energy because the sides are so hard.
The waves criss-cross, some cancelling each other out to become quieter, some adding together to become louder.
What went in as a steady hum comes back out as a series of louder and softer noises which are no longer well-blended.
Our ears, and brains, have evolved to ignore constant background sounds, and tune in to irregular sounds. So when we hold the shell to our ear, we notice a different sound, like a faint roar.
We compare it to the something we've heard before, and voila, we hear the ocean. Or for the less imaginative, a faraway vacuum cleaner.
If you want to experiment with this yourself, stick a sock in the seashell. The sock will absorb the sound waves, the ocean sound will disappear, and you'll wonder why you're standing there listening to a sock.
Gary Payinda MD is an emergency medicine consultant in Whangarei.
Have a science, health topic or question you'd like addressed? Email: drpayinda@gmail.com
(This column provides general information and is not a substitute for the medical advice of your personal doctor.)