You've probably heard that if you hold a seashell up to your ear, you can hear crashing waves...or is it standing waves?
A standing wave is the term physicists use to describe what sound you hear when you don't notice sound at all. See, sound waves are bouncing around us all the time, though we may not notice each one individually. A hollow shell, in our case, does a good job of catching waves out of the air, which is what you're hearing.
Let's think of a straight cardboard tube to keep it simple. Lots of random sound waves come in one end and bounce around the inside. Many will cancel each other out, but a few will be reinforced. The reinforced waves that emerge from the other end are called standing waves. You can get different standing waves depending on the length of the tube or, as you can hear, from the interior dimensions of a seashell.
Next time you hold a shell to your ear, remember it's not crashing waves you are hearing, but standing waves!