What Are the Sounds We Hear in a Shell?
The sounds we hear in the sea shell are really air waves or sounds made by air waves, because all sounds are produced by air waves.
The reason you can hear these sounds in a sea shell is because the shell is so constructed that it forms a natural sounding box. The wooden part of a guitar, zither or violin is a sounding box. They have the faculty of picking up sounds and making them stronger. We call them “resonators,” because they make sounds resound. The construction of a sea shell makes an almost perfect resonator. A perfect resonator will pick up sounds which the human ear cannot hear at all and magnify them so that if you hold a resonator to the ear you can hear sounds you could not otherwise hear. Ear trumpets for the deaf are built upon this principle.
Sometimes when you, with your ear alone, think something is absolutely quiet, you can pick up a sea shell and hear sounds in it. But the sea shell will magnify any sound that reaches it.
It would be possible, of course, to take a sea shell to a place where it would be absolutely quiet and then there would be no sounds.
There are such places, but very few of them. A room can be built which is absolutely sound proof.