Right on Gary.  It is also easy to demonstrate that the source of the
sound is without, not within.  Use an ordinary mug or similar
thick-walled, smooth-rimmed container rather than a shell.  Block one
ear with your finger.  Place the mug over your other ear, and press
firmly, so the rim of the mug "seals" tightly against your head with
no gaps.  You won't hear anything.  Now start to raise one edge of
the mug so there is a gap between the lip and your skin - instant
ocean!  The gap admits outside sound to the interior of the mug.
Note how you can vary the intensity of the sound by tilting the mug,
thereby changing the size of the gap.
 
Paul M.