Art answered the question.  Just wanted to add a personal note.  My
granddaughter, age 2, sometimes likes to go downstairs where my
shells are.  She doesn't talk very well yet, but she has her own sign
language.  She looks at me, points to the door with one hand, and
holds the other hand cupped over her ear.  Once downstairs she picks
out a few shells (hopefully nothing too fragile or expensive), holds
them to her ear, and is then satisfied to go back upstairs.  She
usually selects something that will fit her hand, like an Arabian
Cowrie, which makes no detectable sound whatsoever when held to your
ear.  Maybe her hearing is better than mine.   :-)
 
Paul M.