Good News, fellow CONCH-Lers!
 
As I've been transcribing the HAWAIIAN SHELL NEWS for
THE CAPTURED COWRY archives, I have come across this
timely side item in New Series #88, page 6, lower left
corner -- I quote:
 
--------------------------------------------------
Shell Acts As Sound Mirror
 
The following question and answer was taken from the
International Oceanographic Foundation publication
"Sea Secrets", Vol. 10, No. 7, July-August, 1966.
 
Question: What makes the roaring sound in a sea shell
when it is held to the ear? D.D.N., Cleveland, Ohio.
 
ANSWER: Children who visit the seashore are well aware
of the "sound of the sea" that can be heard by cupping
the ear to a large shell. Unfortunately for romantic
notions, the same sound can be heard in a jar, cup, or
milk bottle. The explanation is that existing sounds in
the air, particularly those of low frequency, are
amplified by the shell, which acts as a "sound mirror."
---------------------------------------------------
unquote
 
A couple of comments from makuabob --
 
Milk bottle?? Oooh, that's stretching it quite a bit.
Seen a milk bottle lately, outside of an antique shop?
 
And "amplify" isn't the correct word. "Selectively
reinforce" is closer. Certain frequencies arrive from
different areas of the "mirror" and cancel each other
(arrive at your ear canal out of phase) while some
arrive in phase and add together. Various others arrive
in varying degrees of adding or cancelling themselves,
depending on how the object is held relative to the ear.
 
Hope that helps with the 'Sea-in-the-shell' question.
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)