Peril of the Conch

I spotted a genuine sea captain's artifact perched high on the wooden shelf: a ridged porcelain conch shell! Naturally, an experienced sailor like me had to sound the night watch. I hoisted myself up beside a heavy bookend, leaned forward, and blew into the shell with everything I had! Nothing came out except a hollow, echoing thrum that made the whole shelf shudder. The sudden reverberation tipped me backward, sending my long legs swinging over a pile of stacked paperbacks! Was I startled? Bah, never! I caught my balance right against the spine of an encyclopedia, crossed my flippers, and puffed out my bill in a grand display of acoustic inspection!