Covert radio pen with bone conduction speaker

https://www.patreon.com/AppliedScience ---- Donate here!
I built a tiny radio into a ballpoint pen that allows the user to hear audio by casually biting the pen.

How a horn amplifies sound (hint: Impedance matching)

A horn-shaped speaker is louder than an equivalent speaker without the horn.  This fact doesn't violate physics because the horn is acting as a matching device between the speaker and the air around it.

Photo Credit for phonograph: Norman Bruderhofer,  Collection of John Lampert-Hopkins