In-building applications might also be considered: masonry eats sound pretty effectively(or echo-mangles it into obscurity if you shout). If I were the local potentate, having a mechanism for telling the servants what I’ll be having for lunch this afternoon without exerting my royal self would totally be worth it…
(It could also have been a tech demo, without any immediately practical use, knocked together by somebody high enough on the food chain to have spare time purely because that person thought that it was pretty cool and wanted a neat effect to show off to others. While ‘R&D’ in the contemporary, organized-laboratories-and-big-science-with-emphasis-on-industry-and-war sense is suspected of being relatively new, ‘Natural philosophers’ showing each other neat tricks that they are poking at is much older, and craftsmen wowing the nobility with clockworks and things are older still.
We will probably never be able to get inside the head of whoever made it; but if the 3rd-rear-priest-of-the-feathered-serpent learned as a kid about how much better sound carries when you press your ear against the door/wall and spent his spare time trying to understand that better, that’d be a totally understandable, totally human, process that might culminate in a cups-and-string apparatus, without there being any broader ‘utility’ aside from the satisfaction of the builder’s curiosity.)
