I looked up "transducer" and it seems to be what you'd need. But! You seem to have found where I was wrong: the horn. The horn seems to be the actual expensive part here. They could start a business I guess selling these en-masse. If anyone has any ideas how a maker could create this within reason please reply.

You can't really sell them in big quantities because they're typically custom to the task. They need to be custom shaped for each application.

Portafilters are made in mass so you could make horns for common portafilters.

> A woofer from AliExpress is not a speaker with amp you can use for University level audio work.

We're talking about coffee here...

Exactly!

Wars have been fought over coffee, people regularly spend thousands for coffee machines. There are entire companies that exist just to cater to this market.

Me, I'm a tea guy.

A wholly different set of wars