I use small passive buzzers (12mm diameter) with young people, usually connecting them to a BBC micro:bit (v1) to make sounds and play musical notes.

They are very quiet, and I like them that way (don't want 30 loud buzzers going at once!).

For a new project, I was putting the buzzers inside a box, so needed a way of guiding the sound so it didn't get lost inside.

I designed this simple horn to slot over the buzzer, which does the job nicely (and improves the quality of the sound as well).