The horn is a Delta Airzound. https://deltacycle.com/airzound-horn

I've been using it about 3 years and at this point, I'm not entirely comfortable commuting without it. The thing is incredibly loud, almost obnoxious, and not passive. I mostly limit its use to the major suburban streets, when I'm in traffic, have a green light, and passing through big intersections at hi-speed. Sometimes I go railroad style and offer up a long blast just to let people know I'm coming. The horn's impact is striking. Drivers immediately take notice. It's a helpful tool when someone is attempting a left/right hook.

The tank pressurizes to about 80psi and lasts about 10 rides. Mounting can get a little creative but it's worth it. To me, the horn is as critical as decent lights. You should get one.