The motor is cooled by having air blown through it, and those horns are just the outlets. The one below is the intake for the cooling blower. No reason for the shape other than horny sirens just look better.
The dampers on the ports are just there because I was basing this off a siren I saw in a dream. It had dampers on the ports, but since it didn't sound, I don't know what they would do. It could just be an elaborate way to enable coded signals or regulate the volume.
No idea about the decibel output, I don't know how I'd calculate that based on the specs of my model. I would guess it could match or beat a chrysler, as there would be a ton of air moving through that thing and the ports are huge (you could chop off your leg in one).
