While on my trip to Atlanta yesterday with my mom, we saw a couple of Federal/Whelen combos of these sirens: (I forgot the name of the street and I didn't take pictures).
Federal Thunderbolt 1000/1000T with Whelen Vortex
Federal STH-10 with Whelen Vortex
Federal STH-10 with Whelen Vortex
On another street (I forgot that name too) we also saw a Whelen 2910.
Does anyone know when Atlanta tests there sirens? I like to see if I can hear one before we leave on sunday.