The mother of all sirens is the Federal Signal Corporation
Q-Siren mounted on the front of most all fire apparatus in all it's chrome glory, as mentioned by Dangerous Dan.
Rated at 123 decibels at 10 feet.
The Engineer and the Officer up front both have a foot pedal to actuate it. I was driving a brand new engine up to a bad multi car, multi department extrication. A big parade of emergency response on both sides of the interstate. Our Q Siren decided to have a stuck switch or relay or something and would not wind down. As soon as we got there and got stopped it was my responsibility to get out and get a bolt cutter and cut the high amp power supply about a foot behind that screaming sob. I did have my headset on. Shutting the engine down and hitting the master kill switch is never an option.
The Medics around here don't have Q sirens but have an electronic versions with electronic imitation "air horns". Still loud.