7. All those different sirens serve different purposes
The classic "wail" is used for open roads when an officer is traveling at a high speed and approaching an intersection, because it's better at penetrating the cabin of a vehicle, meaning you'll hear it. The "yelp," basically a sped-up wail, is used in high-traffic situations, and if you still don't get out of the way, you'll likely get an earful of airhorn.
8. As technology advances, sirens might be on their way out
Short-range FM transmitters mean officers could be able to broadcast straight into your radio, and even if that fails, they've got something called a rumbler -- think of it like subwoofer that you can feel from 300ft away.