Why do some tornado sirens have 3 different sirens?

Question

http://youtube.com/watch?v=rCLdEC8-S44

Answer 1

There are three different sirens for: danger is here, danger is here, and danger is gone. The first siren signifies a funnel cloud and/or tornado is approaching. The second siren signifies the funnel cloud and/or tornado is here. The third siren signifies all clear.

In case you are wondering, the siren was being rotated so that it can send sound out in all directions for everyone to hear. If it sent sound in only one direction, then it would be difficult for people in other directions to hear it.