I was doing a search of the sirens in San Francisco's sirens. I found a site that had some sound on it and it turns out the sirens run in attack. Not the 15 second test tone repeatedly.
Here is a post I put up that will explain what I am talking about.
http://www.airraidsirens.com/forums/vie ... php?t=6791
And is is a site with the attack. (Note this is what they use for all warnings)
http://72hours.org/sirens.html
Click the button that says "listen" with the speaker.