With many thanks to YouTube forumer Gramcracker115 who recorded this, plus another one of the same siren from closer range.
From Quincy, Illinois.
Attack @ 3 cycles starts at 10 seconds in sharp, followed by a approx. 25 second alert blast at about 1:12 in.
http://www.youtube.com/watch?v=tWQuvvSMcmI
The subtle echo is killer, too.