I would have to say that it would be any dual-tone T-bolt. Very sadly, it's a sound that is disappearing from the world with each passing day, fading into the history books. However, those us here on this board will put up a fight to prolong it.
Sincerely yours,
Ron W.
"When your siren's a failin', chances are it's a Whelen."
I'd have to say the 500AT becasue before they replaced the 500at about a half mile from me...during tornado warnings it was erie sounding...but i also had a Model 5 and 2001 sounding along with it...still erie none the less.