I like the Thunderbolts, a lot. They're awesome to me as well and I do wish that my city had them. My personal #1 favorites though, among all, are
the Whelen 4004's and Whelen 3016's. The thunderbolts have always been my second favorite... there's a third, but I forget what it is...
... alright, just a reminder, the main reason why I posted this topic was to get some insight on why tornado sirens are typically in tune with one another.
I'm glad the Thunderbolt has been brought up, because they are among this as well, as I've seen in many, many YouTube videos over the years.
But why is it that a lot of times tornado sirens harmonize? Are they purposely set up to do this? Or is it some sort of coincidence?

If it's electronic, I don't like it.  Don't get me wrong, I love Whelen engineering.  I have been using their products for emergency vehicles for 30+ years.  But a mechanical siren vs electronic will always win. 

Regardless of whether it's mechanical or electronic, I've noticed tornado sirens harmonize with each other. Is there a reason for this?
Or is it merely a coincidence?