Question: I’ve never experienced a warning siren, even though I live in “tornado alley.” I’m not even sure if my town has one. Can you explain how they work? — DG, Liberal KS
Answer: I’m guessing you do have a siren, given your location, so you should be glad you’ve never heard it. Consider yourself lucky. Puzzled, though, why they don’t use it as a noon signal, which is more of a daily test to ensure it’s working. That’s the way it was done when I was living in Wisconsin as a child.
Also, the Emergency Alert System (EAS) is increasingly relying on alerts sent to cell phones, but that’s no help if cell service is out or your phone is off. Warning sirens are completely self-contained, with a backup power supply, so it’s the most reliable system.
The video, courtesy of the Smithsonian, shows a demo of how early sirens made their sound. What it involves is simply blowing air at a spinning disc that has holes punched in it. This video shows a disc with two rows of holes driven into rotation by a hand-cranked belt. An assistant blows air through the holes of the spinning disc using a flexible rubber hose.
Every time the blown air hits a hole, a pulse of air gets through. This creates waves in the air that we hear as sound. All musical instruments utilize the same principle. The faster the disc is spun, the higher the pulse frequency (pitch). The harder the assistant blows, the higher the sound pressure (volume).
There’s a delightful video from the 1950s TV show “Mr. Wizard” where he demonstrates a variation of that to one of his students:
https://www.youtube.com/watch?v=Lsfczwy3KOE&t=155s
Many existing sirens use this design, developed during the Cold War in the early 1950s. It’s called the Federal Signal 2T22 and was produced by Federal Signal Corporation in Illinois:
This particular design employs dual rotors (to push the air) and stators (where the holes are located) to sound each pitch. Because the sound power output of this type of siren is the same in every direction, it is described as omnidirectional. That’s good for a warning siren. The 2T22 can produce 113db (measured 100 feet away from the siren, and (depending on terrain and siren height above ground) can be heard for miles from the source. Of course, larger populated areas will require multiple sirens.
The best siren deployment is to have them capable of operating when the grid is down. That means either setting them up with solar panels and batteries, or using a generator that kicks in as needed. And they need to be hardwired to the local emergency personnel or connected by wifi. Both options carry significant up-front costs as well as regular maintenance. Unfortunately, not all small communities can afford this.
That’s what happened in Kerrville, TX in early July. The city managers had proposed adding sirens to the area. But because the Guadalupe River twists and turns, with dense foliage along its banks, multiple sirens would have been needed. The citizens of Kerrville rejected the proposal based on cost.
There are grants available through FEMA for early warning sirens, but you have to jump through a lot of hoops to get them. Many small communities lack the capabilities to do that. And even if they do, FEMA funding seems to be in a state of flux currently, adding further uncertainty.
Next Week in Sky Lights ⇒ 50,000 Years Ago