sirens - how do they work?

Factual Questions
1

How do sirens (like the kind on ambulances or fire/rescue equipment) work? It sounds, to my unpracticed ears, like there are two different kinds. What’s the difference between the two?

2

Google, “how sirens work”, Hit #2.

http://www.airraidsirens.com/tech_howtheywork.html

You’re welcome. :smiley:

3

First, they start singing. Then sailors hear them and go, “Hey, what’s that?” Then they turn the ship in the direction of the singing. Then…

Wha’?

4

There was only * one * reason I even bothered to look at this thread. And Spritle beat me to it. :slight_smile:

5

There are two kinds of sirens used by emergency vehicles.

The first, also being the cheapest (and therefore most common) is the electrical siren. All it is is a speaker with some sort of electrical driver behind it. Inside the cab of the vehicle, there is a selector switch so we can vary between “wail”, “yelp”, and “hi-lo.” Some drivers can also retransmit the radio through the siren speakers, or be used as an intercom. Since the electric siren doesn’t require all that much electrical power to run, its the type thats most commonly found on police cars and ambulances.

The second type, my personal favorite, is the electro-mechanical siren, known to any firefighter as the Federal Q2 (or Q2B if it has a brake on it). This type works as DDG’s link above shows, a spinning rotor inside a stator makes the sound. To make this one work, there’s a swtich in the cab (either a foot pedal or a button (even the horn ring on some of our trucks)) which you press to make the siren speed up (higher pitch), let it go and it slows down (the lower pitch). But, since its a rather beefy electric motor, usually its reserved for fire apparatus (larger alternator to run the motor).

More on the Q2

6

My apologies, I posted the wrong link. Here’s the right one:

http://www.fedsig.com/fire/fire/products/sirens/qsiren/index.html

7
      • Siren Story: A pal of mine got a hand-crank siren when he was in the military, stationed in Saudi during the gulf war. He stole it off an air-raid tower as a souvenier when he found out he was coming home (he said everybody stole stuff that day).
  • It’s about five inches across and seven inches long with a large crank handle sticking out the center of one end, the whole thing made of metal. When you crank it, it starts out low-pitched and not-loud, then rises in both-- it sounds like the old WWII British air-raid sirens.
    When cranked fast, it is amazingly loud for as small as it is. - DougC