Everything you ever wanted to know about San Francisco's noon siren

S.F. siren, alerting you to danger hypothetical and real. S.F. siren, alerting you to danger hypothetical and real. Photo: PM Images, Getty Images Photo: PM Images, Getty Images Image 1 of / 3 Caption Close Everything you ever wanted to know about San Francisco's noon siren 1 / 3 Back to Gallery

It's Tuesday at noon. You're in downtown San Francisco and you hear the wailing of a siren and an ominous voice telling you, "This is a test."

So what is it exactly? Here's everything you ever wondered — but forgot to google — about the San Francisco sirens.

When were the sirens first installed and why?

Logically enough, the first 50 sirens were installed in 1942 to warn of air raids during World War II. In 2005, the city used a federal grant to start repairing the existing sirens and adding new ones too. Now they're up to 109. Ten of them are solar-powered.

Where are the speakers?! I can't tell where that danged sound is coming from.

There are 1600 and 3200 watt sirens in use and they're located in different spots. The 1600 watt ones are, as you'd expect, in residential neighborhoods and they're mounted on poles. The louder 3200 watt sirens are mounted on building roofs in business areas, like downtown. Here's a full map of the speaker locations.

What kind of emergencies does the city imagine these being used for?

Any number of disasters, natural or otherwise. One use case is if a major earthquake triggers a tsunami. The sirens would sound and instructions would be given. In any event, officials say that hearing the siren at a time other than Tuesday at noon means you should go inside immediately and check the local news for more information.

But how can I tell if it's a real emergency?

The siren will go off for five minutes in a real emergency, and it will be accompanied by a real human giving instructions in English, Spanish and Cantonese.

Have they ever been used in a real emergency?

Nope, luckily no emergency has necessitated their use. You may remember they went off accidentally in the middle of the night in Nov. 2014, freaking everyone out. It was just a technical glitch.

Video: Emergency Sirens Startle San Francisco Residents

Where can I get more information?

The S.F. Department of Emergency Management has more information here, as does SF72, a site for emergency preparedness.