Outdoor Public Warning System Fact Sheet
- 109 sirens with tone and voice capability installed in the City as of August 2011.
- 50 original sirens with tone capability were installed in 1942.
- Plans are in place to bring the total number of sirens to 110.
- In order to achieve optimum coverage, acoustic modeling software has been purchased and employed by qualified sound engineers and professionals to assess gaps.
- Each time a siren is installed, the acoustic modeling is updated to reflect changes in sound created by the new installation. This ensures that the mapping is current.
- Locations are selected for installation based on an identified coverage gap.
- There are two types of siren devices used: 1600 watts that have four speakers and 3200 watts that have eight speakers. The 1600 watt devices are generally used in residential areas and mounted on poles. The 3200 watt devices are larger and are generally mounted on building roofs in business areas.
- Currently, ten sirens are solar powered.
- The Department of Technology attempts to ensure that new pole installations do not adversely affect residents by blocking their views or by being placed in areas that would result in excessively loud exposure to the public.
- Ambient noise, traffic noise, terrain, structures and the interaction of devices with one another makes the modeling and placement of devices very complex.
- New construction of high rise buildings may affect sound propagation of the Outdoor Warning System.
- Tones are generated at a higher decibel and will always be easier to hear than voice messaging. The difference in the range of frequency between electronic tones and voice messaging is difficult to reconcile with the same technical equipment in the devices.
- Every Tuesday at noon, the tone and voice messaging of each device are tested manually by Department of Technology staff working at the Department of Emergency Management headquarters. The test consists of a “wail” tone played for 15 seconds followed by a recorded message that says, “This is a test. This is a test of the Outdoor Public Warning System. This is only a test.”
- Each device is capable of playing up to seven different tones. The most common one is a “wail”.
- Voice messaging can either be: 1) pre-recorded on a chip installed in each device; 2) broadcast from the Department of Emergency Management through a recorded message or a live message; or 3) broadcast through the use of a mobile transmitter.
- The City has two deployable portable transmitters for use with the system while out in the field. These control the activation of some or all of the devices. One unit is in operation at the Central Radio Station as a live back-up system and the other is located at the Department of Emergency Management and can be signed out for use by a qualified public safety representative. The operator has the ability to play a tone and/or provide a live voice message of up to three minutes in length. This unit has been employed by the San Francisco Police Department for public address during large events.
- Public safety mobile and portable radios can be remotely programmed to patch into the siren devices to allow the operator to make emergency announcements.
- Siren devices can be pre-programmed into a variety of groups for specific announcements. One such group is the Tsunami Warning group for sirens located in the inundation areas of the City.
- Sirens can broadcast messages in English, Spanish, and Cantonese.
- In a real emergency, the tone will last for five minutes followed by live messaging. Disaster preparedness education materials direct the public who hear this tone not to call 911 and to tune to a local AM radio station such as 740 KCBS or KGO 810.
You can help improve the Outdoor Public Warning System!
If you are an Amateur Radio Operator, you can check in to Siren Net and give us a report of what you hear during the weekly test. This net is conducted by the Auxiliary Communications Service and you can find more information here on the ACS website.
If you do not have an Amateur Radio License, we have another way for you to report the weekly test of your siren - SurveyMonkey! This link will take you to the survey form to report your siren performance. Information is compiled from these reports to assist with the sound modeling.