page_title: Civil defense siren
section_title: Civil defense sirens around the world
hierarchical_section_title: Civil defense siren / Civil defense sirens around the world
caption_attribution_description: English: One of the various outdoor public address systems installed at various locations around Sydney's Central Business District. Primarily installed for any emergencies that were as a result of APEC Australia 2007 but now remain as a permanant outdoor public address system. Typically this speaker array is installed on traffic light installations as shown. This particular setup can be found on the corner of Castlereagh and Park Streets in Sydney, Australia. This particular system was manufactured by Bosch, as printed on the speaker horn and superimposed the driver backing.
context_page_description: A civil defense siren is a siren used to provide an emergency population warning to the general population of approaching danger. It is sometimes sounded again to indicate the danger has passed. Some sirens are also used to call the volunteer fire department when needed. Initially designed to warn city dwellers of air raids in World War II, they were later used to warn of nuclear attack and natural destructive weather patterns such as tornadoes. The generalized nature of sirens led to many of them being replaced with more specific warnings, such as the broadcast-based Emergency Alert System and the Cell Broadcast-based Wireless Emergency Alerts and EU-Alert mobile technologies.
  A mechanical siren generates sound by spinning a slotted chopper wheel to interrupt a stream of air at a regular rate. Modern sirens can develop a sound level of up to 135 decibels at 100 feet. The Chrysler air raid siren, driven by a 331-cubic-inch Chrysler Hemi gasoline engine, generates 138 dB at 100 feet.
  By use of varying tones or binary patterns of sound, different alert conditions can be signaled. Electronic sirens can transmit voice announcements in addition to alert tone signals.
caption: One of the many public address systems set up in Sydney, Australia, for APEC Australia 2007
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