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Civil defense siren

A civil defense siren is a siren used to provide an emergency population warning to the general population of approaching danger. Initially designed to warn city dwellers of air raids (air-raid sirens) during World War II, they were later used to warn of nuclear attack and natural disasters, such as tornadoes (tornado sirens). 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.

By use of varying tones or binary patterns of sound, different alert conditions can be called. Electronic sirens can transmit voice announcements in addition to alert tone signals. Siren systems may be electronically controlled and integrated into other warning systems.

Pre-raid warning: a 36-second high-tone followed by a 24-second low-tone, with three cycles per period. This warning signifies an air raid is likely about to take place.

Raid warning: a 6-second high-tone followed by a 6-second low tone, with 15 cycles per period. This signifies that an air raid is imminent.

Post-raid warning: a single 3-minute high-tone. When sounded, it signifies an end to the raid or a cool-down of the wartime situation.

List of civil defense sirens

Emergency Alert System

Fire alarm system

Long Range Acoustic Device

Rotary woofer

Air Raid Sirens – outdoor warning siren website

Civil Defense Museum – Overview of sirens since their inception

Archived 13 May 2016 at the Wayback Machine

Los Angeles air raid sirens – Pictures of unused nuclear-era civil defense sirens still extant in Los Angeles, California

The Siren Archive – Over 1,000 siren photographs coupled with a few recordings from around the world

The world's loudest and largest sirens ever

(CBC Radio News Special, 13 Nov 1961)

"Tocsin B", Canada's dry run of its Nuclear Warning System from 1961

Toronto Star – The mystery of the air raid sirens

Michigan Civil Defense Museum

Old German World War II Air Raid Siren YouTube

British World War II Air Raid Siren with all-clear YouTube

Federal Thunderbolt 1000T air raid siren in alert signal YouTube

Media related to Sounds of warning sirens at Wikimedia Commons