Air raid sirens have been used for decades as civil defence warnings, designed to be heard over long distances and to prompt people to seek shelter. Their rising and falling wail became widely recognisable through news footage and films about wartime emergencies. Online, short recordings of the siren are now reused as a dramatic shorthand for danger, often alongside other warning sounds such as a fire alarm or a smoke detector beep. Creators keep returning to it because the sound needs no explanation: listeners instantly understand that something urgent is about to happen.