"We can hear when the air raids are coming"
Air Raid Siren
Creative Commons Attribution Share AlikeAir Raid Siren - Credit: MdE (de)
As well as placing listening devices on the frontiers, the German army employed look-outs in all major cities and urban areas, who sounded a siren when they saw enemy planes approaching. All lights in the target area would be extinguished, while radio announcements warned of the danger.