English: “Defending the Moscow sky”. Soviet soldiers manning an acoustic aircraft detector near Moscow, in 1941 during World War 2, to detect German bombers. Before radar was developed in World War 2, air defense forces used acoustic horn sound detectors like this one to listen for the sound of approaching enemy planes. The sound collected by the horns was conducted by rubber tubes to "stethoscope-like" earphones on the technician at left. Separate horns connected to each ear allowed the technician to judge the direction of the aircraft by stereophonic hearing. The 4 horns were used in pairs; the horizontal pair to determine direction and the vertical pair to determine elevation.