English: Soldiers operating an acoustic airplane locator during World War 2, Trelleborg, Sweden, 1940. Before radar was invented, acoustic locator equipment was used to detect approaching enemy aircraft by listening for the sound of their engines. The detection horns were used in pairs; the side horns used for determining the bearing of the aircraft and the top and bottom horns the elevation. Each pair of horns was attached by rubber hoses to a stethoscope-type earphone worn by a technician. By rotating the horizontal and vertical axes of the horns until the aircraft sounded "centered" stereophonically in the earphones, the bearing and elevation of the aircraft could be determined.