Horchger Flak: one of the giant listening devices that can capture even the slightest noise of an engine." October 1939 This is an acoustic aircraft detection device. In World War 2, before radar was developed, these devices were used to detect approaching enemy aircraft by listening for the sound of their engines. It consists of 4 acoustic horns, a horizontal pair and a vertical pair, connected by rubber tubes to stethoscope type earphones worn by the two technicians left and right. The stereo earphones enabled one technician to determine the direction and the other the elevation of the aircraft.