This early accoustic warning device was from WW1 and still in use at the start of World War Two. This listening device named “Barbier, Bénard and Turenne” was used as an early warning system for incomming airplanes. Used for military and civil protection or with Anti-aircraft batteries as fire guidiance. The Germans used them, for exmaple, with 88 mm Flak batteries together with SW 34 Search Lights and these types of accoustic hearing devices to help guide the search lights and Flak guns.
They became obsolete after the radar came into play, radar was more accurate and had a wider detections range.
