English: Photo of an acoustic aircraft detection apparatus at Bolling Field, (now Joint Base Anacostia-Bolling), Washington D. C., USA, in 1921. Before radar was developed in WW2, acoustic horns like this were used by air defense forces to detect the sound of approaching enemy aircraft at a distance. The system shown here, apparently being tested, consisted of two conical horns attached by rubber tubes to a stethoscope earpiece in the observer's ears. The stereo receptor allowed the observer to determine direction of the aircraft. The building in the background is the Army War College at Fort McNair.