English: An acoustic aircraft detection device used by the Dutch army, in the Legermuseum Delft. Before the development of radar, acoustic horns like this were employed for air defense, to listen for the engine sounds of incoming enemy aircraft. It consists of two pairs of horn detectors, connected by rubber tubes to stethoscope type earphones worn by a technician (represented by mannikin). The horizontal pair detected the horizontal position (azimuth) of the aircraft, the vertical pair detected the vertical position (elevation). They were mounted on a gimbaled frame and could be scanned across the sky.