English: A French acoustic aircraft locater near Paris in World War 1, 1915. Before radar was developed, air defense forces used acoustic horn devices like this to detect incoming enemy aircraft by listening for the sound of their engines. The horn receptors, mounted on a gimbaled stand, are attached by rubber hoses to stethoscope-type earphones, worn by the military observer. The horns were used in pairs, with a horizontal pair used to determine the aircraft's azimuth (bearing) and a vertical pair used to determine the craft's elevation. By rotating the horns until the aircraft sounded "centered" stereophonically in the earphones the observer could determine its bearing and elevation.

Caption: "The most interesting of the special instruments employed for the defense of Paris from aerial attack are the "listening posts", one of which is shown in the illustration. This consists of four huge horns, which gather up the slightest sound and magnify it by means of a microphone, so it is impossible for an airplane to approach unheard."