Photo: Noise for Airports
Noise for Airports has a gallery of early sound amplifier/locator technologies. He quotes a 1939 issue of Science News Letter about these efforts:
The picturesque triple or quadruple sets of horns, looking like gigantic versions of old-fashioned ear trumpets, that are used by listeners for airplanes, are only artificial external ears that can be cocked in the direction of suspected approach, just as a rabbit or a donkey can tun his ears. Only they are more nearly perfect, mechanically, than any animal ear, because they were made to order along mathematically calculated lines, not slowly evolved out of folds of flesh.
During the World War, many blind men, with ears trained to special acuteness in compensation for loss of sight, volunteered for this service in Britain, and it is likely that such sightless soldiers are again helping their companions to locate enemies in the dark.
Link via Gizmodo