Background information from Wikipedia:
Ringtrichter Richtungshörer Horchgerät, or literally ‘hearing aid for anti-aircraft guns’ were developed in the First World War as military reconnaissance devices to locate guns on the battlefields. The successes were modest, however, because artillery fire is only short-term sound events, and there were several gun positions on the battlefields so that the localization was difficult. The directional receiver was much better suited for the localization of aircraft. By means of several devices, the true position of a relatively slow sound source moving in the subsonic range could be determined by cross-bearing. They were still used during the Second World War in order to locate enemy aircraft, even in weather such as fog or darkness. With the increasing speed of aircraft and the invention of radar technology, the Richtungshörer was largely obsolete.