Radar is wonderful, sure, but militaries didn't always have the benefit of radar to pinpoint enemy locations. Before that, there was sonar. Underwater is one thing, though. On land, you needed big victrola-like contraptions that looked uncomfortable as all hell.
These acoustic locators worked in much the same way as a submarine or bat's sonar would—through active and passive echolocation. Operators could merely listen to the surrounding battlefield to analyze for aircraft or engines, or produce a signal that would reverberate off the distant artillery and give a sense of the enemy's location.
This method was, of course, replaced by the much more accurate and probably lighter radar. Can you imagine lugging these things around? Still, it was a start. I'd like to own one of these things just as an ice breaker. [A Daily Trends via Buzzfeed]