What technology was used for detecting oncoming airplanes before RADAR was developed?
Acoustic Location -- that's what. Here is a website with some info and photographs. Two examples:
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Acoustic locators in Japan: 1930s.
This remarkable picture may have been reproduced before, but I make no apology for showing it here. The impressive array of tubas belong to at least two acoustic locators mounted on 4-wheel carriages. It is a little difficult to work exactly what is connected to what, not least because the background appears to have been erased by some unsubtle retouching, but I think that the format is the same as the British model; there are two horns in a horizontal plane, and on one side of the mounting there are two more in a vertical plane.
To the right, one of the figures is the Japanese emperor Horohito. Behind him are the AA guns intended to be used in conjunction with the locators. The only Japanese gun that I have found documented as being used with a sound locator is the Type 88 dual-purpose AA/coast-defence 75mm; there is not enough visible detail to verify that these are the guns shown in the picture, but they look about the right size.
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A US Army sound locator in use.
This photograph was dated January 1943, and was presented by the American media as being current equipment. This was another piece of misinformation as radar sets were already in widespread use for searchlight control at that date.
Note the large diameter acoustic tubes leading to the operator's headset.
