RSS
The museum "Waalsdorp" in the Netherlands has a ton of amazing gear designed for military purposes from the 30's, check out the listening equipment section - "From the first world war until the 30's air acoustics played an important role in the air defence. Air vehicles carrying a weapon could not be located from the ground e.g. at night time or under cloudy conditions. As radar was still to be discovered, vision had to be supplemented by hearing using the sound of the engines." [via] - Link.
Miniaturized listening equipment - Link.
Acoustics were also used to locate artillery. A line of microphones along the front would pick up the sound of artillery firing. The time difference of the sound at any two microphones defined a hyperbola with all the possible locations of the artillery. A different set of microphones gave a different hyperbola that intersected the first at the location of the artillery. Counter battery fire to follow.
I know about this because I had a class in college with a math professor who worked one of these systems during WWII.
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