Found photo via Make.
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."