“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.”

As the original listening equipment “Waalsdorp” for the Air Watch Service was too complicated and too bulky, a miniature version of the parabolic reflectors was developed to be worn round the head.
Via mediateletipos.