Sound or acoustic mirrors were one of the first early warning detection systems invented to give advanced notice of an approaching enemy aircraft. These worked by focusing the sound from the plane’s engine so it could be heard before it was visible.
Sound mirrors worked using a curved surface to concentrate sound waves into a central point, which were picked up by a sound collector and later by microphones. An operator using a stethoscope would be stationed near the sound mirror, and would need specialist training in identifying different sounds. Distinguishing the complexity of sound was so difficult that the operators could only listen for around 40 minutes.