Acoustics.

This is coolbert:

Before there was radar there was the blind? Acousticians at war.

"How the blind defended Leningrad in World War II"

From https://www.rbth.com/ | HISTORY | FEB 21 2023 | BORIS EGOROV.

"Blind 'eavesdroppers' detected German bombers approaching the city long before they came into sight. They could identify the type and model of aircraft - and sometimes their approximate number - just from the sound of their engines."

[....]

"Along with radars, so-called sound detectors were the main method for detecting enemy aircraft. These cumbersome devices consisted of a system of tubes of various sizes that made it possible to hear the hum of approaching enemy aircraft at a great distance."

THE BLIND THEIR HEARING ESPECIALLY ACUTE MAKING EXCELLENT OPERATORS OF THE ACOUSTIC DETECTION SYSTEMS OF THE PERIOD PRE-RADAR. ABLE TO TELL DIRECTION, NUMBERS AND TYPES OF APPROACHING ENEMY WARPLANES!

See from an Internet web site images of the acoustic detectors as deployed in that era between WW1 and WW2. Apparatus in the modern context seemingly comical but at the time all they had to go with!

coolbert.