BEFORE RADAR!

This is how air attacks were detected before the advent of radar, it was called "Acoustic Detection"!

Acoustic detection continued to be employed well after WW II. The Ground Observer Corps, a USA Fauxiliary comprised of civilian volunteers, used both sight and sound to note the passing of aircraft from a network of sites along the periphery of the United States in the 1950s. Those contacts were phoned in to regional coordination centers, where they were plotted and time correlated to establish tracks.

In the 1960s and 1970s, the Viet Cong and North Vietnamese used listening sites to detect American aircraft. In some cases they were nothing more than shaped pits in the ground with a listener sitting on a stool at the approximate focal point. Most, however, used mechanical devices functionally similar to the equipment illustrated above. These folks had figured out that passive detection techniques didn't cause Shrikes and Standard ARMs to come raining down on their heads!

The first Japanese attack on the fortress island of Corregidor (in the Philippines) on 29th December 1941was detected by American acoustic locators.

There are many more weird looking devices...Google to find more!