Sound Mirrors
Before RADAR the British erected these Sound collecting dishes in hopes of catching airplanes before they made it to the coast and were able to bomb the countryside.
They didn’t really take off (haha, pun) because RADAR was invented, and that allowed for much further then the 20 mi (on a good day) range of the sound dishes.
They also had a giant wall meant to do the same thing but on a bigger scale.
these things look far bigger in real life! shame radar came out too early… I could imagine people building their own if they had the concrete.