Congratulations to @skankdelvar who is on a roll, and , as it happens, after doing a double, is approaching the hallowed ground of a treble! I'm no stato , but that's a rare thing. Without further ado ..........    Before radar was perfected, aircraft were detected using acoustic locators which enhanced and amplified the sound of an aircraft's engine. The smallest version was a pair of acoustic mirrors which the user strapped either side of his head.