Britain's huge concrete "ears"

These "acoustic mirrors" were constructed along the British coast in the pre-radar era of the 1920s as early-warning devices for the approach of enemy aircraft. Note this was only a decade after the first powered flight.

The photo shows three such constructions, including a large rectangular one in the background. The squiggly object in the center of the foremost mirror is the metal remnants of what was originally a microphone used to pick up and transmit the collected sound.

The mirrors worked, but were obviously immobile, and in any case were made obsolete when radar was invented a decade later.

The website for these devices is here, with additional coverage here. The Museum Waalsdorp in the Netherlands has a lot of information of acoustic air defense devices. Via BoingBoing.

And finally I get to post these two photos that I have stored for years in my miscellaneous file wondering how I could ever use them. Both of them are widely distributed on the 'net; I don't know the original source or their actual purposes. This webpage has a large collection of photos of such acoustic devices.