Aircraft were first extensivly used for war during World War I, but until the implementation of radar nearly thirty years later the only early warning system for aircraft attacks was listening. The unaided human ear isn’t keen enough to be especially helpful, so lots of people put lots of effort into advancing the science of acoustics. The result of all that work was the acoustic mirror. Acoustic mirrors work similarly to the external part of the human ear but on a much larger scale. They are large devices shaped so that they reflect and focus sound waves so that they can be more easily heard. It’s a method used by modern day parabolic microphones and satellite dishes.
The British built an impressive system of acoustic mirrors for coastline defense along the English Channel before World War II. These mirrors consisted of large structures built of concrete. There were several different shapes as the British experimented with the best design, but each shape reflected sound waves and focused them in a particular area where a microphone or human listener could be located. Focusing the sound waves allowed listeners to hear aircraft much farther away than the unaided human ear and gave the defenders a little more time to call up their own aircraft for defense. If placed properly multiple acoustic mirrors could be used to determine an approximate distance and location for the incoming aircraft through a triangulation process similar. The British acoustic mirrors were rendered obsolete when radar was successfully implemented, but several are still in place along the Channel coastline as the below satellite images from Google Maps show.
The Japanese had a similar, more mobile, system humorously called ‘war tubas’. I’m not going to try and describe them, the picture speaks for itself. The picture is mostly laughable at this point, but it was serious business 80 years ago as you can judge by the presence of the large anti-aircraft guns in the background nearby.
I originally wrote this post after running across a random picture of the war tubas last fall. Investigating the war tubas naturally led to acoustic mirrors which led to reading up on acoustic location and made for an interesting line of reading. I should have posted in last November because in the course of updating my links so I could post it this week I discovered io9 had scooped me a month ago. Bad Science has an interesting post as well.