Acoustic Mirrors
I was all teed up to write about how you can't find Internet information on anything anymore because search engines return nothing but astroturf marketing sites, but fortunately for you, I stumbled on something much more interesting.
Before radar was invented, there was no way to detect enemy aircraft beyond looking up in the sky. That wasn't particularly useful. Oh, and there was also no way to detect zeppelins (zeppelins!).
During World War I, Britain developed an interesting detection system: acoustic mirrors. Here's a description of how they worked:
When a planar sound wave strikes a reflective, concave surface, the surface will reflect the sound and also focus it to a point. The larger the surface, the longer the wavelength that will be reflected. Now, suppose that you wanted to know when enemy airplanes are approaching. You set up a number of large reflecting concave surfaces facing the English Channel, place microphones at the focal points. Then you build a communications system to relay that information to a central command, where you can instruct your defense forces. You have an early-warning system of sound mirrors.
The level of ingenuity is really remarkable.
Physically, they were quite interesting. Some were shaped like huge satellite dishes, but one built at the Denge RAF facility was curved wall that was 230' wide and 16' high, which is massive.
How effective were they? Well, theoretically, they could detect enemy aircraft up to 25 miles away (usually it was closer to 15), and could also establish the approaching direction. This was an enormous improvement over "nothing," but the weather could have a significant effect on the detection capabilities, so it wasn't consistent.
They were used until radar was invented in 1935 and a nationwide radar system was put in place.
Many of these mirrors still exist today, and if you want to see a video of one in action, there's one at the bottom of this article, and it's excellent: Giant Concrete “Sound Mirrors” were Used to Detect Enemy Aircraft Before Radar.
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