If you visit Denge, a former Royal Air Force base on the Eastern coastline of Kent, England, you’ll find three enormous concrete structures. Two of the structures resemble a hollow ball, cut in half and propped up on a base. The third is a solid concrete square with a half-sphere scooped out of the center. The big, brutalist structures look both ancient and eerily futuristic; they are a cross between a ruin and satellite dish to the cosmos.
In fact, they are remnants of WWI and WWII-era surveillance devices—the massive concrete precursors to radar. And though they ceased operation decades ago, the influence of their brilliant design can be seen across modern technology today—from tiny, eavesdropping parabolic microphones used in surveillance, to a 1,000-foot-wide radio telescope nestled in a sink hole in Puerto Rico.
Pioneering Location Technology
Called either sound mirrors or acoustic mirrors, over a dozen of these structures dot the coast of southern England and Malta. During WWI, Major William Sansome Tucker, then director of acoustical research for the Air Force, invented the acoustic mirror to detect oncoming enemy aircraft by the noise of their engines. Tucker developed the mirrors after inventing a system of movable microphones that would later be incorporated into this project.
Before fighter jets advanced to have quieter engines, the concrete bowl would reflect the engine sound, which would be amplified by the microphone. As sound waves reached the concave sphere in the center of a concrete structure, movable microphones situated in front of it would pick up the engine noise and transmit it to the headphones of an operator staked out in a trench nearby. The most sophisticated of these devices could detect the sounds of aircraft up to 25 miles away. This would give the English artillery about 15 minutes to prepare for the attack.
Yet the acoustic mirrors in Denge and along the English coast—colloquially known as “listening ears”—never had the chance to work as they were intended. The network of structures was still being built when radar stopped the project dead in its tracks. By 1938, the year before the start of WWII, a ring of coastal radar stations with the codename of Chain Home (CH) began full-time operation. News had reached England that the Germans were working on a radio-based weapon, which led to physicist and radar pioneer Robert Watson-Watt to create a prototype system that used commercial shortwave radio hardware to detect passing aircraft. Combined with the first integrated ground-controlled interception network, known as the Dowding System, Chain Home could detect aircraft over 100 miles away.
By the onset of WWII, the invention of radar gave the Air Force the ability to locate aircraft much more precisely, and the acoustic mirror project was abandoned. But the fact that the British artillery was already familiar with a system that relied on linking stations and plotting aircraft movement contributed to their success in the war–and the design also ended up being influential far beyond the battlefield.
The Legacy of “Listening Ears”
Parabolic microphones make up the most direct lineage of Tucker’s revolutionary technology. Like acoustic mirrors, the microphones use a parabolic dish to reflect sound from a specific direction into the mic at the center (the U.K.’s war devices were technically spherical, not parabolic, though the technology is essentially the same). Parabolic microphones are still used in surveillance and espionage because their design makes them extremely sensitive to sound in one direction, and they can be manufactured very small.
On the other end of the size spectrum, enormous radio telescopes make similar use of the parabolic shape to amplify sounds from outer space. Puerto Rico’s Arecibo radio telescope uses an enormous collecting dish to detect wayward asteroids and discover some of the most remote bodies in the cosmos. Arecibo–which is 1,000 feet wide–and China’s 1,600-foot Aperture Spherical Radio Telescope are the largest observatories in the world, thanks to the natural sinkholes that they are built into.
Meanwhile, you’ll see the echo of Tucker’s ideas in a common exhibition fixture in science museums: two wall-sized parabolic acoustic mirrors called “whispering dishes,” installed facing each other to demonstrate focusing of sound.
The listening mirrors have also been influential across the design industry and art world–for instance, serving as inspiration for these fancy concrete lamps created by Matali Crasset, who told Co.Design‘s Mark Wilson that the design is a tribute. “When I discovered this radar project I was captivated by the beauty,” he said. And in Houston, Texas, artist Doug Hollis created two stone Listening Vessels that employ the same shape and design of acoustic mirrors for a public art piece.
These days, visitors to the U.K. take special trips just to see the hulking defense ruins, which are now pieces of art in and of themselves. They’re made even more incredible by their backstory, and the future technologies they inspired.