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earlier the enemy could be located, and the warning sounded, the better. The problem was that visual sighting was notoriously difficult—especially when the approximate location of an intruder was not known. Even on a bright and clear day, the probability of making a sighting fell dramatically after a distance of only five miles. On hazy or foggy days, or at night, such invaders were effectively invisible.

Harnessing sound

Clearly, a means of locating enemy aircraft at greater ranges, and in all sighting conditions, was desperately needed. Under the direction of a Professor Mather, the British began experimenting with acoustic mirrors in order to detect the sound of the aircraft. Mather cut a semispherical depression into a horizontal chalk wall at Binbury Manor. A listening trumpet was placed on an adjustable mount near the focus. An operator would listen to the concentrated sound using rubber tubes connected to the trumpet, stethoscope style; he moved the trumpet to maximize the sound, and the azimuth and elevation could be read off the gridmarks. This explains why the depression wasn’t parabolized: Because the mirror itself wasn’t movable, “aiming” had to be done by moving the detector around, and it was best to keep the depression symmetric.

Mather claimed that this device would detect noise from as far away as 20 miles, and aircraft engines were certainly loud enough to be picked up. Although the British Army didn’t necessarily agree when they conducted their own tests of the mirrors, they kept the program in place, possibly because no one had anything else to suggest. Larger mirrors were built, and the surfaces were lined with concrete for better sound reflection.

Later, the mirrors were built to be freestanding and made entirely out of reinforced concrete. In 1917 and 1918, the mirrors near Dover proved their worth, detecting airplanes at a distance of 12 to 15 miles. In October of 1917, airplanes headed for London were detected in time to provide several minutes of warning to the military.

British artists have
begun to construct their
own acoustic mirrors
in public parks. To my
knowledge, these only
exist in Britain, which is
the sole country to have
actually used the tech-
nology in a practical way.

Even after the war ended, the experiments continued, since there was still no other viable solution to the problem of long-range aircraft detection. Larger acoustic mirrors were constructed, and microphones and stethoscopes were tried.

In 1925, W.S. Tucker was put in charge of the program. He directed the construction of 20-foot mirrors, and then 30-foot ones, along the south coast, along with structures for personnel and equipment and booths for operators, who controlled the detector position with handwheels and foot pedals. Eventually mirrors as long as 200 feet were built to detect long wavelengths (although these were not complete hemispheres). They could detect aircraft at a range of 30 miles.

Operation of the acoustic mirrors did not occur without considerable difficulties. Because the task required intense concentration, operators took shifts of no more than 40 minutes each. Wind blowing across the mirror could mask faint sounds (curtains were hung across the ends to decrease the effect), and the structures also picked up ambient noises in front of them, interfering with aircraft detection.

In 1935, the onset of radar technology derailed plans to build an extensive series of mirrors. In July of that year, a radar installation set up at Orford Ness could detect aircraft at a range of 40 miles, without causing any strain on the operators. At that point, construction of further acoustic mirrors was cancelled, although the existing ones were still used occasionally; the last ones were not phased out until 1939.

But the story doesn’t end there. Acoustic mirrors continued to be used in the second world war as a useful “cover” for Britain’s still-secret radar systems and a back-up detection method for the times when German scientists began jamming radar signals later in the war.

Historical markers
and works of art

Most of the acoustic mirrors used by the British still exist today, although some have been buried or destroyed by time and weathering. Others are on private land that is not legally accessible. Many of those that are accessible have been covered with graffiti; efforts are under way to preserve them. The mirrors have been used in videos and works of conceptual art.

In an interesting modern twist, British artists have begun to construct their own acoustic mirrors in public parks. To my knowledge, these only exist in Britain, which is the sole country to have actually used the technology in a practical way. Paired listening and speaking concrete mirrors have been built along the Palmarsh Footbridge over the Royal Military Canal in Kent, not far from a set of the original WW I mirrors. More mirrors, also made of concrete, have been erected at Wat Tyler Country Park in Essex, not far from the coast, and at St. James’ Mount in Liverpool. Yet another set of mirrors, this one constructed from aluminum and parabolic in shape, is featured in the River Colme Sculpture Park. t

Stephen R. Wilk ( swilk@comcast.net) is an optical engineer in Lexington, Mass., U.S.A.

[ References and Resources ]

>> O. Gillie. “Listing of Sound Mirrors Urged,” The Independent, 3 July 1993: www.independent.co.uk/news/uk/ listing-of-sound-mirrors-urged-ollie-gillie-reports-on-the-preradar-detection-devices-that-enthusiasts-want-to-see- preserved-1482621.html.