Report - - Acoustic Mirrors - Denge, Kent 19-07-09 | Military Sites | 28DaysLater.co.uk

Report - Acoustic Mirrors - Denge, Kent 19-07-09

The Ancient Mariner

28DL Full Member
28DL Full Member
This was an arranged visit but I thought it relevant so decided to do a small report. Visited with sneaky fox and another friend.

The three concrete structures at Denge are the remains of part of an acoustic early warning system, designed to detect engine sounds from approaching aircraft. They are known as 'sound mirrors' and were constructed in the years 1928-1930.

The three mirrors:

They consist of three designs as follows:

20 ft Mirror:

The small mirror, a 20 foot concave dish, dates from 1928. A 'listener' stood on a platform in front of the mirror, controlling a moveable sound collector in the focal area. The collector was connected by tubing to a stethoscope worn by the listener, who was able to detect the loudest reflected sound as he manipulated the collector. Via scales on the equipment he obtained a bearing on the aircraft.

20ft Mirror:

20 ft Mirror:

Back of 20 and 30 ft Mirrors:

30 ft Mirror:

The 30 foot concave dish mirror with its increased surface area was completed in April 1930. The method of collecting the sound was similar to that as the 20 ft mirror, but in design the listener was protected from the elements in his enclosed chamber under the bowl. His stethoscope was connected to an overhead sound collector mounted on a vertical shaft passing through the roof and into the bowl, which he controlled by a wheel and foot pedals. By searching the focal area of the mirror the listener could read off the bearing of an approaching aircraft from horizontal and vertical scales mounted on the equipment.

Side of 30 ft Mirror:

Front of 30 ft Mirror:

Back of 30 ft Mirror:

Remains of telecoms, back of 30 ft Mirror:

200 ft Wall

The 200 ft mirror was completed in June 1930. Aircraft engines tended to emit long wave sounds and it was an advantage to design large detecting surfaces, culminating in the 200 foot mirror. It consists of a segment of a sphere curved in plan and elevation, measuring 200 by 26 feet. Multiple microphones connected to a central control room were placed behind a front wall concentric with the main mirror. The microphones were very directional, and as they responded to sounds reflected from particular areas of the the mirror it was possible for an operator in the control room to assess the bearing of an approaching aircraft.

200 ft Mirror:

Front of 200 ft Mirror:

200 ft Mirror:

View showing sphere construction of 200 ft Mirror:

With the invention of Radar the mirrors soon became a dead end technology. They stand now as protected ancient monuments, a reminder of a time between the wars when invasion was a real possibility.

'Mariner' at the mirrors:


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