Denge acoustic mirrors

View of all three mirrors

Denge is a former Royal Air Force site near Dungeness, in Kent, England. It is best known for the early experimental acoustic mirrors which remain there.

The acoustic mirrors, known colloquially as 'listening ears', at Denge are located between the Greatstone-on-Sea and Lydd airfields, on the banks of a now disused gravel pit. The mirrors were built in the 1920s as an experimental early warning system for incoming aircraft, developed by Dr. William Sansome Tucker. Several were built along the south and east coasts, but the complex at Denge is the best preserved.

Denge complex

The acoustic mirrors at Denge. Left to right, the 200 foot, 20 foot and 30 foot mirrors.

There are three acoustic mirrors in the complex, each consisting of a single concrete hemispherical reflector.

• The 200 foot mirror is a near vertical, curved wall, 200 feet (60m) long. It is one of only two similar acoustic mirrors in the world, the other being in Maghtab, Malta.
• The 30 foot mirror is a circular dish, similar to a deeply curved satellite dish, 9 m (30 ft) across, supported on concrete buttresses. This mirror still retains the metal microphone pole at its centre.
• The 20 foot mirror is similar to the 30 foot mirror, with a smaller, shallower dish 6 m (20 ft) across. The design is close to that of an acoustic mirror in Kilnsea, East Riding of Yorkshire.

Acoustic mirrors could effectively be used to detect slow-moving enemy aircraft before they came into sight. They worked by concentrating sound waves towards a central point, where a microphone would have been located.

Abandonment

However, the acoustic mirrors' use was limited as aircraft became faster. Operators also found it difficult to distinguish between aircraft and seagoing vessels. In any case, they quickly became obsolete due to the invention of radar in 1932. The experiment was abandoned, and the mirrors left to decay. The gravel extraction works caused some undermining of at least one of the structures.

Restoration

The mirrors are in the process of being transferred from the aggregate quarry owner to become part of a nature reserve. In 2003, English Heritage secured £500,000 from the Aggregates Levy Sustainability Fund and from the EU's Interreg programme under the Historic Fortifications Network, as administered by Kent County Council. This money was spent to restore the damage caused by the gravel works, as well as to install a swing bridge which now is the only means of access, reducing the monument's exposure to vandalism. The site is now only accessible during guided tours.