Denge Sound Mirrors

Added 1606965627 by Photographer Gino Cinganelli.

The Sound Mirrors, also known as Acoustic Mirrors, Concrete Dishes, or Listening Ears, are large concrete structures designed as an early warning system for Britain to detect enemy aircraft.

These worked by focusing the sound from the plane’s engine so it could be heard before it was visible. There were three designs of mirrors, 20ft, 30ft, and 200ft, and all three can be seen in Greatstone, located at Greatstone Lakes on the northeast side of the Dungeness Nature Reserve.

How the Sound Mirrors Work

The Sound Mirrors worked by using their curved surface to concentrate sound waves by capturing the noise of incoming enemy aircraft approaching from the European mainland and focus it onto a microphone or a human listener equipped with then state-of-the-art stethoscopes. .

Once the receivers were adjusted for best reception, the results would be compared with those from one of the other ‘ears’ and used to calculate an aircraft’s height, speed, and flight path. They worked best at a range of between eight and 24 miles

An operator using a stethoscope would be stationed near the sound mirror, and would need specialist training in identifying different sounds.

Distinguishing the complexity of sound was so difficult that the operators could only listen for around 40 minutes.

Listening at a Sound Mirror

Early Days

Experiments starting in WW1 took place with four types of sound detectors. They were listening wells, discs, mirrors and trumpets.

The story of acoustic early warning systems began in 1914 at the start of WW1 when attempts were made to detect the position of the enemy's artillery guns, barrages from which often preceded attacks on the trenches.

The use of microphones was a significant step forward in 1916 and when placed first in holes and then deep shafts in the ground, picked up sounds hitherto unheard

These became known as listening wells but they were somewhat primitive, with men confined to the bottom of deep shafts and were replaced by horizontal discs and microphones at ground level

The key point about the change to discs was that the disc replaced the well and the microphone, placed at the centre of the disc, replaced the observer.

The low lying Romney Marsh was well suited to these discs with them being installed in 1924 in two long lines about three miles apart (see picture right).

Sound detectors shaped liked trumpets were also trialled and used during WW1. Following trials of various types of sound locator, a simple locator with four trumpets became universally used by the army.

But research was also taking place about the use of large concrete mirrors with an experimental one being built at Hythe in 1922, with the first one (20 feet) being built at Greatstone in 1928. By this date, the effectiveness of the disc system was being questioned leading to the abandonment of the discs in 1932.

NB You can find out more about these early days from Richard N Scarth's book Echoes from the Sky: A Story of Acoustic Defence

Sound Mirrors at Greatstone

The three concrete " listening ears" at Greatstone range in size from 20 to 200 feet in size. Built between 1928-30, the sound mirrors were part of Britain's national defence strategy. They were designed to pick up the sound of approaching enemy aircraft. Sound waves were caught in the belly of the mirror and relayed back through microphones and a stethoscope to an operator who raised the alarm. Anti-aircraft defences were then deployed. The mirrors effectively gave Britain a fifteen-minute warning of an impending attack.

The 20 feet(diameter) mirror was the first to be built in 1928. It was precast as one huge slab of curved concrete. Lessons were quickly learnt and a 30 feet(diameter) mirror, set at a different angle and providing greater accuracy, was built in early 1930 alongside the 20 foot mirror.

The 30ft mirror gave no advantage as to range but it was able to track an aircraft a lot more accurately than the 20ft mirror, which was only able to give estimations.

The third mirror is 200 feet in length and 26 feet high, and was built in 1930 alongside the other two smaller sound mirrors. Microphones were attached to the curved surfaces and in favorable conditions could pick up the sounds of aircraft up to 24 miles away.

The mirrors did work, and 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 the microphone would have been located. (note the microphone in the picture of the 30 foot mirror below). However, their 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 1935, being finally abandoned in 1939.

The sound mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to deal with by the time they had been detected. The development of radar put an end to further experimentation with the technique.

Nevertheless, there were long-lasting benefits. The sound mirror programs, led by Dr William Sansome Tucker, had given Britain the methodology to use interconnected stations to pinpoint the position of an enemy in the sky. The system they developed for linking the stations and plotting aircraft movements was given to the early radar team and contributed to their success in World War II; although the British radar was less sophisticated than the German system, the British system was used more successfully.

A spur line to the Greatstone Mirrors was built off the RH&DR in 1928, known at the time as the War Department Branch, which was used to transport men, equipment and building materials to the mirrors site.

The sound mirrors are now a scheduled ancient monument .