“The World, according to the best geographers, is divided into Europe, Asia, Africa, America, and Romney Marsh.”

Sound Mirrors

 

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 Books about the Sound Mirrors

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 NB The Sound Mirrors at Greatstone will be open to the public on Saturday 15 July 2017. find out more

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The Sound Mirrors at Greatstone
The 200ft, 20ft and 30ft Sound Mirrors at Greatstone

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 in the north east side of the Dungeness Nature Reserve.   

How the Sound Mirrors Work

Sound mirrors worked using a curved surface to concentrate sound waves into a central point, which were picked up by a sound collector and later by microphones.

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.

 

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.


Listening Disc on Romney Marsh, 1924
Listening Disc on Romney Marsh, 1924 (ack.34)
 

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 (see picture right).

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

Listening Disc System on Romney Marsh, c1924
Listening Disc System on Romney Marsh, c1924 (ack.34)

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 2oft mirro, which was only able to give estiamtions.

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 favourable conditions could pick up the sounds of aircraft up to 24 miles away.


The Sound Mirrors at Greatstone Lakes
The Sound Mirrors at Greatstone Lakes (ack.6)

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.

For more information about the development of the Sound Mirrors in Kent, please see Sound Mirrors Timeline below and the map on the right.

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 (Number 462809). Please see National Monuments Record for more information.

Sound Mirrors Timeline

20 Foot Sound Mirror
20ft Sound Mirror

 
  • 1915 - Early War Office reference to the subject of listening devices for aeroplanes by means of acoustic arrangements
  • 1915 - Trials of a 16ft Sound Mirror dug into the chalk near Maidstone in Kent
  • 1916 - Wooden paraboloid sound collector 60cms diameter used in First World War by the French
  • 1917 - Larger mirror 3 metres in diameter tested with an aircraft
  • 1917/18 - 15 ft diameter Sound Mirrors dug into the chalk at Fan Bay (east of Dover) and Joss Gap saw operational use against enemy aircraft
  • 1922 - 20ft diameter concrete Sound Mirror built at West Hythe
  • 1925 - 20ft Hythe Sound Mirror trials20ft Sound Mirror at Abbots Cliff
  • 1927 - Proposal to erect three similar 20ft Sound Mirrors along the coast for use in 1928. In the event, only two were built, one at Greatstone and one at Abbots Cliff, between Folkestone and Dover
  • 1927 - Plans under consideration for the building of 200ft wide Sound Mirror at Greatstone
  • 1928 - 20ft Sound Mirrors completed at Greatstone and Abbots Cliff
  • 1928 - Spur line to the Sound Mirrors was built off the RH&DR, known at the time as the War Department Branch, which was used to transport men, equipment and building materials to the mirrors site.The branch line was close in 1951
  • 1929/1930 - 30ft Sound Mirrors built at Greatstone and West Hythe
  • 1930 - Proposals for more 30ft Sound Mirrors, to defend Portsmouth. No records have been found of these or any other 30ft mirrors
  • 1930 - 200ft Sound Mirror built and ready for use at Greatstone 200ft Sound Mirror at Greatstone
  • 1932/33/34 - Exercises undertaken to test the six concrete Sound Mirrors in Kent; the three at Greatstone, two at West Hythe and one at Abbots Cliff
  • 1933 - Two storey concrete listening room constructed at the 200ft Sound Mirror
  • 1935 - The first practical Radar system was produced,with trials carried out at Orfordness in Suffolk
  • 1935 - Because of serious problems with noise from the residential development of Greatstone and Lydd-on-Sea, consideration was given to abandoning the site at Greatstone with proposals to move to a new site at Dungeness
  • 1935 - Planned work to build Sound MIrrors in the Thames Estuary were abandoned in   view of the possible development of of alternative methods of detection ie Radar
  • 1936 - Experiments continue at the 200ft Sound Mirror at Greatstone
  • 1937 - Re-alignment of the Southern Railway branch line to New Romney caused problems at the Greatstone site
  • 1937 - 200ft Sound Mirror at Greatstone closed down
  • 1939 - Instructions of the General Staff to destroy the six Kent Sound Mirrors were never carried out. Five still exist, with the sixth, the 20ft Hythe mirror, was only lost  through natural causes.

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How to See the Mirrors

The mirrors are situated on an island at Greatstone Lakes in the Dungeness Nature Reserve. Access to them is via a swing bridge, which is normally closed. The site is managed by the RSPB, who arrange for open days each year, normally in July. 

However they can be seen quite clearly from the footpath which runs along the east side of the south lake and from the land side of the swing bridge.

To see the mirrors, travel to the coast end of Taylor Road in Greatstone (postcode TN29 9PD). Buses, routes 11 & 102, run from New Romney town and RH&DR station, with bus stops and car park in The Parade, just past Hull Road. From the Parade, walk westwards down Taylor Road to an entrance to the Nature Reserve and then follow the signs for The Mirrors.
By the side of this footpath there is a concrete plinth with information about the Sound Mirrors.

                                                    See footpath marked in red on the map on the right 

To see them really close up, however, you need to go on one of the open days or guided walks organised by the RSPB. 

For more information  about open days and guided walks, please contact the RSPB at Dungeness   email   Telephone Icon 01797 320588.


How to get to the Mirrors


Sound Mirrors at Greatstone
Listening at the Listening Ears - 30ft and 200ft Mirrors(Ack. 43)             



 Video of the Sound MIrrors

Working Model
There is a working model of a Sound Mirror by the Royal Military Canal, just south of Hythe; walk south from Hythe RH&DR station down either side of the canal until you came to the footbridge, where you will see the Sound Mirror model on the west bank. Here you can, with the help of a friend, try it out for yourself.  see here
From here you can also see the 30 foot mirror.

Two sound mirrors were also built on the side of a hill just west of Hythe. A 20 foot mirror was built first, then a 30 foot mirror added nearby. The 30 foot mirror is still standing and can be seen from the path alongside the Royal Military Canal. The remains of the 20 foot mirror lie nearby.