Sound Mirrors
Sound mirrors, sometimes called listening ears, were acoustic detection devices, designed to provide early warning of enemy aircraft. Mirrors were upright concave bowls, (like satellite dishes in shape) between 3m and 4m in diameter and often made of concrete.
© Historic England
The aim of a sound mirror was to detect incoming enemy aircraft, especially Zeppelin airships, as they crossed the coast to raid the United Kingdom. Often the Zeppelins came by night and were almost impossible to see; a sound mirror had the potential to give an early warning of up to fifteen minutes.
Sound mirrors worked by detecting the sound and direction of the the approaching aircraft. The sound waves concentrated into a central focal point, were picked up be listening apparatus facing the curved surface. An operator using a stethoscope or microphone would be stationed on a platform at the front of the sound mirror
The sound mirrors were emplaced, as part of a chain of mirrors, on the Southeast and East coasts of the UK. They continued to be developed after the First World War and into the 1920’s. As aircraft became faster so their usefulness became less and as radar was developed, they became obsolete; the idea eventually being abandoned in 1935.
20 foot Sound Mirror, Fan Bay © CITiZAN
Our friends over at Coastal and Intertidal Zone Archaeological Network (CITiZAN) have recently recorded remaining sound mirrors on the South Coast. Lara Band, CITiZAN Archaeologist for Training, describes CITiZAN’s work and findings in this blog post from 12/02/2016.
“Over the last year the CITiZAN South East team have been able to visit the sites of three of these fascinating sonic remains.
During the First World War an experimental station was established at Joss Gap, Kent. It was operated by the Signals Experimental Establishment who had the remit to develop sound mirrors and other sound detection devices. There were originally two sound mirrors here: a rotating mirror 20 feet in diameter, mounted on a wooden gantry, and a 15 foot mirror cut in the chalk cliff. Experimental work continued at Joss Gap until 1922 when the research centre was moved to an area near Hythe. The 15 foot mirror was decommissioned in 1936, the 20 foot mirror in 1939.
Locating the remains of these mirrors proved elusive. Armed with a 1927 aerial photograph from the fantastic Britain from Above archive and with clues and images from various secondary sources we set off to hunt for remains of the mirrors. Eventually we spotted a group of bricks and timber, c 25cm x 25cm, set into the cliff face approximately 70 metres south of the concrete boardwalk that connects Joss Bay with Kingsgate Bay to the north. Comparison with a contemporary photograph suggests that this is all that remains of the gantry that supported the more southerly of the two mirrors. Of the other mirror, nothing is left. The Britain from Above photograph and historic Google Earth satellite imagery, accessed via the time slide feature, show the gradual erosion of the cliff, which has resulted in the total loss of the site.
Fan Bay Sound Mirror © CITiZAN
The two mirrors built at Fan Bay are now part of the National Trust property the White Cliffs of Dover. A mirror 15 feet in diameter was cut into the cliff face by 1917 and saw action the same year, detecting an enemy raid at a range of 12-15 miles. The range of this mirror and of those at Joss Gap intersected at the French coast thus providing cover for the south Kent coast and the English Channel. A second mirror, 20 feet in diameter, was added by 1920, the extra size giving it a greater range. As with the mirrors at Joss Gap, the 15 foot mirror was decommissioned in 1936 and the 20 foot mirror in 1939. Both were buried by the local council in the 1970s. In 2014 they were excavated and restored by the National Trust and can be visited along with the adjacent Second World War Fan Bay Deep Shelter when the National Trust re-opens the site for the season in March this year.
At Warden Point on the Isle of Sheppey are the remains of a 15 foot sound mirror, built some time between 1917 and 1923. Although fragmented, the dish of the mirror can still clearly be seen. The fragmentation also renders visible elements of the mirror’s construction: areas of the dish are covered with a coating of fine concrete c 50mm thick, presumably to provide a better reflective surface. In other areas it’s still possible to see the grain of the wood of the forms used to cast the mirror. This mirror was also decommissioned in 1936. It fell from the cliff top in the late 1970s and now lies on the beach close to remains of the Chain Home Low radar station that replaced it.”
Be sure to check out the CITiZAN project and read the full blog post HERE
Over to you!
We would like you to record passive air raid defences, such as Sound Mirrors. This will help build up a picture of the air raid early warning system that was in place during the First World War.
Watch a video about Locating Zeppelins by Sound during the First World War HERE
