Monument record TR 37 SE 1488 - First World War Sound Mirror, Hackendown Point

A First World War sound mirror site is visible as an earthwork on RAF/26J/BR194 VB 31 10-JAN-1941. Only the platform is visible. A military acoustic station was established at Joss Bay by 1918 in order to listen for hostile German Airforce aircraft approaching on a route from the continent to Britain.

An experimental, moveable, sound mirror was installed on the cliff top. A fixed mirror was cut into the chalk cliff face at the rear of the platform and faced with concrete; it was possibly the first of this type to have provided an early warning function in the First World War. The Joss Bay and Fan Bay (Dover) sound mirrors were part of the Dover Anti-Aircraft Defence system and were used successfully during bombing raids on London in Spring 1918.

A unique disc sound locator was also trialled successfully at Joss Bay in 1918-19. The moveable, vertically-mounted structure was positioned in front of the cliff face on a staging above the beach and used a hot wire microphone to detect the sounds transmitted through the disc structure. The disc was camouflaged to mimic the cliff face. A control hut was situated nearby.

Experiments in direction-finding by sound collection and transmission continued at Joss Bay after the First World War. Despite proposals to move the acoustic station further south onto North Foreland, the Joss Bay installations were abandoned in mid-August 1922 when apparatus was transferred to Hythe.

The platform was recorded as a demolished structure (lost to coastal erosion) on Next Perspectives APGB Imagery 07-APR-2021. It was mapped as part of the Thanet Landscape Mapping Project. (1-3)

The site of a sound mirror located at Joss Gap (TR 398703). Sound mirrors were designed to provide early warning of approaching enemy aircraft by long-distance listening for the sound of their engine. The sound mirror at Joss Gap and its partner mirror at Fan Bay were installed between 1917-1918. The pair of mirrors covered the Kent coast and the North Sea, intersecting at the French coast. The mirrors were 15ft (4.6m) diameter shallow, concave discs that were positioned top and bottom on swivels supported by gantries set into the cliff face. Listening was by means of a central microphone.The mirror at Joss Gap was used continuously during the latter half of the First World War. (4)

The sound mirror site has been located exactly at TR3981170403 from aerial photos. (5)

<1> Historic England Archive, 1941-1960, Historic England Archive RAF vertical aerial photographs, RAF/26J/BR194 VB 31 10-JAN-1941 (Archive). SKE57101.

<2> Next Perspectives, 2003-2021, Next Perspectives APGB orthophotography, Next Perspectives APGB Imagery 07-APR-2021 (Archive). SKE57110.

<3> Richard N. Scarth, 1999, Echoes from the Sky (Bibliographic reference). SKE58482.

<4> Dobinson, C, 2000, Twentieth century fortifications in England, volume 7. Acoustics and radar: England's early warning systems 1915-1945 (Bibliographic reference). SWX23709.

<5> Untitled Source (Photograph). SKE59192.