An anti-aircraft acoustic-detection device. Two mirrors exist on the site. They are examples of a small number of surviving sound mirrors nationally including nearby Greatstone (TR 02 SE 12) and at Abbotscliff (TR 23 NE 32).

One mirror dates from 1917 and is twenty feet in diameter, the other being thirty feet in diameter and dates from the 1920s. They were intended to give early-warning of approaching enemy aircraft by detecting the sound of their engines at long distance and formed part of a chain of partly experimental sound mirrors at points along the Kentish coast. The earlier mirror lies at a shallower angle (two degrees) than the later dish and it appears it was abandoned in favour of the larger mirror which sits at an eight degree angle.

Both are supported at the back and at either side by buttresses of steel girders fixed into concrete ground pedestals and by two intervening webs of concrete, with metal supports and concrete bearers at the front of the bowl. Spalling shows the bowl to have a formwork of steel mesh into which a mortar of sand and cement was plastered. In the lower part of the front of the bowl is the cut-off pillar for the sound trumpet.

Sound location was achieved by collecting and focussing the sounds of an aircraft engine striking the concrete ‘mirror’ in a metal trumpet connected to a stethoscope worn by a listener who sat in a small chamber below the front of the bowl. By moving the trumpet and recording the angle of best reception of sound the direction of the target could be established. By matching this to an angle of sound taken from another of the coastal mirrors, it was possible to establish the position and height of the target aircraft and track it in flight. Fighter interceptors could then be ordered airborne. This system was tested with varying degrees of success during a number of air defence exercises in the early-mid 1930s but was abandoned with the establishment of RADAR stations in the later 1930s.

The mirrors have recently (2012) been excavated during work at the nearby Fan Bay Deep Shelter and are currently (2016) in the process of being conserved by the national Trust.

GIS depiction corrected following communication with John Guy. (1)

<1> John A. Guy, 2013-, Verbal communication from John Guy, defence expert working in the Dover area (Verbal communication). SKE24831.

<2> Dover Archaeological Group, 2015, Fan Hole deep Shelter and Sound Mirrors, St Margaret's at Cliffe, Dover. Watching Brief (Unpublished document). SKE31707.