This ‘bowl’ mirror dates from 1929. It is a survivor of three sound mirrors constructed just below the crest of ‘The Roughs’, an area of rising ground to landward of Hythe Ranges. It was 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 from Greatstone to Dover. 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. Then 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. Situated near the bowl mirror were an earlier 20 ft. diameter slab mirror (1922) and an experimental horizontal disc mirror (1931). There is a residue of the slab mirror, which collapsed over on its face in recent years, but no trace is evident of the disc mirror. A now vanished timber hut erected as a control room was an early feature of the site.A concrete building a short distance SSE of the bowl mirror has been described elsewhere as a replacement control room. Sound location by fixed detectors ceased with the establishment of RADAR stations in the later 1930s.