Sound mirrors

As no one correctly guess the purpose of the concrete defence structures, I going to tell you.

The cylindrical tower shown in the first photo of my earlier post is a Martello Tower .  Martellos are small defensive forts built in the early 19th century and there are quite a few on the south and east coast

However the other structures are more interesting and much rarer.   These are sound mirrors.

The sound mirrors were built in the late 1920s and early 1930s as an experimental early warning system of enemy aircraft.  Several were built on the south and east coast, but the complex at Denge is the best preserved.

As shown in the photos, there are three sound mirrors at Denge:  a 200ft curved wall, a 30ft circular dish with a metal microphone pole in its centre (which is seen in the photo), and a smaller shallower 20ft dish.

Sound mirrors were effective at detecting slow moving aircraft long before they came in sight.  The sound of aircraft engines was concentrated at the focus of the mirror where a microphone was located.

Sound mirrors became less useful as aircraft became faster and they finally became obsolete after radar was invented.  The experiment was abandoned prior the Second World War and the sound mirrors were left to decay.