The Sound Mirrors: A History of the Future of War Noises is a July 2008 article by Ithamar Silver from Le Panoptique.

Although “not part of architectural history proper,” a series of moss and graffiti covered ruins along England’s southeastern coast belies one of the more grandly misguided displays of national insecurity to be produced by the tumult of the twentieth century. The remains are as imposing and impenetrable as any fortress—yet these were not traditional fortifications meant to withstand an enemy onslaught, a fact that renders their solidity largely palliative.

They were, essentially, ears.

(where it says one in Boulby that had somehow been transformed into a private residence, it should presumably read “Selsey”)

Meanwhile, Peter Frost has sent this photo of the Abbot’s Cliff sound mirror, which he came across whilst walking towards Dover from Capel-le-Ferne in Kent.