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Sound Mirrors

            An acoustic mirror is a passive device used to reflect and focus (concentrate) sound waves. Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as "whisper galleries" are displayed in science museums to demonstrate sound focusing.

Between the World Wars, before the invention of radar, parabolic sound mirrors were used experimentally as early-warning devices by military air defence forces to detect incoming enemy aircraft by listening for the sound of their engines.

During World War II on the coast of southern England, a network of large concrete acoustic mirrors was in the process of being built when the project was cancelled owing to the development of the Chain Home radar system. Some of these mirrors are still standing today

Before World War II and the invention of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of detecting incoming enemy aircraft by the sound of their engines. The most famous of these devices still stand at Denge on the Dungeness peninsula and at Hythe in Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby, Kilnsea and Selsey Bill), and Baħar iċ-Ċagħaq in Malta. The Maltese sound mirror is known locally as "the ear" (il-Widna) and appears to be the only sound mirror built outside Great Britain.

The Dungeness mirrors, known colloquially as the "listening ears", consist of three large concrete reflectors built in the 1920s–1930s. Their experimental nature can be discerned by the different shapes of each of the three reflectors: one is a long curved wall about 5 m (16 ft) high by 70 m (230 ft) long, while the other two are dish-shaped constructions approximately 4–5 m (13–16 ft) in diameter. Microphones placed at the foci of the reflectors enabled a listener to detect the sound of aircraft far out over the English Channel. The reflectors are not parabolic, but are actually spherical mirrors.[2] Spherical mirrors can be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory.[3]

Acoustic mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to engage by the time they had been detected. The development of radar put an end to further experimentation with the technique. Nevertheless, there were long-lasting benefits. The acoustic mirror programme, led by Dr William Sansome Tucker, had given Britain the methodology to use interconnected stations to pinpoint the position of an enemy in the sky.

The system they developed for linking the stations and plotting aircraft movements was given to the early radar team and contributed to their success in World War II; although the British radar was less sophisticated than the German system, the British system was used more successfully.

Taken in: UK / England / Shepway District / Greatstone (show map)

Taken on: May 3, 2020

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april may june 2020

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Camera: Nikon D200, f/14.0, 1/25 sec, 26mm (=39mm), ISO 100 (more info) (hide info)

Camera:	Nikon D200
Aperture:	f/14.0
Exposure Time:	0.04 sec (1/25)
Focal Length:	26mm (35mm equivalent: 39mm)
ISO:	100
Exposure Bias:	0 EV
Flash:	No Flash
Exposure Program:	Aperture-priority AE
Exposure Mode:	Auto
Metering Mode:	Multi-segment
White Balance:	Manual
Light Source:	Cloudy
Contrast:	High
Saturation:	High
Sharpness:	Hard
Color Space:	sRGB
Software:	Adobe Photoshop Elements 7.0 Windows
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