Acoustic mirror

4.5 metre high WW1 concrete acoustic mirror near Kilnsea Grange, East Yorkshire, UK. The pipe which held the 'collector head' (microphone) can be seen in front of the structure

An acoustic mirror is a passive device used to reflect and perhaps to focus (concentrate) sound waves.

Overview

Prior to 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 airborne invasions. 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.

Acoustic mirrors at Denge

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 high by 70 m long, while the other two are dish-shaped constructions approximately 4–5 m 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.^[1] Spherical mirrors may be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory; see also^[2]

Acoustic mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to deal with 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 pin point the position of an enemy in the sky. The system they developed for linking the ranging 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.

Acoustic lenses similar to acoustic mirrors are used today as novelty items — "whisper dishes" — in science museums to allow patrons to whisper across long distances, for example at Ontario Science Centre, San Francisco's Exploratorium,^[3] and Parkes Observatory in Australia. Such whisper dishes must be parabolic for greatest effect.

Parabolic microphones appear to use acoustic mirror properties but instead depend on a parabolic dish to reflect sound coming from a specific direction into the microphone placed at the focus. Because of their small, portable size, they can easily be used in the same manner as acoustic mirrors for detection and direction finding of distant noise sources.

Locations

Acoustic mirrors are known to have been built at:-

• Denge
• Abbot’s Cliff Kent at OS grid reference TR27083867
• Boulby Yorkshire
• Dover Kent, at Fan Bay OS grid reference TR352428.
• Hartlepool in the Clavering area
• Hythe, Kent
• Malta Five sound mirrors were planned for Malta, serialled alphabetically, but only the Magħtab wall is known to have been built:
  • A. Magħtab (colloquially Il Widna - The Ear)
  • B. Zonkor
  • C. Ta Karach
  • D. Ta Zura
  • E. Tal Merhla
• Joss Gap, Kent
• Kilnsea
• The Brickyard (NC State) North Carolina State University campus
• Redcar
• Romney Marsh - a series of horizontal discs.
• Seaham
• Selsey - converted into a residence.
• Sunderland, at Namey Hill (OS grid reference NZ38945960^[4])
• Warden Point, Isle of Sheppey - The Warden Point mirror, sited on a cliff-top, fell onto the beach below ca 1978-9.
• Pennypot - Royal Military Canal - A civilian installation for entertainment.
• Wat Tyler country park - Modern sculpture in the form of functional sonic mirrors.

See also

• Acoustic location
• Japanese war tuba
• Sound ranging
• Parabolic microphone

References

1. ^ DrPhysics.com. "Sound Mirrors". http://www.drphysics.com/sound_mirror/sound_mirrors.html. Retrieved 2010-06-08. 
2. ^ The Arecibo Observatory - Past, Present and Future
3. ^ Exploratorium Listening Vessels exhibit
4. ^ Sunderland City Council. "World War 1 early warning acoustic mirror on Namey Hill". http://www.sunderland.gov.uk/index.aspx?articleid=3242. Retrieved 2011-01-14. 

• Richard Newton Scarth, Echoes from the Sky: A Story of Acoustic Defence (Hythe Civic Society, 1999) (ISBN 1-900101-30-0) (As of January 2007^[update], this book is out of print)

External links

Wikimedia Commons has media related to: Acoustic mirrors

• Acoustic mirrors in Britain
• Military acoustic locators
• White Cliffs Underground further details of variety of East Kent defences
• Visiting information for UK (and Maltese) sound mirrors