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

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, 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

Denge

Denge is a former Royal Air Force site near Dungeness, in Kent, England. It is best known for the early experimental acoustic mirrors which remain there....

 on the Dungeness peninsula and at Hythe

Hythe, Kent

Hythe , is a small coastal market town on the edge of Romney Marsh, in the District of Shepway on the south coast of Kent. The word Hythe or Hithe is an Old English word meaning Haven or Landing Place....

 in Kent

Kent

Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...

. Other examples exist in other parts of Britain (including Sunderland, Redcar

Redcar

Redcar is a seaside resort in the north east of England, and a major town in the unitary authority of Redcar and Cleveland in the ceremonial county of North Yorkshire. It lies east-northeast of Middlesbrough by the North Sea coast...

, Boulby, Kilnsea

Kilnsea

Kilnsea is a hamlet in the East Riding of Yorkshire, England, in an area known as Holderness. It is situated approximately south of the village of Easington, on the north bank of the Humber Estuary.It forms part of the civil parish of Easington....

) and Selsey Bill

Selsey Bill

Selsey Bill is a headland into the English Channel on the south coast of England in the county of West Sussex.The southern most town in Sussex is Selsey which is at the end of the Selsey Peninsula and Selsey Bill is situated on the towns southerncoastline...

, and Baħar iċ-Ċagħaq in Malta

Malta

Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...

. 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

Concrete

Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 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. Microphone

Microphone

A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

s placed at the foci

Focus (geometry)

In geometry, the foci are a pair of special points with reference to which any of a variety of curves is constructed. For example, foci can be used in defining conic sections, the four types of which are the circle, ellipse, parabola, and hyperbola...

 of the reflectors enabled a listener to detect the sound of aircraft far out over the English Channel

English Channel

The English Channel , often referred to simply as the Channel, is an arm of the Atlantic Ocean that separates southern England from northern France, and joins the North Sea to the Atlantic. It is about long and varies in width from at its widest to in the Strait of Dover...

. The reflectors are not parabolic, but are actually spherical mirrors.
Spherical mirrors may be used for direction finding by moving the sensor rather than the mirror; another unusual example is the Arecibo Observatory

Arecibo Observatory

The Arecibo Observatory is a radio telescope near the city of Arecibo in Puerto Rico. It is operated by SRI International under cooperative agreement with the National Science Foundation...

; see also

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

William Sansome Tucker

Major William Sansome Tucker, DSc, OBE was a British pioneer in acoustical research. He was born in Kidderminster, the son of William Tucker, an artist painter, and his wife Anna....

, 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

Ontario Science Centre

Ontario Science Centre is a science museum in Toronto, Ontario, Canada, near the Don Valley Parkway about northeast of downtown on Don Mills Road just south of Eglinton Avenue East...

, San Francisco's Exploratorium

Exploratorium

The Exploratorium is a museum in San Francisco with over 475 participatory exhibits, all of them made onsite, that mix science and art. It also aims to promote museums as informal education centers....

, and Parkes Observatory

Parkes Observatory

The Parkes Observatory is a radio telescope observatory, 20 kilometres north of the town of Parkes, New South Wales, Australia. It was one of several radio antennas used to receive live, televised images of the Apollo 11 moon landing on 20 July 1969....

 in Australia. Such whisper dishes must be parabolic for greatest effect.

Parabolic microphone

Parabolic microphone

A parabolic microphone is a microphone that uses a parabolic reflector to collect and focus sound waves onto a receiver, in much the same way that a parabolic antenna does with radio waves...

s 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
  Denge
  Denge is a former Royal Air Force site near Dungeness, in Kent, England. It is best known for the early experimental acoustic mirrors which remain there....
  
  
• Abbot’s Cliff Kent at OS grid reference TR27083867
• Boulby Yorkshire
• Dover
  Dover
  Dover is a town and major ferry port in the home county of Kent, in South East England. It faces France across the narrowest part of the English Channel, and lies south-east of Canterbury; east of Kent's administrative capital Maidstone; and north-east along the coastline from Dungeness and Hastings...
  
   Kent, at Fan Bay OS grid reference TR352428.
• Hartlepool
  Hartlepool
  Hartlepool is a town and port in North East England.It was founded in the 7th century AD, around the Northumbrian monastery of Hartlepool Abbey. The village grew during the Middle Ages and developed a harbour which served as the official port of the County Palatine of Durham. A railway link from...
  
   in the Clavering area
• Hythe, Kent
  Hythe, Kent
  Hythe , is a small coastal market town on the edge of Romney Marsh, in the District of Shepway on the south coast of Kent. The word Hythe or Hithe is an Old English word meaning Haven or Landing Place....
  
  
• Malta
  Malta
  Malta , officially known as the Republic of Malta , is a Southern European country consisting of an archipelago situated in the centre of the Mediterranean, south of Sicily, east of Tunisia and north of Libya, with Gibraltar to the west and Alexandria to the east.Malta covers just over in...
  
   Five sound mirrors were planned for Malta, serialled alphabetically, but only the Magħtab
  Magħtab
  Maghtab is a village in Malta. Maghtab is mostly known for Malta's largest landfill. Since Malta joined the E.U it has been closed down.-Zones in Maghtab:*Bajjad*Bingjala*Ġebel San Pietru*Ġjovadu*Ghallis*Ghallis Island*Ħatja il-Bajda*Il-Widna*Il-Mejjieli...
  
   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
  Kilnsea
  Kilnsea is a hamlet in the East Riding of Yorkshire, England, in an area known as Holderness. It is situated approximately south of the village of Easington, on the north bank of the Humber Estuary.It forms part of the civil parish of Easington....
  
  
• The Brickyard (NC State)
  The Brickyard (NC State)
  University Plaza, or more commonly The Brickyard, is a public plaza at the heart of North Carolina State University's North Campus in Raleigh, North Carolina. It is the university's most active court and has been the site of numerous special events....
  
   North Carolina State University campus
• Redcar
  Redcar
  Redcar is a seaside resort in the north east of England, and a major town in the unitary authority of Redcar and Cleveland in the ceremonial county of North Yorkshire. It lies east-northeast of Middlesbrough by the North Sea coast...
  
  
• Romney Marsh
  Romney Marsh
  Romney Marsh is a sparsely populated wetland area in the counties of Kent and East Sussex in the south-east of England. It covers about 100 mi ² .-Quotations:*“As Egypt was the gift of the Nile, this level tract .....
  
   - a series of horizontal discs.
• Seaham
  Seaham
  Seaham, formerly Seaham Harbour, is a small town in County Durham, situated south of Sunderland and east of Durham. It has a small parish church, St Mary the Virgin, with a late 7th century Anglo Saxon nave resembling the church at Escomb in many respects. St Mary the Virgin is regarded as one of...
  
  
• Selsey
  Selsey
  Selsey is a seaside town and civil parish, about seven miles south of Chichester, in the Chichester District of West Sussex, England. Selsey lies at the southernmost point of the Manhood Peninsula, almost cut off from mainland Sussex by the sea...
  
   - converted into a residence.
• Sunderland, at Namey Hill (OS grid reference NZ38945960)
• 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
  Royal Military Canal
  The Royal Military Canal is a canal running for 28 miles between Seabrook near Folkestone and Cliff End near Hastings, following the old cliff line bordering Romney Marsh.-Construction:...
  
   - A civilian installation for entertainment.
• Wat Tyler country park - Modern sculpture in the form of functional sonic mirrors.

See also

• Acoustic location
  Acoustic location
  Acoustic location is the science of using sound to determine the distance and direction of something. Location can be done actively or passively, and can take place in gases , liquids , and in solids .* Active acoustic location involves the creation of sound in order to produce an echo, which is...
  
  
• Japanese war tuba
  Japanese War Tuba
  The Japanese war tuba is a colloquial name sometimes applied to Imperial Japanese Army acoustic locators due to the visual resemblance to the musical tuba...
  
  
• Sound ranging
  Sound ranging
  In land warfare, sound ranging is a method of determining the coordinates of a hostile artillery battery using data derived from the sound of its guns firing...
  
  
• Parabolic microphone
  Parabolic microphone
  A parabolic microphone is a microphone that uses a parabolic reflector to collect and focus sound waves onto a receiver, in much the same way that a parabolic antenna does with radio waves...
  
  

External links

• 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