Download Mirrors (acoustic)

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.

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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.[1]

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 Baar i-agaq in Malta. The Maltese sound mirror is known locally as "the ear" (il-Widna).

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.

Parabolic acoustic mirrors called "whisper dishes" are used as participatory exhibits in science museums to demonstrate focusing of sound. Examples are located at Bristol's We The Curious, Ontario Science Centre, Albuquerque's Explora!, Baltimore's Maryland Science Center, Oklahoma City's Science Museum Oklahoma, San Francisco's Exploratorium,[4] the Science Museum of Minnesota, the Museum of Science and Industry in Chicago, Pacific Science Center in Seattle, Jodrell Bank Observatory, St. Louis Science Center, Parkes Observatory in Australia and on the north campus lawn of North Carolina State University.

The sound mirrors at Denge in Kent have become quite famous, but it is less well known that a number of other mirrors existed, built to a range of different designs. These webpages bring together photographs of the surviving mirrors, and some details of where they are if you are interested in visiting them.

Sensory traps pose a considerable and often fatal risk for animals, leading them to misinterpret their environment. Bats predominantly rely on their echolocation system to forage, orientate, and navigate. We found that bats can mistake smooth, vertical surfaces as clear flight paths, repeatedly colliding with them, likely as a result of their acoustic mirror properties. The probability of collision is influenced by the number of echolocation calls and by the amount of time spent in front of the surface. The echolocation call analysis corroborates that bats perceive smooth, vertical surfaces as open flyways. Reporting on occurrences with different species in the wild, we argue that it is necessary to more closely monitor potentially dangerous locations with acoustic mirror properties (such as glass fronts) to assess the true frequency of fatalities around these sensory traps.

These vast concrete dishes, which can be found along the northern and easterly British coastline, are sound mirrors. Originally designed to capture the sounds of incoming enemy aircraft as they approached the United Kingdom from across the English Channel and the North Sea (although one was also built at Baar i-agaq in Malta), these military listening devices acted as a rudimentary early warning system in the decades before Radar was developed and deployed.

Conceived by William Sansome Tucker, and operated at differing scales between around 1915 and 1935, the acoustic mirrors were able to signal an aircraft from up to 24 kilometers (15 miles) away, allowing for enough time to allow British defence to prepare for counterattack. The concave structures responded to sound by focusing the waves to a single point, whereupon a microphone would be positioned. Not only were the structures able to announce the arrival of an aircraft, but they could also determine the incoming direction of attack of the plane to an accuracy of 1.5 degrees. With the development of faster aircraft in the 1930s, these sound mirrors became obsolete.

The objective of this paper is to show that time-reversalinvariance can be exploited in acoustics to accurately controlwave propagation through random propagating media as well asthrough waveguides or reverberant cavities. To illustrate theseconcepts, several experiments are presented. They show that,contrary to long-held beliefs, multiple scattering in randommedia and multi-pathing in waveguides and cavities enhancesspatial resolution in time-reversal acoustics by making theeffective size of time-reversal mirrors much larger than theirphysical size.

Applications of time-reversal mirrors in pulse-echo detection arethen described and, for a multi-target medium, the iterativetime-reversal mode is presented. It will be shown how the studyof the time-reversal operator allows us to select and to focus oneach target through a distorting medium or inside a reverberantmedium.

This apparatus (image 1) consists of a pair of brass parabolic reflectors mounted on wooden stands. One of the reflectors has a bracket with a hook for hanging a pocket watch. The demonstrator would use this apparatus to show how the sound of a ticking watch may be heard at a considerable distance having first been made into a beam, projected over a distance and refocused. Very little is known about this pair of mirrors in the Whipple's collection. It is thought that they were made in France or Germany during the second quarter of the 19th century.

Sound waves can, however, be reflected as a focused beam, just like light waves. When a sound wave encounters a plane hard surface at a given angle, it will be reflected at the same angle. But when the surface is curved different parts of the wave may be reflected in different directions. Parabolic mirrors exploit this fact in order to focus oncoming wave fronts into a single point of high energy intensity - the focus, F (image 2). They may also be used to do the opposite: if a wave source is held at the focus of a parabolic mirror, then that mirror will project the sound as a series of parallel wave fronts (imagine reversing the arrowheads in image 2), which may be collected and refocused by another parabolic mirror.

These mirrors could also be used to demonstrate the wave nature of radiant heat, as shown in image 3. A hot coal is held at the focus of one mirror (A) while a flammable substance is held at the other (B). Infrared radiation emitted by the coal is concentrated into a beam, transmitted and then focused on to the target causing it to burn.

Today, parabolic mirrors find many uses, from collecting satellite TV signals to creating light beams in car headlights, and even as massive radio telescopes peering into the deepest regions of the universe.

During the first world war, and until the invention of radar, huge parabolic sound mirrors were used as early warning detectors to warn of approaching enemy aircraft - many of these still exist along the British coast (image 4).

Mirrors is a sound installation that follows the acoustic journey of the Minke Whale as it travels from Antarctica to Namibia. A collaboration between HIFMB based artist-researcher Geraint Rhys and Marine Acoustic scientist Dr Ilse van Opzeeland it charts how the vocalizations of the Minke changes with a change in location as it migrates across the oceans.

Lasting 5 minutes, Mirrors combines real acoustic data collected by the Ocean Acoustic Group at the Alfred Wegener Institute for Polar and Marine Research, with a stroyline narrative that emphasises the importance of sound in the marine world.

Another topic covered very briefly was coastal defences which included a short film sequence showing a concrete sound or acoustic mirror (given as being in the area of Spurn Point) with the commentary saying that it was in use during WW1. I know that sound detection was used for ranging purposes on the Western Front, but had always understood that the static, concrete type shown to be an inter-war, mainly experimental, development that was rendered obsolete by the increasing speed of aircraft making their detection too late to be of any real benefit, and the invention of radar.

The programme is correct. Tucker microphones were being installed in cliffs etc from 1917 but did not become really effective until 1918 when they could be coupled with newly developed amplifiers using vacuum tubes I posted a link to a Royal Society article on the use of optical mirrors for tracking air craft and AA fire only yesterday. This included a section on sound mirrors

Thanks for that Centurion; an interesting link. I have since found the English Heritage listing for the one that was shown (Click) which confirms the date of construction and also gives that some of those on the Kent coast also dated back to WW1, but those at Denge to the 1920s; my understanding is that Denge was a research site, so possibly the one there that resembled that shown in the programme was built to the same design in the early days as a starting point for the project there. Strangely, although the listing gives that all the remaining acoustic mirror sites 'are considered to be of national importance' the site at Denge (near Greatstone, Hythe), which has the largest surviving example in the UK (an impressive 200ft version), appears to be unlisted . I thought a copy of the Echoes from the sky. A story of acoustic defence (published by the Hythe Civic Society in 1999 & given in the references of the link) might be interesting, but unfortunately it's out of print with silly money (100+) being asked for used copies. ff782bc1db