An acoustic mirror is a passive device used to reflect and focus sound waves. Parabolic acoustic mirrors are 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 2 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. Many 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, Baħar iċ-Ċagħaq in Malta; the Maltese sound mirror is known locally as "the ear" 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 high by 70 m long, while the other two are dish-shaped constructions 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 spherical mirrors. Spherical mirrors can be used for direction finding by moving the sensor rather than the mirror. Acoustic mirrors had a limited effectiveness, the increasing speed of aircraft in the 1930s meant that they would 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. 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. 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, Baltimore's Maryland Science Center, Oklahoma City's Science Museum Oklahoma, San Francisco's Exploratorium, the Science Museum of Minnesota, the Museum of Science and Industry in Chicago, Pacific Science Center in Seattle, Jodrell Bank Observatory, Parkes Observatory in Australia. A pair of dishes 2 to 3 m in diameter, is installed facing each other, separated by around a hundred metres.
A person standing at the focus of one can hear another person speaking in a whisper at the focus of the other, despite the wide separation between them. Parabolic microphones depend on a parabolic dish to reflect sound coming from a specific direction into the microphone placed at the focus, they are directional: sensitive to sounds coming from a specific direction. However, they have poor bass response because a dish small enough to be portable cannot focus long wavelengths. Small portable parabolic microphones are used to record wildlife sounds such as birdsongs, in televised sports events to pick up the conversations of players, such as in the huddle during American Football games, or to record the sounds of the sport, in audio surveillance to record speech without the knowledge of the speaker. Acoustic aircraft detection mirrors are known to have been built at: Denge, Kent Abbot’s Cliff, Kent Boulby, Yorkshire Dover, Kent, at Fan Bay Hartlepool, Co. Durham, 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 B.
Zonkor C. Ta Karach D. Ta Zura E. Tal Merhla Joss Gap, Kent Kilnsea, Yorkshire Redcar, Yorkshire Romney Marsh, Kent – a series of horizontal discs Seaham, Co. Durham Selsey, Sussex – converted into a residence Sunderland, at Namey Hill Warden Point, Isle of Sheppey, Kent – the Warden Point mirror, sited on a cliff-top, fell onto the beach below ca 1978-9Modern acoustic mirrors built for entertainment Pennypot, Royal Military Canal Wat Tyler country park, nr. Pitsea, Essex – modern sculpture in the form of functional sonic mirrors The Brickyard – North Carolina State University campus Discovery Green in downtown Houston has a sculpture made of limestone called the Listening Vessels. Acoustic location Sound ranging, for the artillery use Parabolic microphone Richard Newton Scarth, Echoes from the Sky Acoustic mirrors in Britain Military acoustic locators White Cliffs Underground further details of
Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs are the bird sounds. In ornithology and birding, songs are distinguished by function from calls; the distinction between songs and calls is based upon complexity and context. Songs are longer and more complex and are associated with courtship and mating, while calls tend to serve such functions as alarms or keeping members of a flock in contact. Other authorities such as Howell and Webb make the distinction based on function, so that short vocalizations, such as those of pigeons, non-vocal sounds, such as the drumming of woodpeckers and the "winnowing" of snipes' wings in display flight, are considered songs. Still others require song to have syllabic diversity and temporal regularity akin to the repetitive and transformative patterns that define music, it is agreed upon in birding and ornithology which sounds are songs and which are calls, a good field guide will differentiate between the two. Bird song is best developed in the order Passeriformes.
Some groups are nearly voiceless, producing only percussive and rhythmic sounds, such as the storks, which clatter their bills. In some manakins, the males have evolved several mechanisms for mechanical sound production, including mechanisms for stridulation not unlike those found in some insects. Song is delivered from prominent perches, although some species may sing when flying; the production of sounds by mechanical means as opposed to the use of the syrinx has been termed variously instrumental music by Charles Darwin, mechanical sounds and more sonation. The term sonate has been defined as the act of producing non-vocal sounds that are intentionally modulated communicative signals, produced using non-syringeal structures such as the bill, tail and body feathers. In extratropical Eurasia and the Americas all song is produced by male birds; these differences have been known for a long time and are attributed to the much less regular and seasonal climate of Australian and African arid zones requiring that birds breed at any time when conditions are favourable, although they cannot breed in many years because food supply never increases above a minimal level.
With aseasonal irregular breeding, both sexes must be brought into breeding condition and vocalisation duetting, serves this purpose. The high frequency of female vocalisations in the tropics and Southern Africa may relate to low mortality rates producing much stronger pair-bonding and territoriality; the avian vocal organ is called the syrinx. The syrinx and sometimes a surrounding air sac resonate to sound waves that are made by membranes past which the bird forces air; the bird controls the pitch by changing the tension on the membranes and controls both pitch and volume by changing the force of exhalation. It can control the two sides of the trachea independently, how some species can produce two notes at once. One of the two main functions of bird song is mate attraction. Scientists hypothesize that bird song evolved through sexual selection, experiments suggest that the quality of bird song may be a good indicator of fitness. Experiments suggest that parasites and diseases may directly affect song characteristics such as song rate, which thereby act as reliable indicators of health.
The song repertoire appears to indicate fitness in some species. The ability of male birds to hold and advertise territories using song demonstrates their fitness. Therefore, a female bird may select males based on the quality of their songs and the size of their song repertoire; the second principal function of bird song is territory defense. Territorial birds will interact with each other using song to negotiate territory boundaries. Since song may be a reliable indicator of quality, individuals may be able to discern the quality of rivals and prevent an energetically costly fight. In birds with song repertoires, individuals may share the same song type and use these song types for more complex communication; some birds will respond to a shared song type with a song-type match. This may be an aggressive signal, however results are mixed. Birds may interact using repertoire-matches, wherein a bird responds with a song type, in its rival's repertoire but is not the song that it is singing; this may be a less aggressive act than song-type matching.
Song complexity is linked to male territorial defense, with more complex songs being perceived as a greater territorial threat. Communication through bird calls can be between individuals of the same species or across species. Birds communicate alarm through vocalizations and movements that are specific to the threat, bird alarms can be understood by other animal species, including other birds, in order to identify and protect against the specific threat. Mobbing calls are used to recruit individuals in an area where an owl or other predator may be present; these calls are characterized by wide-frequency spectra, sharp onset and termination, repetitiveness that are common across species and are believed to be helpful to other potential "mobbers" by being easy to locate. The alarm calls of most species, on the other hand, are characteristically high-pitched, making the caller difficult to locate. Individual birds may be sensitive enough to identify each other through their calls. Many birds t
Espionage or spying is the act of obtaining secret or confidential information without the permission of the holder of the information. Spies help agencies uncover secret information. Any individual or spy ring, in the service of a government, company or independent operation, can commit espionage; the practice is clandestine, as it is by definition unwelcome and in many cases illegal and punishable by law. Espionage is a method of intelligence gathering which includes information gathering from public sources. Espionage is part of an institutional effort by a government or commercial concern. However, the term tends to be associated with state spying on potential or actual enemies for military purposes. Spying involving corporations is known as industrial espionage. One of the most effective ways to gather data and information about the enemy is by infiltrating the enemy's ranks; this is the job of the spy. Spies can return information concerning the strength of enemy forces, they can find dissidents within the enemy's forces and influence them to defect.
In times of crisis, spies sabotage the enemy in various ways. Counterintelligence is the practice of thwarting enemy intelligence-gathering. All nations have strict laws concerning espionage and the penalty for being caught is severe. However, the benefits gained through espionage are so great that most governments and many large corporations make use of it. Information collection techniques used in the conduct of clandestine human intelligence include operational techniques, asset recruiting, tradecraft. Today, espionage agencies target terrorists as well as state actors. Since 2008, the United States has charged at least 57 defendants for attempting to spy for China. Intelligence services value certain intelligence collection techniques over others; the former Soviet Union, for example, preferred human sources over research in open sources, while the United States has tended to emphasize technological methods such as SIGINT and IMINT. In the Soviet Union, both political and military intelligence officers were judged by the number of agents they recruited.
Espionage agents are trained experts in a targeted field so they can differentiate mundane information from targets of value to their own organizational development. Correct identification of the target at its execution is the sole purpose of the espionage operation. Broad areas of espionage targeting expertise include: Natural resources: strategic production identification and assessment. Agents are found among bureaucrats who administer these resources in their own countries Popular sentiment towards domestic and foreign policies. Agents recruited from field journalistic crews, exchange postgraduate students and sociology researchers Strategic economic strengths. Agents recruited from science and technology academia, commercial enterprises, more from among military technologists Military capability intelligence. Agents are trained by military espionage education facilities, posted to an area of operation with covert identities to minimize prosecution Counterintelligence operations targeting opponents' intelligence services themselves, such as breaching confidentiality of communications, recruiting defectors or moles Although the news media may speak of "spy satellites" and the like, espionage is not a synonym for all intelligence-gathering disciplines.
It is a specific form of human source intelligence. Codebreaking, aircraft or satellite photography, research in open publications are all intelligence gathering disciplines, but none of them is considered espionage. Many HUMINT activities, such as prisoner interrogation, reports from military reconnaissance patrols and from diplomats, etc. are not considered espionage. Espionage is the disclosure of sensitive information to people who are not cleared for that information or access to that sensitive information. Unlike other forms of intelligence collection disciplines, espionage involves accessing the place where the desired information is stored or accessing the people who know the information and will divulge it through some kind of subterfuge. There are exceptions to physical meetings, such as the Oslo Report, or the insistence of Robert Hanssen in never meeting the people who bought his information; the US defines espionage towards itself as "The act of obtaining, transmitting, communicating, or receiving information about the national defense with an intent, or reason to believe, that the information may be used to the injury of the United States or to the advantage of any foreign nation".
Black's Law Dictionary defines espionage as: "... gathering, transmitting, or losing... information related to the national defense". Espionage is a violation of United States law, 18 U. S. C. §§ 792–798 and Article 106a of the Uniform Code of Military Justice". The United States, like most nations, conducts espionage against other nations, under the control of the National Clandestine Service. Britain's espionage activities are controlled by the Secret Intelligence Service. A spy is a person employed to seek out top secret information from a source. Within the United States Intelligence Community, "asset" is a more common usage. A case officer or Special Agent, who may have diplomatic status and directs the human collector. Cutouts are couriers who do not know the case officer but transfer messages. A
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is popularly called a dish antenna or parabolic dish; the main advantage of a parabolic antenna is. It functions to a searchlight or flashlight reflector to direct the radio waves in a narrow beam, or receive radio waves from one particular direction only. Parabolic antennas have some of the highest gains, meaning that they can produce the narrowest beamwidths, of any antenna type. In order to achieve narrow beamwidths, the parabolic reflector must be much larger than the wavelength of the radio waves used, so parabolic antennas are used in the high frequency part of the radio spectrum, at UHF and microwave frequencies, at which the wavelengths are small enough that conveniently-sized reflectors can be used. Parabolic antennas are used as high-gain antennas for point-to-point communications, in applications such as microwave relay links that carry telephone and television signals between nearby cities, wireless WAN/LAN links for data communications, satellite communications and spacecraft communication antennas.
They are used in radio telescopes. The other large use of parabolic antennas is for radar antennas, in which there is a need to transmit a narrow beam of radio waves to locate objects like ships and guided missiles, for weather detection. With the advent of home satellite television receivers, parabolic antennas have become a common feature of the landscapes of modern countries; the parabolic antenna was invented by German physicist Heinrich Hertz during his discovery of radio waves in 1887. He used cylindrical parabolic reflectors with spark-excited dipole antennas at their focus for both transmitting and receiving during his historic experiments; the operating principle of a parabolic antenna is that a point source of radio waves at the focal point in front of a paraboloidal reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point.
A typical parabolic antenna consists of a metal parabolic reflector with a small feed antenna suspended in front of the reflector at its focus, pointed back toward the reflector. The reflector is a metallic surface formed into a paraboloid of revolution and truncated in a circular rim that forms the diameter of the antenna. In a transmitting antenna, radio frequency current from a transmitter is supplied through a transmission line cable to the feed antenna, which converts it into radio waves; the radio waves are emitted back toward the dish by the feed antenna and reflect off the dish into a parallel beam. In a receiving antenna the incoming radio waves bounce off the dish and are focused to a point at the feed antenna, which converts them to electric currents which travel through a transmission line to the radio receiver; the reflector can be of sheet metal, metal screen, or wire grill construction, it can be either a circular "dish" or various other shapes to create different beam shapes.
A metal screen reflects radio waves as well as a solid metal surface as long as the holes are smaller than one-tenth of a wavelength, so screen reflectors are used to reduce weight and wind loads on the dish. To achieve the maximum gain, it is necessary that the shape of the dish be accurate within a small fraction of a wavelength, to ensure the waves from different parts of the antenna arrive at the focus in phase. Large dishes require a supporting truss structure behind them to provide the required stiffness. A reflector made of a grill of parallel wires or bars oriented in one direction acts as a polarizing filter as well as a reflector, it only reflects linearly polarized radio waves, with the electric field parallel to the grill elements. This type is used in radar antennas. Combined with a linearly polarized feed horn, it helps filter out noise in the receiver and reduces false returns. Since a shiny metal parabolic reflector can focus the sun's rays, most dishes could concentrate enough solar energy on the feed structure to overheat it if they happened to be pointed at the sun, solid reflectors are always given a coat of flat paint.
The feed antenna at the reflector's focus is a low-gain type such as a half-wave dipole or more a small horn antenna called a feed horn. In more complex designs, such as the Cassegrain and Gregorian, a secondary reflector is used to direct the energy into the parabolic reflector from a feed antenna located away from the primary focal point; the feed antenna is connected to the associated radio-frequency transmitting or receiving equipment by means of a coaxial cable transmission line or waveguide. At the microwave frequencies used in many parabolic antennas, waveguide is required to conduct the microwaves between the feed antenna and transmitter or receiver; because of the high cost of waveguide runs, in many parabolic antennas the RF front end electronics of the receiver is located at the feed antenna, the received signal is converted to a lower intermediate frequency so it can be conducted to the receiver through cheaper coaxial cable. This is called a low-noise block downconverter. In transmitting dishes, the microwave transmitter may be located at the feed point.
An advantage of parabolic antennas is that most of the structure of the antenna is nonresonant, so it can function over a wide range of frequencies, a wide bandwidth. All, necessary to change the frequency of operation is to replace the feed antenna with one that works at the new frequen
Eavesdropping is the act of secretly or stealthily listening to the private conversation or communications of others without their consent. The practice is regarded as unethical, in many jurisdictions is illegal; the verb eavesdrop is a back-formation from the noun eavesdropper, formed from the related noun eavesdrop. An eavesdropper was someone who would hang from the eave of a building so as to hear what is said within; the PBS documentaries, Inside the Court of Henry VIII and Secrets of Henry VIII’s Palace include segments that display and discuss "eavedrops", carved wooden figures Henry VIII had built into the eaves of Hampton Court to discourage unwanted gossip or dissension from the King's wishes and rule, to foment paranoia and fear, demonstrate that everything said there was being overheard. Eavesdropping vectors include telephone lines, cellular networks and other methods of private instant messaging. VoIP communications software is vulnerable to electronic eavesdropping via infections such as trojans.
Network eavesdropping is a network layer attack that focuses on capturing small packets from the network transmitted by other computers and reading the data content in search of any type of information. This type of network attack is one of the most effective as a lack of encryption services are used, it is linked to the collection of metadata. Those who perform this type of attack are black-hat hackers; the dictionary definition of eavesdropping at Wiktionary Media related to Eavesdropping at Wikimedia Commons
Sound recording and reproduction
Sound recording and reproduction is an electrical, electronic, or digital inscription and re-creation of sound waves, such as spoken voice, instrumental music, or sound effects. The two main classes of sound recording technology are analog digital recording. Acoustic analog recording is achieved by a microphone diaphragm that senses changes in atmospheric pressure caused by acoustic sound waves and records them as a mechanical representation of the sound waves on a medium such as a phonograph record. In magnetic tape recording, the sound waves vibrate the microphone diaphragm and are converted into a varying electric current, converted to a varying magnetic field by an electromagnet, which makes a representation of the sound as magnetized areas on a plastic tape with a magnetic coating on it. Analog sound reproduction is the reverse process, with a bigger loudspeaker diaphragm causing changes to atmospheric pressure to form acoustic sound waves. Digital recording and reproduction converts the analog sound signal picked up by the microphone to a digital form by the process of sampling.
This lets the audio data be transmitted by a wider variety of media. Digital recording stores audio as a series of binary numbers representing samples of the amplitude of the audio signal at equal time intervals, at a sample rate high enough to convey all sounds capable of being heard. A digital audio signal must be reconverted to analog form during playback before it is amplified and connected to a loudspeaker to produce sound. Prior to the development of sound recording, there were mechanical systems, such as wind-up music boxes and player pianos, for encoding and reproducing instrumental music. Long before sound was first recorded, music was recorded—first by written music notation also by mechanical devices. Automatic music reproduction traces back as far as the 9th century, when the Banū Mūsā brothers invented the earliest known mechanical musical instrument, in this case, a hydropowered organ that played interchangeable cylinders. According to Charles B. Fowler, this "...cylinder with raised pins on the surface remained the basic device to produce and reproduce music mechanically until the second half of the nineteenth century."
The Banū Mūsā brothers invented an automatic flute player, which appears to have been the first programmable machine. Carvings in the Rosslyn Chapel from the 1560s may represent an early attempt to record the Chladni patterns produced by sound in stone representations, although this theory has not been conclusively proved. In the 14th century, a mechanical bell-ringer controlled by a rotating cylinder was introduced in Flanders. Similar designs appeared in barrel organs, musical clocks, barrel pianos, music boxes. A music box is an automatic musical instrument that produces sounds by the use of a set of pins placed on a revolving cylinder or disc so as to pluck the tuned teeth of a steel comb; the fairground organ, developed in 1892, used a system of accordion-folded punched cardboard books. The player piano, first demonstrated in 1876, used a punched paper scroll that could store a long piece of music; the most sophisticated of the piano rolls were hand-played, meaning that the roll represented the actual performance of an individual, not just a transcription of the sheet music.
This technology to record a live performance onto a piano roll was not developed until 1904. Piano rolls were in continuous mass production from 1896 to 2008. A 1908 U. S. Supreme Court copyright case noted that, in 1902 alone, there were between 70,000 and 75,000 player pianos manufactured, between 1,000,000 and 1,500,000 piano rolls produced; the first device that could record actual sounds as they passed through the air was the phonautograph, patented in 1857 by Parisian inventor Édouard-Léon Scott de Martinville. The earliest known recordings of the human voice are phonautograph recordings, called phonautograms, made in 1857, they consist of sheets of paper with sound-wave-modulated white lines created by a vibrating stylus that cut through a coating of soot as the paper was passed under it. An 1860 phonautogram of Au Clair de la Lune, a French folk song, was played back as sound for the first time in 2008 by scanning it and using software to convert the undulating line, which graphically encoded the sound, into a corresponding digital audio file.
On April 30, 1877, French poet, humorous writer and inventor Charles Cros submitted a sealed envelope containing a letter to the Academy of Sciences in Paris explaining his proposed method, called the paleophone. Though no trace of a working paleophone was found, Cros is remembered as the earliest inventor of a sound recording and reproduction machine; the first practical sound recording and reproduction device was the mechanical phonograph cylinder, invented by Thomas Edison in 1877 and patented in 1878. The invention soon spread across the globe and over the next two decades the commercial recording and sale of sound recordings became a growing new international industry, with the most popular titles selling millions of units by the early 1900s; the development of mass-production techniques enabled cylinder recordings to become a major new consumer item in industrial countries and the cylinder was the main consumer format from the late 1880s until around 1910. The next major technical development was the invention of the gramophone record credited to Emile Berliner and patented in 1887, though others had demonstrated simi