Sonic and ultrasonic weapons are weapons of various types that use sound to injure, incapacitate, or kill an opponent. Some sonic weapons are in limited use or in research and development by military and police forces; some of these weapons have been described as sonic bullets, sonic grenades, sonic mines, or sonic cannons. Some make a focused beam of ultrasound. High-power sound waves can disrupt or destroy the eardrums of a target and cause severe pain or disorientation; this is sufficient to incapacitate a person. Less powerful sound waves can cause humans to experience discomfort; the use of these frequencies to incapacitate persons has occurred both in anti-citizen special operation and crowd control settings. The possibility of a device that produces frequency that causes vibration of the eyeballs—and therefore distortion of vision—was suggested by paranormal researcher Vic Tandy in the 1990s while attempting to demystify a "haunting" in his laboratory in Coventry; this "spook" was characterised by vague glimpses of a grey apparition.
Some detective work implicated a newly installed extractor fan that, Tandy found, was generating infrasound of 18.9 Hz, 0.3 Hz, 9 Hz. A long-range acoustic device has been used by the crew of the cruise ship Seabourn Spirit to deter pirates who chased and attacked the ship. More this device and others of similar design have been used to disperse protesters and rioters in crowd control efforts. A similar system is called a "magnetic acoustic device" "The mosquito".'Mobile' sonic devices have been used in the United Kingdom to deter teenagers from lingering around shops in target areas. The device works by emitting an ultra-high frequency blast that teenagers or people under 20 are susceptible to and find uncomfortable. Age-related hearing loss prevents the ultra-high pitch sound from causing a nuisance to those in their late twenties and above, though this is wholly dependent on a young person's past exposure to high sound pressure levels. High-amplitude sound of a specific pattern at a frequency close to the sensitivity peak of human hearing is used as a burglar deterrent.
Some police forces have used sound cannons against protesters, for example during the 2009 G20 Pittsburgh summit, the 2014 Ferguson unrest, the 2016 Dakota Access Pipeline protest in North Dakota, among others. It has been reported that "sonic attacks" may have taken place in Cuba in 2016 and 2017, leading to health problems, including hearing loss, in US and Canadian government employees at the US and Canadian embassies in Havana; some events as reported would seem to violate the laws of physics, it has been suggested that they are in fact an example of mass psychogenic illness. Cuban investigators have dismissed claims that sonic weapons have been used against American diplomats, describing them as "science fiction"; the US issued an alert to its employees in China after it learned of a possible "sonic attack" on a US citizen working there. The employee complained of symptoms from late 2017 through April 2018 returned to the US and was examined by a doctor who said the person had suffered a mild traumatic brain injury.
The US diplomat likened it to the incident. In early June, 2018 several employees were evacuated from their work in the U. S. Consulate offices in China and sent back to Pennsylvania for testing and diagnosis after sharing similar concerns. Studies have found that exposure to high intensity ultrasound at frequencies from 700 kHz to 3.6 MHz can cause lung and intestinal damage in mice. Heart rate patterns following vibroacoustic stimulation has resulted in serious negative consequences such as atrial flutter and bradycardia; the extra-aural bioeffects on various internal organs and the central nervous system included auditory shifts, vibrotactile sensitivity change, muscle contraction, cardiovascular function change, central nervous system effects, vestibular effects, chest wall/lung tissue effects. Researchers found that low frequency sonar exposure could result in significant cavitations and tissue shearing. No follow up experiments were recommended. Tests performed on mice show the threshold for both lung and liver damage occurs at about 184 dB.
Damage increases as intensity is increased. The American Institute of Ultrasound in Medicine has stated that there have been no proven biological effects associated with an unfocused sound beam with intensities below 100 mW/cm² SPTA or focused sound beams below an intensity level of 1 mW/cm² SPTA. Noise-induced neurologic disturbances in scuba divers exposed to continuous low frequency tones for durations longer than 15 minutes has involved in some cases the development of immediate and long-term problems affecting brain tissue; the symptoms resembled those of individuals. One theory for a causal mechanism is that the prolonged sound exposure resulted in enough mechanical strain to brain tissue to induce an encephalopathy. Divers and aquatic mammals may suffer lung and sinus injuries from high intensity, low frequency sound; this is due to the ease with which low frequency sound passes from water into a body, but not into any pockets of gas in the body, which reflect the sound due to mismatched acoustic impedance.
Brown note Sone Sound pressure Sound energy flux Sound power Sound intensity Infrasound Ultrasound LED Incapacitator The Hum Directional sound Parametric array Electronic harassment Ultrasonic welding Goodman, Steve. Sonic Warfare: Sound and the Ecology of Fear. MIT Press. ISBN 978-0-262-51795-9. ‘’USA Today’’ report
In meteorology, visibility is a measure of the distance at which an object or light can be discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads and aviation. Meteorological visibility refers to transparency of air: in dark, meteorological visibility is still the same as in daylight for the same air. ICAO Annex 3 Meteorological Service for International Air Navigation contains the following definitions and note: a) the greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background. Note.— The two distances have different values in air of a given extinction coefficient, the latter b) varies with the background illumination. The former a) is represented by the meteorological optical range. Annex 3 defines Runway Visual Range as: The range over which the pilot of an aircraft on the centre line of a runway can see the runway surface markings or the lights delineating the runway or identifying its centre line.
In clean air in Arctic or mountainous areas, the visibility can be up to 240 km where there are large markers such as mountains or high ridges. However, visibility is reduced somewhat by air pollution and high humidity. Various weather stations report this as mist. Fog and smoke can reduce visibility to near zero, making driving dangerous; the same can happen with forest fires. Heavy rain not only causes low visibility, but the inability to brake due to hydroplaning. Blizzards and ground blizzards are defined in part by low visibility. To define visibility the case of a black object being viewed against a white background is examined; the visual contrast, CV, at a distance x from the black object is defined as the relative difference between the light intensity of the background and the object C V = F B − F F B where FB and F are the intensities of the background and the object, respectively. Because the object is assumed to be black, it must absorb all of the light incident on it, thus when x=0, F = 0 and CV = 1.
Between the object and the observer, F is affected by additional light, scattered into the observer's line of sight and the absorption of light by gases and particles. Light scattered by particles outside of a particular beam may contribute to the irradiance at the target, a phenomenon known as multiple scattering. Unlike absorbed light, scattered light is not lost from a system. Rather, it can contribute to other directions, it is only lost from the original beam traveling in one particular direction. The multiple scattering's contribution to the irradiance at x is modified by the individual particle scattering coefficient, the number concentration of particles, the depth of the beam; the intensity change dF is the result of these effects over a distance dx. Because dx is a measure of the amount of suspended gases and particles, the fraction of F, diminished is assumed to be proportional to the distance, dx; the fractional reduction in F is d F = − b ext F d x. The scattering of background light into the observer's line of sight can increase F over the distance dx.
This increase is defined as b' FB dx. The overall change in intensity is expressed as d F = d x Since FB represents the background intensity, it is independent of x by definition. Therefore, d F B = 0 = d x It is clear from this expression that b' must be equal to bext. Thus, the visual contrast, CV, obeys the Beer–Lambert law d C V d x = − b ext C V which means that the contrast decreases exponentially with the distance from the object: C V = exp ( − b ext
Terrorism is, in the broadest sense, the use of intentionally indiscriminate violence as a means to create terror among masses of people. It is used in this regard to refer to violence during peacetime or in war against non-combatants; the terms "terrorist" and "terrorism" originated during the French Revolution of the late 18th century but gained mainstream popularity in the 1970s in news reports and books covering the conflicts in Northern Ireland, the Basque Country and Palestine. The increased use of suicide attacks from the 1980s onwards was typified by the September 11 attacks in New York City and Washington, D. C. in 2001. There are different definitions of terrorism. Terrorism is a charged term, it is used with the connotation of something, "morally wrong". Governments and non-state groups denounce opposing groups. Varied political organizations have been accused of using terrorism to achieve their objectives; these organizations include right-wing and left-wing political organizations, nationalist groups, religious groups and ruling governments.
Legislation declaring terrorism a crime has been adopted in many states. There is no consensus as to; the Global Terrorism Database, maintained by the University of Maryland, College Park, has recorded more than 61,000 incidents of non-state terrorism, resulting in at least 140,000 deaths between 2000 and 2014. Etmologically, the word terror is derived from the Latin verb Tersere, which becomes Terrere; the latter form appears in European languages as early as the 12th century. By 1356 the word terreur is in use. Terreur is the origin of the Middle English term terrour, which becomes the modern word "terror"; the term terroriste, meaning "terrorist", is first used in 1794 by the French philosopher François-Noël Babeuf, who denounces Maximilien Robespierre's Jacobin regime as a dictatorship. In the years leading up to the Reign of Terror, the Brunswick Manifesto threatened Paris with an "exemplary, never to be forgotten vengeance: the city would be subjected to military punishment and total destruction" if the royal family was harmed, but this only increased the Revolution's will to abolish the monarchy.
Some writers attitudes about French Revolution grew less favorable after the French monarchy was abolished in 1792. During the Reign of Terror, which began in July 1793 and lasted thirteen months, Paris was governed by the Committee of Public safety who oversaw a regime of mass executions and public purges. Prior to the French Revolution, ancient philosophers wrote about tyrannicide, as tyranny was seen as the greatest political threat to Greco-Roman civilization. Medieval philosophers were occupied with the concept of tyranny, though the analysis of some theologians like Thomas Aquinas drew a distinction between usurpers, who could be killed by anyone, legitimate rulers who abused their power – the latter, in Aquinas' view, could only be punished by a public authority. John of Salisbury was the first medieval Christian scholar. Most scholars today trace the origins of the modern tactic of terrorism to the Jewish Sicarii Zealots who attacked Romans and Jews in 1st century Palestine, they follow its development from the Persian Order of Assassins through to 19th-century anarchists.
The "Reign of Terror" is regarded as an issue of etymology. The term terrorism has been used to describe violence by non-state actors rather than government violence since the 19th-century Anarchist Movement. In December 1795, Edmund Burke used the word "Terrorists" in a description of the new French government called'Directory': At length, after a terrible struggle, the Troops prevailed over the Citizens To secure them further, they have a strong corps of irregulars, ready armed. Thousands of those Hell-hounds called Terrorists, whom they had shut up in Prison on their last Revolution, as the Satellites of Tyranny, are let loose on the people; the terms "terrorism" and "terrorist" gained renewed currency in the 1970s as a result of the Israeli–Palestinian conflict, the Northern Ireland conflict, the Basque conflict, the operations of groups such as the Red Army Faction. Leila Khaled was described as a terrorist in a 1970 number of Life magazine. A number of books on terrorism were published in the 1970s.
The topic came further to the fore after the 1983 Beirut barracks bombings and again after the 2001 September 11 attacks and the 2002 Bali bombings. There are over 109 different definitions of terrorism. American political philosopher Michael Walzer in 2002 wrote: "Terrorism is the deliberate killing of innocent people, at random, to spread fear through a whole population and force the hand of its political leaders". Bruce Hoffman, an American scholar, has noted that It is not only individual agencies within the same governmental apparatus that cannot agree on a single definition of terrorism. Experts and other long-established scholars in the field are incapable of reaching a consensus. C. A. J. Coady has written that the question of how to define terrorism is "irresolvable" because "its natural home is in polemical and propagandist contexts". French historian Sophie Wahnich distinguishes between the revolutionary terror of the French Revolution and the terrorists of the September 11 attacks: Revolutionary terror is not terrorism.
To make a moral equivalence between the Revolution's year II and September 2001 is historical and philosophical nonsense... The violence exercised on 11 September 2001 aimed neither at liberty. Nor did the preventive war announced by the president of the United States. Experts
Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, spacecraft, guided missiles, motor vehicles, weather formations, terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object. Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of small systems with sub-meter resolution; the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anticollision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring and flight control systems, guided missile target locating systems, ground-penetrating radar for geological observations, range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from high noise levels. Radar is a key technology that the self-driving systems are designed to use, along with sonar and other sensors. Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is "lidar". With the emergence of driverless vehicles, Radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes; the next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation; the German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter, he obtained a patent for his detection device in April 1904 and a patent for a related amendment for estimating the distance to the ship.
He got a British patent on September 23, 1904 for a full radar system, that he called a telemobiloscope. It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap, his system used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U. K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U. S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not continue the work. Eight years Lawrence A. Hyland at the Naval Research Laboratory observed similar fading effects from passing aircraft. Before the Second World War, researchers in the United Kingdom, Germany, Japan, the Netherlands, the Soviet Union, the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, New Zealand, South Africa followed prewar Great Britain's radar development, Hungary generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locatin
Sonar is a technique that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels. Two types of technology share the name "sonar": passive sonar is listening for the sound made by vessels. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may be used in air for robot navigation, SODAR is used for atmospheric investigations; the term sonar is used for the equipment used to generate and receive the sound. The acoustic frequencies used in sonar systems vary from low to high; the study of underwater sound is known as underwater hydroacoustics. The first recorded use of the technique was by Leonardo da Vinci in 1490 who used a tube inserted into the water to detect vessels by ear, it was developed during World War I to counter the growing threat of submarine warfare, with an operational passive sonar system in use by 1918.
Modern active sonar systems use an acoustic transponder to generate a sound wave, reflected back from target objects. Although some animals have used sound for communication and object detection for millions of years, use by humans in the water is recorded by Leonardo da Vinci in 1490: a tube inserted into the water was said to be used to detect vessels by placing an ear to the tube. In the late 19th century an underwater bell was used as an ancillary to lighthouses or light ships to provide warning of hazards; the use of sound to "echo-locate" underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912. The world's first patent for an underwater echo-ranging device was filed at the British Patent Office by English meteorologist Lewis Fry Richardson a month after the sinking of the Titanic, a German physicist Alexander Behm obtained a patent for an echo sounder in 1913; the Canadian engineer Reginald Fessenden, while working for the Submarine Signal Company in Boston, built an experimental system beginning in 1912, a system tested in Boston Harbor, in 1914 from the U.
S. Revenue Cutter Miami on the Grand Banks off Newfoundland. In that test, Fessenden echo ranging; the "Fessenden oscillator", operated at about 500 Hz frequency, was unable to determine the bearing of the iceberg due to the 3-metre wavelength and the small dimension of the transducer's radiating face. The ten Montreal-built British H-class submarines launched in 1915 were equipped with Fessenden oscillators. During World War I the need to detect; the British made early use of underwater listening devices called hydrophones, while the French physicist Paul Langevin, working with a Russian immigrant electrical engineer Constantin Chilowsky, worked on the development of active sound devices for detecting submarines in 1915. Although piezoelectric and magnetostrictive transducers superseded the electrostatic transducers they used, this work influenced future designs. Lightweight sound-sensitive plastic film and fibre optics have been used for hydrophones, while Terfenol-D and PMN have been developed for projectors.
In 1916, under the British Board of Invention and Research, Canadian physicist Robert William Boyle took on the active sound detection project with A. B. Wood, producing a prototype for testing in mid-1917; this work, for the Anti-Submarine Division of the British Naval Staff, was undertaken in utmost secrecy, used quartz piezoelectric crystals to produce the world's first practical underwater active sound detection apparatus. To maintain secrecy, no mention of sound experimentation or quartz was made – the word used to describe the early work was changed to "ASD"ics, the quartz material to "ASD"ivite: "ASD" for "Anti-Submarine Division", hence the British acronym ASDIC. In 1939, in response to a question from the Oxford English Dictionary, the Admiralty made up the story that it stood for "Allied Submarine Detection Investigation Committee", this is still believed, though no committee bearing this name has been found in the Admiralty archives. By 1918, Britain and France had built prototype active systems.
The British tested their ASDIC on HMS Antrim in 1920 and started production in 1922. The 6th Destroyer Flotilla had ASDIC-equipped vessels in 1923. An anti-submarine school HMS Osprey and a training flotilla of four vessels were established on Portland in 1924; the U. S. Sonar QB set arrived in 1931. By the outbreak of World War II, the Royal Navy had five sets for different surface ship classes, others for submarines, incorporated into a complete anti-submarine attack system; the effectiveness of early ASDIC was hampered by the use of the depth charge as an anti-submarine weapon. This required an attacking vessel to pass over a submerged contact before dropping charges over the stern, resulting in a loss of ASDIC contact in the moments leading up to attack; the hunter was firing blind, during which time a submarine commander could take evasive action. This situation was remedied by using several ships cooperating and by the adoption of "ahead-throwing weapons", such as Hedgehogs and Squids, which proj
Maritime Safety and Security Team
A Maritime Safety and Security Team or MSST is a United States Coast Guard anti-terrorism team established to protect local maritime assets. It is a harbor and inshore patrol and security team that includes detecting and if necessary stopping or arresting submerged divers, using the Underwater Port Security System, it is the only special operations group. MSSTs were created under the Maritime Transportation Security Act of 2002 in direct response to the terrorist attacks on 11 Sept. 2001, are a part of the United States Department of Homeland Security's layered strategy directed at protecting seaports and waterways. MSSTs provide waterborne and a modest level of shoreside antiterrorism force protection for strategic shipping, high interest vessels, critical infrastructure. MSSTs are a quick response force capable of rapid nationwide deployment via air, ground or sea transportation in response to changing threat conditions and evolving Maritime Homeland Security mission requirements. Multi-mission capability facilitates augmentation for other selected Coast Guard missions.
MSST personnel receive training in Advanced Tactical Boat Operations and Anti-terrorism Force protection at the Joint Maritime Training Center at Camp Lejeune, North Carolina. The MSSTs operate as part of the Coast Guard's Deployable Operations Group now called Deployable Specialized Forces, they are under operational control of the Coast Guard's corresponding Pacific and Atlantic Area commands as well as an active reserve component of The White House on request. Modeled after the Port Security Unit and Law Enforcement Detachment programs, MSSTs provide a complementary non-redundant capability designed to close critical security gaps in United States strategic seaports. MSSTs are staffed to support continuous law enforcement operations both ashore and afloat. In addition, MSSTs: Jointly staffed to maximize effectiveness executing Port and Coastal Security operations. Provide enhanced port safety and security and law enforcement capabilities to the economic or military significant port where they are based.
Deploy in support of National Special Security Events requiring Coast Guard presence, such as OpSail, Republican & Democratic National Conventions, major disasters or storm recovery operations. Prototype/employ specialized capabilities to enhance mission performance. Deploy on board cutters and other naval vessels for port safety and security, drug law enforcement, migrant interdiction, or other maritime homeland security mission requirements. Support Naval Coastal Warfare requirements during Homeland Defense and in accordance with long standing agreements with DOD and the Combatant Commanders Capabilities Maritime interdiction and law enforcement Anti-terrorism/Force Protection CBRN-E Detection Search and Rescue Port Protection/Anti-sabotage Underwater Port Security Canine Handling Teams Tactical Boat Operations Non-compliant boarding operations MSST 91101 -- Seattle MSST 91102 -- Chesapeake, Va. MSST 91103 -- Los Angeles/Long Beach MSST 91104 -- Houston/Galveston MSST 91105 -- San Francisco MSST 91106 -- Fort Wadsworth, Staten Island, NY MSST 91107 -- Honolulu, HI MSST 91108 -- St. Marys, Georgia MSST 91109 -- San Diego, CA MSST 91110 -- Boston, MA MSST 91111 -- Anchorage, Alaska MSST 91112 -- New Orleans MSST 91114 -- Miami, FL Each MSST has 65 active duty personnel.
Law Enforcement Detachments Port Security Unit Maritime Safety and Security Team 91104's web page Maritime Safety and Security Team 91114's web page ShadowSpear Special Operations: Maritime Security Response Team
A loudspeaker is an electroacoustic transducer. The most used type of speaker in the 2010s is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice; the dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move back and forth due to Faraday's law of induction, which causes a diaphragm attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound; the sound source must be amplified or strengthened with an audio power amplifier before the signal is sent to the speaker. Speakers are housed in a speaker enclosure or speaker cabinet, a rectangular or square box made of wood or sometimes plastic.
The enclosure's materials and design play an important role in the quality of the sound. Where high fidelity reproduction of sound is required, multiple loudspeaker transducers are mounted in the same enclosure, each reproducing a part of the audible frequency range. In this case the individual speakers are referred to as "drivers" and the entire unit is called a loudspeaker. Drivers made for reproducing high audio frequencies are called tweeters, those for middle frequencies are called mid-range drivers, those for low frequencies are called woofers. Smaller loudspeakers are found in devices such as radios, portable audio players and electronic musical instruments. Larger loudspeaker systems are used for music, sound reinforcement in theatres and concerts, in public address systems; the term "loudspeaker" may refer to individual transducers or to complete speaker systems consisting of an enclosure including one or more drivers. To adequately reproduce a wide range of frequencies with coverage, most loudspeaker systems employ more than one driver for higher sound pressure level or maximum accuracy.
Individual drivers are used to reproduce different frequency ranges. The drivers are named subwoofers; the terms for different speaker drivers differ, depending on the application. In two-way systems there is no mid-range driver, so the task of reproducing the mid-range sounds falls upon the woofer and tweeter. Home stereos use the designation "tweeter" for the high frequency driver, while professional concert systems may designate them as "HF" or "highs"; when multiple drivers are used in a system, a "filter network", called a crossover, separates the incoming signal into different frequency ranges and routes them to the appropriate driver. A loudspeaker system with n separate frequency bands is described as "n-way speakers": a two-way system will have a woofer and a tweeter. Loudspeaker driver of the type pictured are termed "dynamic" to distinguish them from earlier drivers, or speakers using piezoelectric or electrostatic systems, or any of several other sorts. Johann Philipp Reis installed an electric loudspeaker in his telephone in 1861.
Alexander Graham Bell patented his first electric loudspeaker as part of his telephone in 1876, followed in 1877 by an improved version from Ernst Siemens. During this time, Thomas Edison was issued a British patent for a system using compressed air as an amplifying mechanism for his early cylinder phonographs, but he settled for the familiar metal horn driven by a membrane attached to the stylus. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air. A few companies, including the Victor Talking Machine Company and Pathé, produced record players using compressed-air loudspeakers. However, these designs were limited by their poor sound quality and their inability to reproduce sound at low volume. Variants of the system were used for public address applications, more other variations have been used to test space-equipment resistance to the loud sound and vibration levels that the launching of rockets produces; the first experimental moving-coil loudspeaker was invented by Oliver Lodge in 1898.
The first practical moving-coil loudspeakers were manufactured by Danish engineer Peter L. Jensen and Edwin Pridham in 1915, in Napa, California. Like previous loudspeakers these used horns to amplify the sound produced by a small diaphragm. Jensen was denied patents. Being unsuccessful in selling their product to telephone companies, in 1915 they changed their target market to radios and public address systems, named their product Magnavox. Jensen was, for years after the invention of a part owner of The Magnavox Company; the moving-coil principle used today in speakers was patented in 1924 by Chester W. Rice and Edward W. Kellogg; the key difference between previous attempts and the patent by Rice and Kell