In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between those layers. Turbulence is observed in everyday phenomena such as surf, fast flowing rivers, billowing storm clouds, or smoke from a chimney, most fluid flows occurring in nature or created in engineering applications are turbulent. Turbulence is caused by excessive kinetic energy in parts of a fluid flow, which overcomes the damping effect of the fluid's viscosity. For this reason turbulence is realized in low viscosity fluids. In general terms, in turbulent flow, unsteady vortices appear of many sizes which interact with each other drag due to friction effects increases; this increases the energy needed to pump fluid through a pipe. Turbulence can be exploited, for example, by devices such as aerodynamic spoilers on aircraft that "spoil" the laminar flow to increase drag and reduce lift.
The onset of turbulence can be predicted by the dimensionless Reynolds number, the ratio of kinetic energy to viscous damping in a fluid flow. However, turbulence has long resisted detailed physical analysis, the interactions within turbulence create a complex phenomenon. Richard Feynman has described turbulence as the most important unsolved problem in classical physics. Smoke rising from a cigarette. For the first few centimeters, the smoke is laminar; the smoke plume becomes turbulent as its Reynolds number increases with increases in flow velocity and characteristic lengthscale. Flow over a golf ball. If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable to unfavorable, creating a large region of low pressure behind the ball that creates high form drag. To prevent this, the surface is dimpled to promote turbulence; this results in higher skin friction, but it moves the point of boundary layer separation further along, resulting in lower drag.
Clear-air turbulence experienced during airplane flight, as well as poor astronomical seeing. Most of the terrestrial atmospheric circulation; the oceanic and atmospheric mixed intense oceanic currents. The flow conditions in many industrial equipment and machines; the external flow over all kinds of vehicles such as cars, airplanes and submarines. The motions of matter in stellar atmospheres. A jet exhausting from a nozzle into a quiescent fluid; as the flow emerges into this external fluid, shear layers originating at the lips of the nozzle are created. These layers separate the fast moving jet from the external fluid, at a certain critical Reynolds number they become unstable and break down to turbulence. Biologically generated. Snow fences work by inducing turbulence in the wind, forcing it to drop much of its snow load near the fence. Bridge supports in water. In the late summer and fall, when river flow is slow, water flows smoothly around the support legs. In the spring, when the flow is faster, a higher Reynolds number is associated with the flow.
The flow may start off laminar but is separated from the leg and becomes turbulent. In many geophysical flows, the flow turbulence is dominated by the coherent structures and turbulent events. A turbulent event is a series of turbulent fluctuations that contain more energy than the average flow turbulence; the turbulent events are associated with coherent flow structures such as eddies and turbulent bursting, they play a critical role in terms of sediment scour and transport in rivers as well as contaminant mixing and dispersion in rivers and estuaries, in the atmosphere. In the medical field of cardiology, a stethoscope is used to detect heart sounds and bruits, which are due to turbulent blood flow. In normal individuals, heart sounds are a product of turbulent flow as heart valves close. However, in some conditions turbulent flow can be audible due to other reasons, some of them pathological. For example, in advanced atherosclerosis, bruits can be heard in some vessels that have been narrowed by the disease process.
Turbulence in porous media became a debated subject. Turbulence is characterized by the following features: Irregularity Turbulent flows are always irregular. For this reason, turbulence problems are treated statistically rather than deterministically. Turbulent flow is chaotic. However, not all chaotic flows are turbulent. Diffusivity The available supply of energy in turbulent flows tends to accelerate the homogenization of fluid mixtures; the characteristic, responsible for the enhanced mixing and increased rates of mass and energy transports in a flow is called "diffusivity". Turbulent diffusion is described by a turbulent diffusion coefficient; this turbulent diffusion coefficient is defined in a phenomenological sense, by analogy with the molecular diffusivities, but it does not have a true physical meaning, being dependent on the flow conditions, not a property of the fluid itself. In addition, the turbulent diffusivity concept assumes a con
Europe is a continent located in the Northern Hemisphere and in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west and the Mediterranean Sea to the south, it comprises the westernmost part of Eurasia. Since around 1850, Europe is most considered to be separated from Asia by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas and the waterways of the Turkish Straits. Although the term "continent" implies physical geography, the land border is somewhat arbitrary and has been redefined several times since its first conception in classical antiquity; the division of Eurasia into two continents reflects East-West cultural and ethnic differences which vary on a spectrum rather than with a sharp dividing line. The geographic border does not follow political boundaries, with Turkey and Kazakhstan being transcontinental countries. A strict application of the Caucasus Mountains boundary places two comparatively small countries and Georgia, in both continents.
Europe covers 2 % of the Earth's surface. Politically, Europe is divided into about fifty sovereign states of which the Russian Federation is the largest and most populous, spanning 39% of the continent and comprising 15% of its population. Europe had a total population of about 741 million as of 2016; the European climate is affected by warm Atlantic currents that temper winters and summers on much of the continent at latitudes along which the climate in Asia and North America is severe. Further from the sea, seasonal differences are more noticeable than close to the coast. Europe, in particular ancient Greece, was the birthplace of Western civilization; the fall of the Western Roman Empire in 476 AD and the subsequent Migration Period marked the end of ancient history and the beginning of the Middle Ages. Renaissance humanism, exploration and science led to the modern era. Since the Age of Discovery started by Portugal and Spain, Europe played a predominant role in global affairs. Between the 16th and 20th centuries, European powers controlled at various times the Americas all of Africa and Oceania and the majority of Asia.
The Age of Enlightenment, the subsequent French Revolution and the Napoleonic Wars shaped the continent culturally and economically from the end of the 17th century until the first half of the 19th century. The Industrial Revolution, which began in Great Britain at the end of the 18th century, gave rise to radical economic and social change in Western Europe and the wider world. Both world wars took place for the most part in Europe, contributing to a decline in Western European dominance in world affairs by the mid-20th century as the Soviet Union and the United States took prominence. During the Cold War, Europe was divided along the Iron Curtain between NATO in the West and the Warsaw Pact in the East, until the revolutions of 1989 and fall of the Berlin Wall. In 1949 the Council of Europe was founded, following a speech by Sir Winston Churchill, with the idea of unifying Europe to achieve common goals, it includes all European states except for Belarus and Vatican City. Further European integration by some states led to the formation of the European Union, a separate political entity that lies between a confederation and a federation.
The EU originated in Western Europe but has been expanding eastward since the fall of the Soviet Union in 1991. The currency of most countries of the European Union, the euro, is the most used among Europeans. In classical Greek mythology, Europa was a Phoenician princess; the word Europe is derived from her name. The name contains the elements εὐρύς, "wide, broad" and ὤψ "eye, countenance", hence their composite Eurṓpē would mean "wide-gazing" or "broad of aspect". Broad has been an epithet of Earth herself in the reconstructed Proto-Indo-European religion and the poetry devoted to it. There have been attempts to connect Eurṓpē to a Semitic term for "west", this being either Akkadian erebu meaning "to go down, set" or Phoenician'ereb "evening, west", at the origin of Arabic Maghreb and Hebrew ma'arav. Michael A. Barry, professor in Princeton University's Near Eastern Studies Department, finds the mention of the word Ereb on an Assyrian stele with the meaning of "night, sunset", in opposition to Asu " sunrise", i.e. Asia.
The same naming motive according to "cartographic convention" appears in Greek Ἀνατολή. Martin Litchfield West stated that "phonologically, the match between Europa's name and any form of the Semitic word is poor." Next to these hypotheses there is a Proto-Indo-European root *h1regʷos, meaning "darkness", which produced Greek Erebus. Most major world languages use words derived from Europa to refer to the continent. Chinese, for example, uses the word Ōuzhōu. In some Turkic languages the Persian name Frangistan is used casually in referring to much of Europe, besides official names such as Avrupa or Evropa; the prevalent definition of Europe as a geographical term has been in use since the mid-19th century. Europe is taken to be bounded by large bodies of water
A diacritic – diacritical mark, diacritical point, diacritical sign, or accent – is a glyph added to a letter, or basic glyph. The term derives from the Ancient Greek διακριτικός, from διακρίνω. Diacritic is an adjective, though sometimes used as a noun, whereas diacritical is only an adjective; some diacritical marks, such as the acute and grave, are called accents. Diacritical marks may appear above or below a letter, or in some other position such as within the letter or between two letters; the main use of diacritical marks in the Latin script is to change the sound-values of the letters to which they are added. Examples are the diaereses in the borrowed French words naïve and Noël, which show that the vowel with the diaeresis mark is pronounced separately from the preceding vowel. In other Latin-script alphabets, they may distinguish between homonyms, such as the French là versus la that are both pronounced /la/. In Gaelic type, a dot over a consonant indicates lenition of the consonant in question.
In other alphabetic systems, diacritical marks may perform other functions. Vowel pointing systems, namely the Arabic harakat and the Hebrew niqqud systems, indicate vowels that are not conveyed by the basic alphabet; the Indic virama and the Arabic sukūn mark the absence of vowels. Cantillation marks indicate prosody. Other uses include the Early Cyrillic titlo stroke and the Hebrew gershayim, which mark abbreviations or acronyms, Greek diacritical marks, which showed that letters of the alphabet were being used as numerals. In the Hanyu Pinyin official romanization system for Chinese, diacritics are used to mark the tones of the syllables in which the marked vowels occur. In orthography and collation, a letter modified by a diacritic may be treated either as a new, distinct letter or as a letter–diacritic combination; this varies from language to language, may vary from case to case within a language. English is the only major modern European language requiring no diacritics for native words.
In some cases, letters are used as "in-line diacritics", with the same function as ancillary glyphs, in that they modify the sound of the letter preceding them, as in the case of the "h" in the English pronunciation of "sh" and "th". Among the types of diacritic used in alphabets based on the Latin script are: accents ◌́ – acute ◌̀ – grave ◌̂ – circumflex ◌̌ – caron, wedge ◌̋ – double acute ◌̏ – double grave ◌̃ - tilde dots ◌̇ – overdot ◌̣ – an underdot is used in Rheinische Dokumenta and in Hebrew and Arabic transcription ◌·◌ – interpunct tittle, the superscript dot of the modern lowercase Latin i and j ◌̈ – diaeresis or umlaut ◌ː – triangular colon, used in the IPA to mark long vowels. Curves ◌̆ – breve ◌̑ - inverted breve ◌͗ – sicilicus, a palaeographic diacritic similar to a caron or breve ◌̃ – tilde ◌҃ – titlo vertical stroke ◌̩ – syllabic a subscript vertical stroke is used in IPA to mark syllabicity and in Rheinische Dokumenta to mark a schwa macron or horizontal line ◌̄ – macron ◌̱ – underbar overlays ◌⃓ – vertical bar through the character ◌̷ – slash through the character ◌̵ – crossbar through the character ring ◌̊ – overring superscript curls ◌̓ – apostrophe ◌̉ – hoi ◌̛ – horn subscript curls ◌̦ – undercomma ◌̧ – cedilla ◌̡ ◌̢ – hook, left or right, sometimes superscript ◌̨ – ogonek double marks ◌͝◌ – double breve ◌͡◌ – tie bar or top ligature ◌᷍◌ – double circumflex ◌͞◌ – longum ◌͠◌ – double tilde double sub/superscript diacritics ◌̧ ̧ - double cedilla ◌̨ ̨ - double ogonek ◌̈ ̈ - double diaeresisThe tilde, comma, apostrophe and colon are sometimes diacritical marks, but have other uses.
Not all diacritics occur adjacent to the letter. In the Wali language of Ghana, for example, an apostrophe indicates a change of vowel quality, but occurs at the beginning of the word, as in the dialects ’Bulengee and ’Dolimi; because of vowel harmony, all vowels in a word are affected, so the scope of the diacritic is the entire word. In abugida scripts, like those used to write Hindi and Thai, diacritics indicate vowels, may occur above, before, after, or around the consonant letter they modify; the tittle on the letter i of the Latin alphabet originated as a diacritic to distinguish i from the minims of adjacent letters. It first appeared in the 11th century in the sequence ii spread to i adjacent to m, n, u, to all lowercase i's; the j a variant of i, inherited the tittle. The shape of the diacritic developed from resembling today's acute accent to a long flourish by the 15th century. With the advent of Roman type it was reduced to the round. Hamza: indicates a glottal stop. Tanwīn symbols: Serve a grammatical role in Arabic.
The sign ـً is most written in combination with alif, e.g. ـًا. Shadda: Gemination of consonants. Waṣla: Comes most at the beginning of a word. Indicates a type of hamza, pronounced only when the letter is read at the beginnin
Western Asia, West Asia, Southwestern Asia or Southwest Asia is the westernmost subregion of Asia. The concept is in limited use, as it overlaps with the Middle East, the main difference being the exclusion of the majority of Egypt and the inclusion of the Caucasus; the term is sometimes used for the purposes of grouping countries in statistics. The total population of Western Asia is an estimated 300 million as of 2015. Although the term "Western Asia" is used as a convenient division of contemporary sovereign states into a manageable number of world regions for statistical purposes, it is sometimes used instead of the more geopolitical term "Middle East". In an unrelated context, the term is used in ancient history and archaeology to divide the Fertile Crescent into the "Asiatic" or "Western Asian" cultures as opposed to ancient Egypt; as a geographic concept, Western Asia includes the Levant, Anatolia, the Armenian Highlands, the South Caucasus, the Arabian peninsula as well as the Sinai Peninsula, making Egypt a transcontinental country.
The term is used pragmatically and has no "correct" or agreed-upon definition. The National Geographic Style Manual as well as Maddison's The World Economy: Historical Statistics by the Organisation for Economic Co-operation and Development only includes Bahrain, Iraq, Jordan, Lebanon, Qatar, Palestinian territories, Saudi Arabia, Turkey, UAE, Yemen as West Asian countries. In contrast to this definition, the United Nations Industrial Development Organization in its 2015 yearbook includes Armenia and Azerbaijan, excludes Israel and Turkey. Unlike the UNIDO, the United Nations Statistics Division excludes Iran from Western Asia and includes Turkey and Cyprus in the region. In the United Nation's geopolitical Eastern European Group and Georgia are included in Eastern Europe, whereas Cyprus and East Thracian Turkey are in Southern Europe; these three nations are listed in the European category of the United Nations Educational and Cultural Organisation. National members of West Asian sports governing bodies are limited to Bahrain, Iraq, Kuwait, Syria, Palestine, Saudi Arabia, United Arab Emirates, Yemen.
The Olympic Council of Asia's multi-sport event West Asian Games are contested by athletes representing these thirteen countries. Among the region's sports organisations are the West Asia Basketball Association, West Asian Billiards and Snooker Federation, West Asian Football Federation, the West Asian Tennis Federation. "Western Asia" was in use as a geographical term in the early 19th century before "Near East" became current as a geopolitical concept. In the context of the history of classical antiquity, "Western Asia" could mean the part of Asia known in classical antiquity, as opposed to the reaches of "interior Asia", i.e. Scythia, "Eastern Asia" the easternmost reaches of geographical knowledge in classical authors, i.e. Transoxania and India. In the 20th century, "Western Asia" was used to denote a rough geographical era in the fields of archaeology and ancient history as a shorthand for "the Fertile Crescent excluding Ancient Egypt" for the purposes of comparing the early civilizations of Egypt and the former.
Use of the term in the context of contemporary geopolitics or world economy appears to date from at least the mid-1960s. Western Asia is located directly south of Eastern Europe; the region is surrounded by seven major seas. To the north, the region is delimited from Europe by the Caucasus Mountains, to the southwest, it is delimited from Africa by the Isthmus of Suez, while to the east, the region adjoins Central Asia and South Asia; the Dasht-e Kavir and Dasht-e Lut deserts in eastern Iran delimit the region somewhat from Asia itself. Three major tectonic plates converge on Western Asia, including the African and Arabian plates; the boundaries between the tectonic plates make up the Azores-Gibraltar Ridge, extending across North Africa, the Red Sea, into Iran. The Arabian Plate is moving northward into the Anatolian plate at the East Anatolian Fault, the boundary between the Aegean and Anatolian plate in eastern Turkey is seismically active. Several major aquifers provide water to large portions of Western Asia.
In Saudi Arabia, two large aquifers of Palaeozoic and Triassic origins are located beneath the Jabal Tuwayq mountains and areas west to the Red Sea. Cretaceous and Eocene-origin aquifers are located beneath large portions of central and eastern Saudi Arabia, including Wasia and Biyadh which contain amounts of both fresh water and saline water. Flood or furrow irrigation, as well as sprinkler methods, are extensively used for irrigation, covering nearly 90,000 km2 across Western Asia for agriculture; the Tigris and Euphrates rivers contribute well. Western Asia is arid and semi-arid, can be subject to drought, but it contains vast expanses of forest and fertile valleys; the region consists of grasslands, rangelands and mountains. Water shortages are a problem in many parts of West Asia, with growing populations increasing demands for water, while salinization and pollution threaten water supplies. Major rivers, including the Tigris and Euphrates, provide sources for irrigation water to support agriculture.
There are two wind phenomena in Western Asia: the shamal. The sharqi is a wind that comes from the south and southea
International Phonetic Alphabet
The International Phonetic Alphabet is an alphabetic system of phonetic notation based on the Latin alphabet. It was devised by the International Phonetic Association in the late 19th century as a standardized representation of the sounds of spoken language; the IPA is used by lexicographers, foreign language students and teachers, speech-language pathologists, actors, constructed language creators and translators. The IPA is designed to represent only those qualities of speech that are part of oral language: phones, phonemes and the separation of words and syllables. To represent additional qualities of speech, such as tooth gnashing and sounds made with a cleft lip and cleft palate, an extended set of symbols, the extensions to the International Phonetic Alphabet, may be used. IPA symbols are composed of one or more elements of two basic types and diacritics. For example, the sound of the English letter ⟨t⟩ may be transcribed in IPA with a single letter, or with a letter plus diacritics, depending on how precise one wishes to be.
Slashes are used to signal broad or phonemic transcription. Letters or diacritics are added, removed or modified by the International Phonetic Association; as of the most recent change in 2005, there are 107 letters, 52 diacritics and four prosodic marks in the IPA. These are shown in the current IPA chart, posted below in this article and at the website of the IPA. In 1886, a group of French and British language teachers, led by the French linguist Paul Passy, formed what would come to be known from 1897 onwards as the International Phonetic Association, their original alphabet was based on a spelling reform for English known as the Romic alphabet, but in order to make it usable for other languages, the values of the symbols were allowed to vary from language to language. For example, the sound was represented with the letter ⟨c⟩ in English, but with the digraph ⟨ch⟩ in French. However, in 1888, the alphabet was revised so as to be uniform across languages, thus providing the base for all future revisions.
The idea of making the IPA was first suggested by Otto Jespersen in a letter to Paul Passy. It was developed by Alexander John Ellis, Henry Sweet, Daniel Jones, Passy. Since its creation, the IPA has undergone a number of revisions. After revisions and expansions from the 1890s to the 1940s, the IPA remained unchanged until the Kiel Convention in 1989. A minor revision took place in 1993 with the addition of four letters for mid central vowels and the removal of letters for voiceless implosives; the alphabet was last revised in May 2005 with the addition of a letter for a labiodental flap. Apart from the addition and removal of symbols, changes to the IPA have consisted of renaming symbols and categories and in modifying typefaces. Extensions to the International Phonetic Alphabet for speech pathology were created in 1990 and adopted by the International Clinical Phonetics and Linguistics Association in 1994; the general principle of the IPA is to provide one letter for each distinctive sound, although this practice is not followed if the sound itself is complex.
This means that: It does not use combinations of letters to represent single sounds, the way English does with ⟨sh⟩, ⟨th⟩ and ⟨ng⟩, or single letters to represent multiple sounds the way ⟨x⟩ represents /ks/ or /ɡz/ in English. There are no letters that have context-dependent sound values, as do "hard" and "soft" ⟨c⟩ or ⟨g⟩ in several European languages; the IPA does not have separate letters for two sounds if no known language makes a distinction between them, a property known as "selectiveness". Among the symbols of the IPA, 107 letters represent consonants and vowels, 31 diacritics are used to modify these, 19 additional signs indicate suprasegmental qualities such as length, tone and intonation; these are organized into a chart. The letters chosen for the IPA are meant to harmonize with the Latin alphabet. For this reason, most letters modifications thereof; some letters are neither: for example, the letter denoting the glottal stop, ⟨ʔ⟩, has the form of a dotless question mark, derives from an apostrophe.
A few letters, such as that of the voiced pharyngeal fricative, ⟨ʕ⟩, were inspired by other writing systems. Despite its preference for harmonizing with the Latin script, the International Phonetic Association has admitted other letters. For example, before 1989, the IPA letters for click consonants were ⟨ʘ⟩, ⟨ʇ⟩, ⟨ʗ⟩, ⟨ʖ⟩, all of which were derived either from existing IPA letters, or from Latin and Greek letters. However, except for ⟨ʘ⟩, none of these letters were used among Khoisanists or Bantuists, as a result they were replaced by the more widespread symbols ⟨ʘ⟩, ⟨ǀ⟩, ⟨ǃ⟩, ⟨ǂ⟩, ⟨ǁ⟩ at the IPA Kiel Convention in 1989. Although the IPA diacritics are featural, there is little systemicity in the letter forms. A retroflex articulation is indicated with a right-swinging tail, as in ⟨ɖ ʂ ɳ⟩, implosion by a top hook, ⟨ɓ ɗ ɠ⟩, but other pseudo-featural elements are due to haphazard derivation and coincidence. For example, all nasal consonants but uvular ⟨ɴ⟩ are based on the form ⟨n⟩: ⟨m ɱ n ɳ ɲ ŋ⟩.
However, the similarity between ⟨m⟩ and ⟨n⟩ is a historical accident. Some of the new letters were ordinary Latin letters tu
Africa is the world's second largest and second most-populous continent, being behind Asia in both categories. At about 30.3 million km2 including adjacent islands, it covers 6% of Earth's total surface area and 20% of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, the Isthmus of Suez and the Red Sea to the northeast, the Indian Ocean to the southeast and the Atlantic Ocean to the west; the continent includes various archipelagos. It contains 54 recognised sovereign states, nine territories and two de facto independent states with limited or no recognition; the majority of the continent and its countries are in the Northern Hemisphere, with a substantial portion and number of countries in the Southern Hemisphere. Africa's average population is the youngest amongst all the continents. Algeria is Africa's largest country by area, Nigeria is its largest by population. Africa central Eastern Africa, is accepted as the place of origin of humans and the Hominidae clade, as evidenced by the discovery of the earliest hominids and their ancestors as well as ones that have been dated to around 7 million years ago, including Sahelanthropus tchadensis, Australopithecus africanus, A. afarensis, Homo erectus, H. habilis and H. ergaster—the earliest Homo sapiens, found in Ethiopia, date to circa 200,000 years ago.
Africa encompasses numerous climate areas. Africa hosts a large diversity of ethnicities and languages. In the late 19th century, European countries colonised all of Africa. African nations cooperate through the establishment of the African Union, headquartered in Addis Ababa. Afri was a Latin name used to refer to the inhabitants of then-known northern Africa to the west of the Nile river, in its widest sense referred to all lands south of the Mediterranean; this name seems to have referred to a native Libyan tribe, an ancestor of modern Berbers. The name had been connected with the Phoenician word ʿafar meaning "dust", but a 1981 hypothesis has asserted that it stems from the Berber word ifri meaning "cave", in reference to cave dwellers; the same word may be found in the name of the Banu Ifran from Algeria and Tripolitania, a Berber tribe from Yafran in northwestern Libya. Under Roman rule, Carthage became the capital of the province it named Africa Proconsularis, following its defeat of the Carthaginians in the Third Punic War in 146 BC, which included the coastal part of modern Libya.
The Latin suffix -ica can sometimes be used to denote a land. The Muslim region of Ifriqiya, following its conquest of the Byzantine Empire's Exarchatus Africae preserved a form of the name. According to the Romans, Africa lay to the west of Egypt, while "Asia" was used to refer to Anatolia and lands to the east. A definite line was drawn between the two continents by the geographer Ptolemy, indicating Alexandria along the Prime Meridian and making the isthmus of Suez and the Red Sea the boundary between Asia and Africa; as Europeans came to understand the real extent of the continent, the idea of "Africa" expanded with their knowledge. Other etymological hypotheses have been postulated for the ancient name "Africa": The 1st-century Jewish historian Flavius Josephus asserted that it was named for Epher, grandson of Abraham according to Gen. 25:4, whose descendants, he claimed, had invaded Libya. Isidore of Seville in his 7th-century Etymologiae XIV.5.2. Suggests "Africa comes from the Latin aprica, meaning "sunny".
Massey, in 1881, stated that Africa is derived from the Egyptian af-rui-ka, meaning "to turn toward the opening of the Ka." The Ka is the energetic double of every person and the "opening of the Ka" refers to a womb or birthplace. Africa would be, for the Egyptians, "the birthplace." Michèle Fruyt in 1976 proposed linking the Latin word with africus "south wind", which would be of Umbrian origin and mean "rainy wind". Robert R. Stieglitz of Rutgers University in 1984 proposed: "The name Africa, derived from the Latin *Aphir-ic-a, is cognate to Hebrew Ophir." Ibn Khallikan and some other historians claim that the name of Africa came from a Himyarite king called Afrikin ibn Kais ibn Saifi called "Afrikus son of Abrahah" who subdued Ifriqiya. Africa is considered by most paleoanthropologists to be the oldest inhabited territory on Earth, with the human species originating from the continent. During the mid-20th century, anthropologists discovered many fossils and evidence of human occupation as early as 7 million years ago.
Fossil remains of several species of early apelike humans thought to have evolved into modern man, such as Australopithecus afarensis (radiometrically dated to 3.9–3.0 million years BP, Paranthropus boisei and Homo ergaster have been discovered. After the evolution of Homo sapiens sapiens 150,000 to 100,000 years BP in Africa, the continent was populated by groups of hunter-gatherers; these first modern humans left Africa and populated the rest of the globe during the Out of Africa II migration dated to 50,000 years BP, exiting the continent eith
The term phonation has different meanings depending on the subfield of phonetics. Among some phoneticians, phonation is the process by which the vocal folds produce certain sounds through quasi-periodic vibration; this is the definition used among those who study laryngeal anatomy and physiology and speech production in general. Phoneticians in other subfields, such as linguistic phonetics, call this process voicing, use the term phonation to refer to any oscillatory state of any part of the larynx that modifies the airstream, of which voicing is just one example. Voiceless and supra-glottal phonations are included under this definition; the phonatory process, or voicing, occurs when air is expelled from the lungs through the glottis, creating a pressure drop across the larynx. When this drop becomes sufficiently large, the vocal folds start to oscillate; the minimum pressure drop required to achieve phonation is called the phonation threshold pressure, for humans with normal vocal folds, it is 2–3 cm H2O.
The motion of the vocal folds during oscillation is lateral, though there is some superior component as well. However, there is no motion along the length of the vocal folds; the oscillation of the vocal folds serves to modulate the pressure and flow of the air through the larynx, this modulated airflow is the main component of the sound of most voiced phones. The sound that the larynx produces is a harmonic series. In other words, it consists of a fundamental tone accompanied by harmonic overtones, which are multiples of the fundamental frequency. According to the source–filter theory, the resulting sound excites the resonance chamber, the vocal tract to produce the individual speech sounds; the vocal folds will not oscillate if they are not sufficiently close to one another, are not under sufficient tension or under too much tension, or if the pressure drop across the larynx is not sufficiently large. In linguistics, a phone is called voiceless. In speech, voiceless phones are associated with vocal folds that are elongated tensed, placed laterally when compared to vocal folds during phonation.
Fundamental frequency, the main acoustic cue for the percept pitch, can be varied through a variety of means. Large scale changes are accomplished by increasing the tension in the vocal folds through contraction of the cricothyroid muscle. Smaller changes in tension can be effected by contraction of the thyroarytenoid muscle or changes in the relative position of the thyroid and cricoid cartilages, as may occur when the larynx is lowered or raised, either volitionally or through movement of the tongue to which the larynx is attached via the hyoid bone. In addition to tension changes, fundamental frequency is affected by the pressure drop across the larynx, affected by the pressure in the lungs, will vary with the distance between the vocal folds. Variation in fundamental frequency is used linguistically to produce tone. There are two main theories as to how vibration of the vocal folds is initiated: the myoelastic theory and the aerodynamic theory; these two theories are not in contention with one another and it is quite possible that both theories are true and operating to initiate and maintain vibration.
A third theory, the neurochronaxic theory, was in considerable vogue in the 1950s, but has since been discredited. The myoelastic theory states that when the vocal cords are brought together and breath pressure is applied to them, the cords remain closed until the pressure beneath them, the subglottic pressure, is sufficient to push them apart, allowing air to escape and reducing the pressure enough for the muscle tension recoil to pull the folds back together again; the pressure builds up once again until the cords are pushed apart, the whole cycle keeps repeating itself. The rate at which the cords open and close, the number of cycles per second, determines the pitch of the phonation; the aerodynamic theory is based on the Bernoulli energy law in fluids. The theory states that when a stream of breath is flowing through the glottis while the arytenoid cartilages are held together, a push-pull effect is created on the vocal fold tissues that maintains self-sustained oscillation; the push occurs during glottal opening, when the glottis is convergent, the pull occurs during glottal closing, when the glottis is divergent.
Such an effect causes a transfer of energy from the airflow to the vocal fold tissues which overcomes losses by dissipation and sustain the oscillation. The amount of lung pressure needed to begin phonation is defined by Titze as the oscillation threshold pressure. During glottal closure, the air flow is cut off until breath pressure pushes the folds apart and the flow starts up again, causing the cycles to repeat; the textbook entitled Myoelastic Aerodynamic Theory of Phonation by Ingo Titze credits Janwillem van den Berg as the originator of the theory and provides detailed mathematical development of the theory. This theory states that the frequency of the vocal fold vibration is determined by the chronaxie of the recurrent nerve, not by breath pressure or muscular tension. Advocates of this theory thought that every single vibration of the vocal folds was due to an impulse from the recurrent laryngeal nerves and that the acoustic center in the brain regulated the speed of vocal fold vibration.
Speech and voice scientists have long since left this theory as the muscles have been shown to not be able to contract fast enough to accomplish the vibration. In addition, persons with paralyzed vocal folds can produce phonation, whic