In physics, sound is a vibration that propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the reception of such waves and their perception by the brain. Humans can only hear sound waves as distinct pitches when the frequency lies between about 20 Hz and 20 kHz. Sound waves above 20 kHz is not perceptible by humans. Sound waves below 20 Hz are known as infrasound. Different animal species have varying hearing ranges. Acoustics is the interdisciplinary science that deals with the study of mechanical waves in gases and solids including vibration, sound and infrasound. A scientist who works in the field of acoustics is an acoustician, while someone working in the field of acoustical engineering may be called an acoustical engineer. An audio engineer, on the other hand, is concerned with the recording, manipulation and reproduction of sound. Applications of acoustics are found in all aspects of modern society, subdisciplines include aeroacoustics, audio signal processing, architectural acoustics, electro-acoustics, environmental noise, musical acoustics, noise control, speech, underwater acoustics, vibration.
Sound is defined as " Oscillation in pressure, particle displacement, particle velocity, etc. propagated in a medium with internal forces, or the superposition of such propagated oscillation. Auditory sensation evoked by the oscillation described in." Sound can be viewed as a wave motion in air or other elastic media. In this case, sound is a stimulus. Sound can be viewed as an excitation of the hearing mechanism that results in the perception of sound. In this case, sound is a sensation. Sound can propagate through a medium such as air and solids as longitudinal waves and as a transverse wave in solids; the sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium; as the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave. At a fixed distance from the source, the pressure and displacement of the medium vary in time.
At an instant in time, the pressure and displacement vary in space. Note that the particles of the medium do not travel with the sound wave; this is intuitively obvious for a solid, the same is true for liquids and gases. During propagation, waves can be refracted, or attenuated by the medium; the behavior of sound propagation is affected by three things: A complex relationship between the density and pressure of the medium. This relationship, affected by temperature, determines the speed of sound within the medium. Motion of the medium itself. If the medium is moving, this movement may increase or decrease the absolute speed of the sound wave depending on the direction of the movement. For example, sound moving through wind will have its speed of propagation increased by the speed of the wind if the sound and wind are moving in the same direction. If the sound and wind are moving in opposite directions, the speed of the sound wave will be decreased by the speed of the wind; the viscosity of the medium.
Medium viscosity determines the rate. For many media, such as air or water, attenuation due to viscosity is negligible; when sound is moving through a medium that does not have constant physical properties, it may be refracted. The mechanical vibrations that can be interpreted as sound can travel through all forms of matter: gases, liquids and plasmas; the matter that supports the sound is called the medium. Sound cannot travel through a vacuum. Sound is transmitted through gases and liquids as longitudinal waves called compression waves, it requires a medium to propagate. Through solids, however, it can be transmitted as transverse waves. Longitudinal sound waves are waves of alternating pressure deviations from the equilibrium pressure, causing local regions of compression and rarefaction, while transverse waves are waves of alternating shear stress at right angle to the direction of propagation. Sound waves may be "viewed" using parabolic objects that produce sound; the energy carried by an oscillating sound wave converts back and forth between the potential energy of the extra compression or lateral displacement strain of the matter, the kinetic energy of the displacement velocity of particles of the medium.
Although there are many complexities relating to the transmission of sounds, at the point of reception, sound is dividable into two simple elements: pressure and time. These fundamental elements form the basis of all sound waves, they can be used to describe, in every sound we hear. In order to understand the sound more a complex wave such as the one shown in a blue background on the right of this text, is separated into its component parts, which are a combination of various sound wave frequencies. Sound waves are simplified to a description in terms of sinusoidal plane waves, which are characterized by these generic properties: Frequency, or its inverse, wavelength Amplitude, sound pressure or Intensity Speed of sound DirectionSound, perceptible by humans has frequencies from abou
An optical attenuator, or fiber optic attenuator, is a device used to reduce the power level of an optical signal, either in free space or in an optical fiber. The basic types of optical attenuators are fixed, step-wise variable, continuously variable. Optical attenuators are used in fiber optic communications, either to test power level margins by temporarily adding a calibrated amount of signal loss, or installed permanently to properly match transmitter and receiver levels. Sharp bends can cause losses. If a received signal is too strong a temporary fix is to wrap the cable around a pencil until the desired level of attenuation is achieved. However, such arrangements are unreliable; the power reduction is done by such means as absorption, diffusion, deflection and dispersion, etc. Optical attenuators work by absorbing the light, like sunglasses absorb extra light energy, they have a working wavelength range in which they absorb all light energy equally. They should not reflect the light or scatter the light in an air gap, since that could cause unwanted back reflection in the fiber system.
Another type of attenuator utilizes a length of high-loss optical fiber, that operates upon its input optical signal power level in such a way that its output signal power level is less than the input level. Optical attenuators can take a number of different forms and are classified as fixed or variable attenuators. What's more, they can be classified as LC, SC, ST, FC, MU, E2000 etc. according to the different types of connectors. Fixed optical attenuators used in fiber optic systems may use a variety of principles for their functioning. Preferred attenuators use either doped fibers, or mis-aligned splices,or total power since both of these are reliable and inexpensive. Inline style attenuators are incorporated into patch cables; the alternative build out style attenuator is a small male-female adapter that can be added onto other cables. Non-preferred attenuators use gap loss or reflective principles; such devices can be sensitive to: modal distribution, contamination, temperature, damage due to power bursts, may cause back reflections, may cause signal dispersion etc.
Loopback fiber optic attenuator is designed for testing and the burn-in stage of boards or other equipment. Available in SC/UPC, SC/APC, LC/UPC, LC/APC, MTRJ, MPO for singlemode application.900um fiber cable inside of the black shell for LC and SC type. No black shell for MTRJ and MPO type. Built-in variable optical attenuators may electrically controlled. A manual device is useful for one-time set up of a system, is a near-equivalent to a fixed attenuator, may be referred to as an "adjustable attenuator". In contrast, an electrically controlled attenuator can provide adaptive power optimization. Attributes of merit for electrically controlled devices, include speed of response and avoiding degradation of the transmitted signal. Dynamic range is quite restricted, power feedback may mean that long term stability is a minor issue. Speed of response is a major issue in dynamically reconfigurable systems, where a delay of one millionth of a second can result in the loss of large amounts of transmitted data.
Typical technologies employed for high speed response include liquid crystal variable attenuator, or lithium niobate devices. There is a class of built-in attenuators, technically indistinguishable from test attenuators, except they are packaged for rack mounting, have no test display. Variable optical test attenuators use a variable neutral density filter. Despite high cost, this arrangement has the advantages of being stable, wavelength insensitive, mode insensitive, offering a large dynamic range. Other schemes such as LCD, variable air gap etc. have been tried over the years, but with limited success. They may be either manually or motor controlled. Motor control give regular users a distinct productivity advantage, since used test sequences can be run automatically. Attenuator instrument calibration is a major issue; the user would like an absolute port to port calibration. Calibration should be at a number of wavelengths and power levels, since the device is not always linear; however a number of instruments do not in fact offer these basic features in an attempt to reduce cost.
The most accurate variable attenuator instruments have thousands of calibration points, resulting in excellent overall accuracy in use. Test sequences that use variable attenuators, can be time consuming. Therefore, automation is to achieve useful benefits. Both bench and handheld style devices are available. Attenuation Optical fiber cable Optical fiber connector Gap loss - attenuation sources and causes Optical power meter This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C"
Physics is the natural science that studies matter, its motion, behavior through space and time, that studies the related entities of energy and force. Physics is one of the most fundamental scientific disciplines, its main goal is to understand how the universe behaves. Physics is one of the oldest academic disciplines and, through its inclusion of astronomy the oldest. Over much of the past two millennia, chemistry and certain branches of mathematics, were a part of natural philosophy, but during the scientific revolution in the 17th century these natural sciences emerged as unique research endeavors in their own right. Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, the boundaries of physics which are not rigidly defined. New ideas in physics explain the fundamental mechanisms studied by other sciences and suggest new avenues of research in academic disciplines such as mathematics and philosophy. Advances in physics enable advances in new technologies.
For example, advances in the understanding of electromagnetism and nuclear physics led directly to the development of new products that have transformed modern-day society, such as television, domestic appliances, nuclear weapons. Astronomy is one of the oldest natural sciences. Early civilizations dating back to beyond 3000 BCE, such as the Sumerians, ancient Egyptians, the Indus Valley Civilization, had a predictive knowledge and a basic understanding of the motions of the Sun and stars; the stars and planets were worshipped, believed to represent gods. While the explanations for the observed positions of the stars were unscientific and lacking in evidence, these early observations laid the foundation for astronomy, as the stars were found to traverse great circles across the sky, which however did not explain the positions of the planets. According to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, all Western efforts in the exact sciences are descended from late Babylonian astronomy.
Egyptian astronomers left monuments showing knowledge of the constellations and the motions of the celestial bodies, while Greek poet Homer wrote of various celestial objects in his Iliad and Odyssey. Natural philosophy has its origins in Greece during the Archaic period, when pre-Socratic philosophers like Thales rejected non-naturalistic explanations for natural phenomena and proclaimed that every event had a natural cause, they proposed ideas verified by reason and observation, many of their hypotheses proved successful in experiment. The Western Roman Empire fell in the fifth century, this resulted in a decline in intellectual pursuits in the western part of Europe. By contrast, the Eastern Roman Empire resisted the attacks from the barbarians, continued to advance various fields of learning, including physics. In the sixth century Isidore of Miletus created an important compilation of Archimedes' works that are copied in the Archimedes Palimpsest. In sixth century Europe John Philoponus, a Byzantine scholar, questioned Aristotle's teaching of physics and noting its flaws.
He introduced the theory of impetus. Aristotle's physics was not scrutinized until John Philoponus appeared, unlike Aristotle who based his physics on verbal argument, Philoponus relied on observation. On Aristotle's physics John Philoponus wrote: “But this is erroneous, our view may be corroborated by actual observation more than by any sort of verbal argument. For if you let fall from the same height two weights of which one is many times as heavy as the other, you will see that the ratio of the times required for the motion does not depend on the ratio of the weights, but that the difference in time is a small one, and so, if the difference in the weights is not considerable, that is, of one is, let us say, double the other, there will be no difference, or else an imperceptible difference, in time, though the difference in weight is by no means negligible, with one body weighing twice as much as the other”John Philoponus' criticism of Aristotelian principles of physics served as an inspiration for Galileo Galilei ten centuries during the Scientific Revolution.
Galileo cited Philoponus in his works when arguing that Aristotelian physics was flawed. In the 1300s Jean Buridan, a teacher in the faculty of arts at the University of Paris, developed the concept of impetus, it was a step toward the modern ideas of momentum. Islamic scholarship inherited Aristotelian physics from the Greeks and during the Islamic Golden Age developed it further placing emphasis on observation and a priori reasoning, developing early forms of the scientific method; the most notable innovations were in the field of optics and vision, which came from the works of many scientists like Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi and Avicenna. The most notable work was The Book of Optics, written by Ibn al-Haytham, in which he conclusively disproved the ancient Greek idea about vision, but came up with a new theory. In the book, he presented a study of the phenomenon of the camera obscura (his thousand-year-old
Lead is a chemical element with symbol Pb and atomic number 82. It is a heavy metal, denser than most common materials. Lead is soft and malleable, has a low melting point; when freshly cut, lead is silvery with a hint of blue. Lead has the highest atomic number of any stable element and three of its isotopes each include a major decay chain of heavier elements. Lead is a unreactive post-transition metal, its weak metallic character is illustrated by its amphoteric nature. Compounds of lead are found in the +2 oxidation state rather than the +4 state common with lighter members of the carbon group. Exceptions are limited to organolead compounds. Like the lighter members of the group, lead tends to bond with itself. Lead is extracted from its ores. Galena, a principal ore of lead bears silver, interest in which helped initiate widespread extraction and use of lead in ancient Rome. Lead production declined after the fall of Rome and did not reach comparable levels until the Industrial Revolution. In 2014, the annual global production of lead was about ten million tonnes, over half of, from recycling.
Lead's high density, low melting point and relative inertness to oxidation make it useful. These properties, combined with its relative abundance and low cost, resulted in its extensive use in construction, batteries and shot, solders, fusible alloys, white paints, leaded gasoline, radiation shielding. In the late 19th century, lead's toxicity was recognized, its use has since been phased out of many applications. However, many countries still allow the sale of products that expose humans to lead, including some types of paints and bullets. Lead is a toxin that accumulates in soft tissues and bones, it acts as a neurotoxin damaging the nervous system and interfering with the function of biological enzymes, causing neurological disorders, such as brain damage and behavioral problems. A lead atom has 82 electrons, arranged in an electron configuration of 4f145d106s26p2; the sum of lead's first and second ionization energies—the total energy required to remove the two 6p electrons—is close to that of tin, lead's upper neighbor in the carbon group.
This is unusual. The similarity of ionization energies is caused by the lanthanide contraction—the decrease in element radii from lanthanum to lutetium, the small radii of the elements from hafnium onwards; this is due to poor shielding of the nucleus by the lanthanide 4f electrons. The sum of the first four ionization energies of lead exceeds that of tin, contrary to what periodic trends would predict. Relativistic effects, which become significant in heavier atoms, contribute to this behavior. One such effect is the inert pair effect: the 6s electrons of lead become reluctant to participate in bonding, making the distance between nearest atoms in crystalline lead unusually long. Lead's lighter carbon group congeners form stable or metastable allotropes with the tetrahedrally coordinated and covalently bonded diamond cubic structure; the energy levels of their outer s- and p-orbitals are close enough to allow mixing into four hybrid sp3 orbitals. In lead, the inert pair effect increases the separation between its s- and p-orbitals, the gap cannot be overcome by the energy that would be released by extra bonds following hybridization.
Rather than having a diamond cubic structure, lead forms metallic bonds in which only the p-electrons are delocalized and shared between the Pb2+ ions. Lead has a face-centered cubic structure like the sized divalent metals calcium and strontium. Pure lead has a silvery appearance with a hint of blue, it tarnishes on contact with moist air and takes on a dull appearance, the hue of which depends on the prevailing conditions. Characteristic properties of lead include high density, malleability and high resistance to corrosion due to passivation. Lead's close-packed face-centered cubic structure and high atomic weight result in a density of 11.34 g/cm3, greater than that of common metals such as iron and zinc. This density is the origin of the idiom to go over like a lead balloon; some rarer metals are denser: tungsten and gold are both at 19.3 g/cm3, osmium—the densest metal known—has a density of 22.59 g/cm3 twice that of lead. Lead is a soft metal with a Mohs hardness of 1.5. It is somewhat ductile.
The bulk modulus of lead—a measure of its ease of compressibility—is 45.8 GPa. In comparison, that of aluminium is 75.2 GPa. Lead's tensile strength, at 12–17 MPa, is low; the melting point of lead—at 327.5 °C —is low compared to most metals. Its boiling point of 1749 °C is the lowest among the carbon group elements; the electrical resistivity of lead at 20 °C is 192 nanoohm-meters an order of magnitude higher than those of other industrial metals. Lead is a superconductor at temperatures lower than 7.19 K.
Connective tissue is one of the four basic types of animal tissue, along with epithelial tissue, muscle tissue, nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes that envelop the brain and spinal cord are composed of connective tissue, they protect the body. All connective tissue consists of three main components: ground substance and cells. Not all authorities include blood or lymph as connective tissue because they lack the fiber component. All are immersed in the body water; the cells of connective tissue include fibroblasts, macrophages, mast cells and leucocytes. The term "connective tissue" was introduced in 1830 by Johannes Peter Müller; the tissue was recognized as a distinct class in the 18th century. Connective tissue can be broadly subdivided into connective tissue proper, special connective tissue. Connective tissue proper consists of loose connective tissue and dense connective tissue Loose and dense connective tissue are distinguished by the ratio of ground substance to fibrous tissue.
Loose connective tissue has much more ground substance and a relative lack of fibrous tissue, while the reverse is true of dense connective tissue. Dense regular connective tissue, found in structures such as tendons and ligaments, is characterized by collagen fibers arranged in an orderly parallel fashion, giving it tensile strength in one direction. Dense irregular connective tissue provides strength in multiple directions by its dense bundles of fibers arranged in all directions. Special connective tissue consists of reticular connective tissue, adipose tissue, cartilage and blood. Other kinds of connective tissues include fibrous and lymphoid connective tissues. Fibroareolar tissue is a mix of fibrous and areolar tissue. New vascularised connective tissue that forms in the process of wound healing is termed granulation tissue. Fibroblasts are the cells responsible for the production of some CT. Type I collagen is present in many forms of connective tissue, makes up about 25% of the total protein content of the mammalian body.
Characteristics of CT: Cells are spread through an extracellular fluid. Ground substance - A clear and viscous fluid containing glycosaminoglycans and proteoglycans to fix the body water and the collagen fibers in the intercellular spaces. Ground substance slows the spread of pathogens. Fibers. Not all types of CT are fibrous. Examples of non-fibrous CT include adipose blood. Adipose tissue gives "mechanical cushioning" to the body, among other functions. Although there is no dense collagen network in adipose tissue, groups of adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place; the matrix of blood is plasma. Both the ground substance and proteins create the matrix for CT. Connective tissues are derived from the mesenchyme. Types of fibers: Connective tissue has a wide variety of functions that depend on the types of cells and the different classes of fibers involved. Loose and dense irregular connective tissue, formed by fibroblasts and collagen fibers, have an important role in providing a medium for oxygen and nutrients to diffuse from capillaries to cells, carbon dioxide and waste substances to diffuse from cells back into circulation.
They allow organs to resist stretching and tearing forces. Dense regular connective tissue, which forms organized structures, is a major functional component of tendons and aponeuroses, is found in specialized organs such as the cornea. Elastic fibers, made from elastin and fibrillin provide resistance to stretch forces, they are found in the walls of large blood vessels and in certain ligaments in the ligamenta flava. In hematopoietic and lymphatic tissues, reticular fibers made by reticular cells provide the stroma—or structural support—for the parenchyma—or functional part—of the organ. Mesenchyme is a type of connective tissue found in developing organs of embryos, capable of differentiation into all types of mature connective tissue. Another type of undifferentiated connective tissue is mucous connective tissue, found inside the umbilical cord. Various types of specialized tissues and cells are classified under the spectrum of connective tissue, are as diverse as brown and white adipose tissue, blood and bone.
Cells of the immune system, such as macrophages, mast cells, plasma cells and eosinophils are found scattered in loose connective tissue, providing the ground for starting inflammatory and immune responses upon the detection of antigens. There are many types of connective tissue disorders, such as: Connective tissue neoplasms including sarcomas such as hemangiopericytoma and malignant peripheral nerve sheath tumor in nervous tissue. Congenital diseases include Ehlers-Danlos Syndrome. Myxomatous degeneration – a pathological weakening of connective tissue. Mixed connective tissue disease – a disease of the autoimmune system undifferentiated connective tissue disease. Systemic lupus erythematosus – a major autoimmune disease of connective tissue Scurvy, caused by a deficiency of vitamin C, necessary for the synthesis of collagen. For microscopic viewing, most of the connective tissue staining-techniques, colour tissue fibers in contrasting shades. Collagen may be differentially stained by any of the following: Van Gieson's stain Masson's trichrome stain Mallory's t
Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word refers to visible light, the visible spectrum, visible to the human eye and is responsible for the sense of sight. Visible light is defined as having wavelengths in the range of 400–700 nanometres, or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared and the ultraviolet. This wavelength means a frequency range of 430–750 terahertz; the main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars in the form of starches, which release energy into the living things that digest them; this process of photosynthesis provides all the energy used by living things. Another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has replaced firelight; some species of animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, vampire squids use it to hide themselves from prey. The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarization, while its speed in a vacuum, 299,792,458 metres per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation, is experimentally found to always move at this speed in a vacuum. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays and radio waves are light. Like all types of EM radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed; the absorbed energy of the EM waves is called a photon, represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave collapses to a single location, this location is where the photon "arrives."
This is. This dual wave-like and particle-like nature of light is known as the wave–particle duality; the study of light, known as optics, is an important research area in modern physics. EM radiation, or EMR, is classified by wavelength into radio waves, infrared, the visible spectrum that we perceive as light, ultraviolet, X-rays, gamma rays; the behavior of EMR depends on its wavelength. Higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths; when EMR interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. EMR in the visible light region consists of quanta that are at the lower end of the energies that are capable of causing electronic excitation within molecules, which leads to changes in the bonding or chemistry of the molecule. At the lower end of the visible light spectrum, EMR becomes invisible to humans because its photons no longer have enough individual energy to cause a lasting molecular change in the visual molecule retinal in the human retina, which change triggers the sensation of vision.
There exist animals that are sensitive to various types of infrared, but not by means of quantum-absorption. Infrared sensing in snakes depends on a kind of natural thermal imaging, in which tiny packets of cellular water are raised in temperature by the infrared radiation. EMR in this range causes molecular vibration and heating effects, how these animals detect it. Above the range of visible light, ultraviolet light becomes invisible to humans because it is absorbed by the cornea below 360 nm and the internal lens below 400 nm. Furthermore, the rods and cones located in the retina of the human eye cannot detect the short ultraviolet wavelengths and are in fact damaged by ultraviolet. Many animals with eyes that do not require lenses are able to detect ultraviolet, by quantum photon-absorption mechanisms, in much the same chemical way that humans detect visible light. Various sources define visible light as narrowly as 420–680 nm to as broadly as 380–800 nm. Under ideal laboratory conditions, people can see infrared up to at least 1050 nm.
Plant growth is affected by the color spectrum of light, a process known as photomorphogenesis. The speed of light in a vacuum is defined to be 299,792,458 m/s; the fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849.
Seismic waves are waves of energy that travel through the Earth's layers, are a result of earthquakes, volcanic eruptions, magma movement, large landslides and large man-made explosions that give out low-frequency acoustic energy. Many other natural and anthropogenic sources create low-amplitude waves referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields are recorded by hydrophone, or accelerometer; the propagation velocity of the waves depends on elasticity of the medium. Velocity tends to increase with depth and ranges from 2 to 8 km/s in the Earth's crust, up to 13 km/s in the deep mantle. Earthquakes create distinct types of waves with different velocities. In geophysics the refraction or reflection of seismic waves is used for research into the structure of the Earth's interior, man-made vibrations are generated to investigate shallow, subsurface structures. Among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, surface waves, which travel at the Earth's surface.
Other modes of wave propagation exist. Body waves travel through the interior of the Earth. Surface waves travel across the surface. Surface waves decay more with distance than body waves, which travel in three dimensions. Particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage. Body waves travel through the interior of the Earth along paths controlled by the material properties in terms of density and modulus; the density and modulus, in turn, vary according to temperature and material phase. This effect resembles the refraction of light waves. Two types of particle motion result in two types of body waves: Primary and Secondary waves. Primary waves are compressional waves. P waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary"; these waves can travel through any type of material, including fluids, can travel nearly 1.7 times faster than the S waves. In air, they take the form of sound waves, hence they travel at the speed of sound.
Typical speeds are 1450 m/s in water and about 5000 m/s in granite. Secondary waves are shear waves. Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics. S-waves can travel only through solids. S-waves are slower than P-waves, speeds are around 60% of that of P-waves in any given material. Shear waves can't travel through any liquid medium, so the absence of S-wave in earth's outer core suggests a liquid state. Seismic surface waves travel along the Earth's surface, they can be classified as a form of mechanical surface waves. They are called surface waves, they travel more than seismic body waves. In large earthquakes, surface waves can have an amplitude of several centimeters. Rayleigh waves called ground roll, are surface waves that travel as ripples with motions that are similar to those of waves on the surface of water.
The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves 90% of the velocity of S waves for typical homogeneous elastic media. In a layered medium the velocity of the Rayleigh waves depends on their wavelength. See Lamb waves. Love waves are horizontally polarized shear waves, existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness, they are named after A. E. H. Love, a British mathematician who created a mathematical model of the waves in 1911, they travel faster than Rayleigh waves, about 90% of the S wave velocity, have the largest amplitude. A Stoneley wave is a type of boundary wave that propagates along a solid-fluid boundary or, under specific conditions along a solid-solid boundary. Amplitudes of Stoneley waves have their maximum values at the boundary between the two contacting media and decay exponentially towards the depth of each of them; these waves can be generated along the walls of a fluid-filled borehole, being an important source of coherent noise in VSPs and making up the low frequency component of the source in sonic logging.
The equation for Stoneley waves was first given by Dr. Robert Stoneley, Emeritus Professor of Seismology, Cambridge. Free oscillations of the Earth are standing waves, the result of interference between two surface waves traveling in opposite directions. Interference of Rayleigh waves results in spheroidal oscillation S while interference of Love waves gives toroidal oscillation T; the modes of oscillations are specified by three numbers, e.g. nSlm, wher