Frequency dependent attenuation of electromagnetic radiation in standard atmosphere.

Attenuation is the reduction in amplitude and intensity of a signal. Amplitude is the magnitude of change in the oscillating variable with each Oscillation, within an oscillating system In Physics, intensity is a measure of the time-averaged Energy Flux. Signals may attenuate exponentially by transmission through a medium, in which case attenuation is usually reported in dB with respect to distance traveled through the medium. The decibel ( dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity relative to Attenuation can also be understood to be the opposite of amplification. Attenuation is an important property in telecommunications and ultrasound applications because of its importance in determining signal strength as a function of distance. Attenuation is usually measured in units of decibels per unit length of medium (dB/cm, dB/km, etc) and is represented by the attenuation coefficient of the medium in question. The decibel ( dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity relative to In Mathematics, a coefficient is a Constant multiplicative factor of a certain object ^[1]

Ultrasound

One area of research in which attenuation figures strongly is in ultrasound physics. Not to be confused with Supersonic. Ultrasound is cyclic Sound pressure with a Frequency greater than the upper Attenuation in ultrasound is the reduction in amplitude of the ultrasound beam as a function of distance through the imaging medium. Accounting for attenuation effects in ultrasound is important because a reduced signal amplitude can affect the quality of the image produced. By knowing the attenuation that an ultrasound beam experiences travelling through a medium, one can adjust the input signal amplitude to compensate for any loss of energy at the desired imaging depth. ^[2]

• Ultrasound attenuation measurement in heterogeneous systems, like emulsions or colloids yields information on particle size distribution. Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations An emulsion ( IPA: /ɪˈmʌlʃən/ is a mixture of two Immiscible (unblendable liquids A colloid is a type of mechanical Mixture where one substance is dispersed evenly throughout another The particle size distribution (PSD of a powder or granular material or particles dispersed in fluid is a list of values or a mathematical function that defines the relative amounts There is ISO standard on this technique. ^[3]

• Ultrasound attenuation can be used for extensional rheology measurement. Rheology is the study of the flow of matter mainly liquids but also soft solids or solids under conditions in which they flow rather than deform elastically There are acoustic rheometers that employ Stokes' law for measuring extensional viscosity and volume viscosity. Acoustic rheometer employes Piezo-electric Crystal that can easily launch a successive Wave of extensions and contractions into the Fluid In 1851, George Gabriel Stokes derived an expression now known as Stokes' law, for the frictional force — also called Drag force — exerted on Extensional viscosity is a Viscosity coefficient when applied stress is Extensional stress. Volume viscosity (also called bulk viscosity or second viscosity) appears in the Navier-Stokes equation if it is written for Compressible fluid

Attenuation coefficient

Attenuation coefficients are used to quantify different media according to how strongly the transmitted ultrasound amplitude decreases as a function of frequency. For attenuation coefficient as it applies to electromagnetic theory and telecommunications see propagation constant. The attenuation coefficient (α) can be used to determine total attenuation in dB/cm in the medium using the following formula:

As this equation shows, besides the medium length and attenuation coefficient, attenuation is also linearly dependent on the frequency of the incident ultrasound beam. In Mathematics, a coefficient is a Constant multiplicative factor of a certain object Frequency is a measure of the number of occurrences of a repeating event per unit Time. Attenuation coefficients vary widely for different media. In biomedical ultrasound imaging however, biological materials and water are the most commonly used media. The attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below:^[2]

There are two general ways of acoustic energy losses: absorption and scattering, for instance light scattering. Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from ^[4] Ultrasound propagation through homogeneous media is associated only with absorption and can be characterised with absorption coefficient only. The absorption coefficient α is a property of a material It defines the extent to which a material absorbs energy for example that of Sound waves or Electromagnetic Propagation through heterogeneous media requires taking into account scattering. Heterogeneous is an adjective used to describe an object or system consisting of multiple items having a large number of structural variations ^[5]

Earthquake

The energy with which an earthquake affects a location depends on the running distance. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer Distance is a numerical description of how far apart objects are The attenuation in the signal of ground motion intensity plays an important role in the assessment of possible strong ground shaking. A seismic wave loses energy as it propagates through the earth (attenuation). Seismic waves are Waves that travel through the Earth, most often as the result of a tectonic Earthquake, sometimes from an Explosion In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 This phenomenon is tied in to the dispersion of the seismic energy with the distance. A phenomenon (from Greek φαινόμενoν, pl φαινόμενα - phenomena) is any observable occurrence There are two types of dissipated energy:

1. geometric dispersion caused by distribution of the seismic energy to greater volumes
2. dispersion as heat

Electromagnetic

Attenuation decreases the intensity of electromagnetic radiation due to absorption or scattering of photons. In Physics, dissipation embodies the concept of a Dynamical system where important mechanical modes such as Waves or Oscillations lose Energy Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. In Physics, absorption of electromagnetic radiation is the process by which the Energy of a Photon is taken up by matter typically the electrons of an Scattering is a general physical process whereby some forms of Radiation, such as Light, Sound or moving particles for example are forced to deviate from In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Attenuation does not include the decrease in intensity due to inverse-square law geometric spreading. In Physics, an inverse-square law is any Physical law stating that some physical Quantity or strength is inversely proportional Therefore, calculation of the total change in intensity involves both the inverse-square law and an estimation of attenuation over the path.

The primary causes of attenuation in matter are the photoelectric effect, compton scattering and, for photon energies of above 1. Introduction When a Metallic surface is exposed to Electromagnetic radiation above a certain threshold Frequency, the light is absorbed and Electrons The Compton shift formula Klein-Nishina formulaCompton used a combination of three fundamental formulas representing the various aspects of classical and modern physics combining 022MeV, pair production. See also Electron-positron annihilation Meitner–Hupfeld effect Pair instability supernova

Radiography

See Attenuation coefficient article. For attenuation coefficient as it applies to electromagnetic theory and telecommunications see propagation constant.

Optics

Attenuation of light by cloudy water is called turbidity, and by interstellar dust, extinction (astronomy). Turbidity is the cloudiness or haziness of a fluid caused by individual particles ( suspended solids) that are generally invisible to the Naked eye Extinction is a term used in Astronomy to describe the absorption and Scattering of Electromagnetic radiation emitted by Astronomical objects Attenuation in glass or other solid medium is usually studied by telecommunication engineers, hence is called by the same names as the attenuation of electrical signals. In the fields of communications, Signal processing, and in Electrical engineering more generally a signal is any time-varying or spatial-varying quantity

Attenuation is caused by several different factors, but primarily scattering and absorption. The scattering of light is caused by molecular level irregularities in the glass structure. Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation. Attenuation in fibre optics, also known as transmission loss, is the reduction in intensity of the light beam with respect to distance travelled through a transparent medium. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length Attenuation coefficients in fibre optics usually use units of dB/km through the medium due to the great transparency of modern optical media. The medium is usually a fibre of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a light pulse across far distances, and as a result much research has gone into both limiting the attenuation and maximizing the amplification of the fibre optic light beam. ^[6] Attenuation in fibre optics can be quantified using the following equation:^[7]

Applications

In optical fibers, attenuation is the rate at which the signal light decreases in intensity. An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length For this reason, glass fiber (which has a low attenuation) is used for long-distance fiber optic cables; plastic fiber has a higher attenuation and hence shorter range. There also exist optical attenuators which decrease the signal in a fiber optic cable intentionally. An optical attenuator is a device used to reduce the power level of an optical signal, either in free space or in an Optical fiber.

Attenuation of light is also important in physical oceanography. Physical oceanography is the study of physical conditions and physical processes within the Ocean, especially the motions and physical properties of ocean waters Here, attenuation is the decrease in light intensity with depth due to absorption by water molecules and scattering by suspended particulates. Water absorption is a phenomenon in the transmission of Electromagnetic radiation through a medium containing Water molecules This same effect is an important consideration in weather radar as rain drops absorb a part of the emitted beam that is more or less significant depending on the wavelength used. A weather radar is a type of Radar used to locate precipitation, calculate its motion estimate its type ( Rain

The attenuation of photons, particularly of those in the x-ray spectrum, is important in the field of medical physics. Medical physics is the application of Physics to Medicine. It generally concerns physics as applied to Medical imaging and Radiotherapy, although Due to the damaging effects of high energy photons, it is necessary to know how much energy is deposited in tissue during diagnostic treatments involving such radiation. Additionally gamma radiation is used in cancer treatments where it is important to know how much energy will be deposited in healthy and in tumorous tissue. Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled

Radio

Attenuation is an important consideration in the modern world of wireless telecommunication. People are daily affected by it as they rely more and more on mobile phones, television, satellite communication, and wireless internet. Attenuation limits the range of radio signals and is affected by the materials a signal must travel through (e. g. air, wood, concrete, rain). See the article on path loss for more information on signal loss in wireless communication. Path loss (or path attenuation) is the reduction in power density ( attenuation) of an Electromagnetic wave as it propagates through space

See also

• Attenuator (electronics)
• Attenuator (genetics)
• Cross section (physics)
• Environmental remediation for natural attenuation
• Mean free path
• Path loss
• Rain Fade
• Radiation length
• Radiography
• Wave propagation

External links

• NIST's XAAMDI: X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database
• NIST's XCOM: Photon Cross Sections Database
• NIST's FAST: Attenuation and Scattering Tables

References