Attenuation 40735 224987841 2008-07-11T09:39:39Z Lee Carre 919577 /* Radio */ added main article link {{This|physics|Attenuator}} [[Image:Micrwavattrp.png|thumb|Frequency dependent attenuation of electromagnetic radiation in standard atmosphere.]] '''Attenuation''' is the reduction in [[amplitude]] and [[intensity]] of a signal. Signals may attenuate exponentially by transmission through a medium, or by increments calculated in electronic circuitry or set by variable controls. 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.<ref name=Zagzebski>Essentials of Ultrasound Physics, James A. Zagzebski, Mosby Inc., 1996.</ref> ==Ultrasound== One area of research in which attenuation figures strongly is in [[ultrasound]] physics. 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.<ref name="Bushong">Diagnostic Ultrasound, Stewart C. Bushong and Benjamin R. Archer, Mosby Inc., 1991.</ref> *''Ultrasound attenuation'' measurement in [[heterogeneous]] systems, like [[emulsion]]s or [[colloid]]s yields information on [[particle size distribution]]. There is ISO standard on this technique.<ref>ISO 20998-1:2006 "Measurement and characterization of particles by acoustic methods"</ref> *''Ultrasound attenuation'' can be used for [[extensional rheology]] measurement. There are [[acoustic rheometer]]s that employ [[Stokes' law]] for measuring [[extensional viscosity]] and [[volume viscosity]]. ===Attenuation coefficient=== [[Attenuation coefficient]]s are used to quantify different media according to how strongly the transmitted ultrasound amplitude decreases as a function of frequency. The attenuation [[coefficient]] (<math>\alpha</math>) can be used to determine total attenuation in dB/cm in the medium using the following formula: <center> <math>\mathrm{Attenuation[dB]} = \alpha\mathrm{[dB/(MHz*cm)]}\cdot\mathit{l}\mathrm{[cm]}\cdot\mathrm{f[MHz]}</math> </center> As this equation shows, besides the medium length and attenuation coefficient, attenuation is also linearly dependent on the [[frequency]] of the incident ultrasound beam. 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:<ref name="Bushong"/> <center> {| border="1" |+ ! Material !! <math>\alpha\mathrm{(dB/MHz*cm)}</math> |- | Lung | 41 |- | Bone | 20 |- | Kidney | 1.0 |- | Liver | 0.94 |- | Fat | 0.63 |- | Blood | 0.18 |- | Brain | 0.85 |- | Water | 0.0022 |}</center> There are two general ways of acoustic energy losses: [[absorption]] and [[scattering]], for instance [[light scattering]].<ref> Bohren,C. F. and Huffman, D.R. "Absorption and Scattering of Light by Small Particles", Wiley, (1983), isbn= 0-471-29340-7 </ref> Ultrasound propagation through [[homogeneous]] media is associated only with [[absorption]] and can be characterised with [[absorption coefficient]] only. Propagation through [[heterogeneous]] media requires taking into account scattering.<ref> Dukhin, A.S. and Goetz, P.J. "Ultrasound for characterizing colloids", Elsevier, 2002</ref> ==Earthquake== The energy with which an [[earthquake]] affects a location depends on the running [[distance]]. 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). This [[phenomenon]] is tied in to the [[dispersion]] of the seismic energy with the distance. There are two types of [[Dissipation|dissipated]] energy: # geometric dispersion caused by distribution of the seismic energy to greater volumes # dispersion as heat ==Electromagnetic== Attenuation decreases the intensity of [[electromagnetic radiation]] due to [[absorption (electromagnetic radiation)|absorption]] or [[scattering]] of [[photon]]s. Attenuation does not include the decrease in intensity due to [[inverse-square law]] geometric spreading. 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 1.022MeV, [[pair production]]. ===Radiography=== See [[Attenuation coefficient]] article. ===Optics=== Attenuation of light by cloudy water is called [[turbidity]], and by interstellar dust, [[extinction (astronomy)]]. 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 signal]]s. 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. 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.<ref name="Vardeny">Telecommunications: A Boost for Fibre Optics, Z. Valy Vardeny, Nature 416, 489–491, 2002.</ref> Attenuation in fibre optics can be quantified using the following equation:<ref>{{cite news|url=http://web.archive.org/web/20060224231950/http://floti.bell.ac.uk/MATHSPHYSICS/5attenua.htm |title=Fibre Optics|publisher=Bell College}}</ref> <center> <math>\mathrm{Attenuation(dB)} = 10\times\log_{10}\left(\frac{\mathrm{Input\ Intensity(W)}}{\mathrm{Output\ Intensity(W)}}\right)</math> </center> ===Applications=== In [[optical fiber]]s, attenuation is the rate at which the signal light decreases in intensity. 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 attenuator]]s which decrease the signal in a fiber optic cable intentionally. Attenuation of light is also important in [[physical oceanography]]. Here, attenuation is the decrease in light intensity with depth due to [[water absorption|absorption by water molecules]] and scattering by suspended particulates. This same effect is an important consideration in [[weather radar#Attenuation|weather radar]] as rain drops absorb a part of the emitted beam that is more or less significant depending on the wavelength used. The attenuation of photons, particularly of those in the x-ray spectrum, is important in the field of [[medical physics]]. 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. ===Radio=== {{main|Path loss}} 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. ==See also== *[[Attenuator (electronics)]] *[[Attenuator (genetics)]] *[[Cross section (physics)]] *[[Electrical impedance]] *[[Environmental remediation]] for '''natural attenuation''' *[[Mean free path]] *[[Path loss]] *[[Rain Fade]] *[[Radiation length]] *[[Radiography]] *[[Wave propagation]] == External links == *[http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html NIST's XAAMDI: X-Ray Attenuation and Absorption for Materials of Dosimetric Interest Database] *[http://physics.nist.gov/PhysRefData/Xcom/Text/XCOM.html NIST's XCOM: Photon Cross Sections Database] *[http://physics.nist.gov/PhysRefData/FFast/Text/cover.html NIST's FAST: Attenuation and Scattering Tables] ==References== {{reflist}} [[Category:Physics]] [[Category:Communication engineering]] [[Category:Acoustics]] [[de:Dämpfung]] [[fr:Atténuation]] [[it:Attenuazione]] [[nl:Demping]] [[nn:Demping]] [[pl:Tłumienie]]