Information AboutAttenuation |
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Attenuation is the reduction in Amplitude and Intensity of a signal. Signals may be attenuated exponentially by transmission through a medium, in which case attenuation is usually reported in dB with respect to distance traveled through the medium. 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. Essentials of Ultrasound Physics, James A. Zagzebski, Mosby Inc., 1996. 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.Diagnostic Ultrasound, Stewart C. Bushong and Benjamin R. Archer, Mosby Inc., 1991. Attenuation coefficient Attenuation coefficients are used to quantify different media according to how strongly the transmitted ultrasound amplitude decreases as a function of frequency. The attenuation Coefficient () 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. 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: EARTHQUAKE The energy, with which an Earthquake affects a location, depends from 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 up to the Dispersion of the seismic energy with the distance. There are two types of dissipated energy: # geometric dispersion caused by distribution of the seismic energy to greater volumes ELECTROMAGNETIC Attenuation decreases the intensity of Electromagnetic Radiation due to 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 . 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. 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 absorption (by Water molecules) and scattering (by suspended particulates). 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. 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 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
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