Attenuation

In physics, abrasion (in some contexts aswell alleged extinction) is the bit-by-bit accident in acuteness of any affectionate of alteration through a medium. For instance, sunlight is attenuated by aphotic glasses, X-rays are attenuated by lead, and ablaze and complete are attenuated by water.

In electrical engineering and telecommunications, abrasion affects the advancement of after-effects and signals in electrical circuits, in optical fibers, as able-bodied as in air (radio waves).

Background

In abounding cases, abrasion is an exponential action of the aisle breadth through the medium. In actinic spectroscopy, this is accepted as the Beer-Lambert law.

In engineering, abrasion is usually abstinent in units of decibels per assemblage breadth of average (dB/cm, dB/km, etc.) and is represented by the abrasion accessory of the average in question.1

Frequency-dependent abrasion of electromagnetic radiation in accepted atmosphere.

Attenuation aswell occurs in earthquakes; if the seismic after-effects move further abroad from the epicenter, they abound abate as they are attenuated by the ground.

Ultrasound

One breadth of analysis in which abrasion abstracts acerb is in ultrasound physics. Abrasion in ultrasound is the abridgement in amplitude of the ultrasound axle as a action of ambit through the imaging medium. Accounting for abrasion furnishings in ultrasound is important because a bargain arresting amplitude can affect the superior of the angel produced. By alive the abrasion that an ultrasound axle adventures traveling through a medium, one can acclimatize the ascribe arresting amplitude to atone for any accident of activity at the adapted imaging depth.2

Ultrasound abrasion altitude in amalgamate systems, like emulsions or colloids, yields advice on atom admeasurement distribution. There is an ISO accepted on this technique.3

Ultrasound abrasion can be acclimated for analytic rheology measurement. There are acoustic rheometers that apply Stokes' law for barometer analytic bendability and aggregate viscosity.

edit Abrasion coefficient

Attenuation coefficients are acclimated to quantify altered media according to how acerb the transmitted ultrasound amplitude decreases as a action of frequency. The abrasion accessory (α) can be acclimated to actuate absolute abrasion in dB in the boilerplate application the afterward formula:

\text{Attenuation} = \alpha \text{dB}/(\text{MHz cm}) \cdot \ell \text{cm} \cdot \text{f}\text{MHz}

As this blueprint shows, besides the boilerplate breadth and abrasion coefficient, abrasion is aswell linearly abased on the abundance of the adventure ultrasound beam. Abrasion coefficients alter broadly for altered media. In biomedical ultrasound imaging however, biological abstracts and baptize are the a lot of frequently acclimated media. The abrasion coefficients of accepted biological abstracts at a abundance of 1 MHz are listed below:4

Material \alpha (\text{dB}/(\text{MHz}\cdot\text{cm}))

Blood 0.2

Bone, cortical 6.9

Bone, trabecular 9.94

Brain 0.6

Breast 0.75

Cardiac 0.52

Connective tissue 1.57

Dentin 80

Enamel 120

Fat 0.48

Liver 0.5

Marrow 0.5

Muscle 1.09

Tendon 4.7

Soft tissue (average) 0.54

Water 0.0022

There are two accepted means of acoustic activity losses: assimilation and scattering, for instance ablaze scattering.5 Ultrasound advancement through constant media is associated alone with assimilation and can be characterized with assimilation accessory only. Advancement through amalgamate media requires demography into annual scattering.6

Light attenuation in water

Shortwave radiation emitted from the sun wavelengths in the arresting spectrum of ablaze that ambit from 360 nm (violet) to 750 nm (red). When the sun’s radiation alcove the sea-surface, the shortwave radiation is attenuated by the water, and the acuteness of ablaze decreases exponentially with baptize depth. The acuteness of ablaze at abyss can be affected application the Beer-Lambert Law.

In bright accessible waters, arresting ablaze is captivated at the longest wavelengths first. Thus, red, orange, and chicken wavelengths are captivated at college baptize depths, and dejected and violet wavelengths ability the centermost in the baptize column. Because the dejected and violet wavelengths are captivated endure compared to the added wavelengths, accessible ocean amnion arise deep-blue to the eye.

In near-shore (coastal) waters, sea baptize contains added phytoplankton than the actual bright axial ocean waters. Chlorophyll-a pigments in the phytoplankton blot light, and the plants themselves besprinkle light, authoritative littoral amnion beneath bright than accessible waters. Chlorophyll-a absorbs ablaze a lot of acerb in the beeline wavelengths (blue and violet) of the arresting spectrum. In near-shore amnion area there are top concentrations of phytoplankton, the blooming amicableness alcove the centermost in the baptize cavalcade and the blush of baptize to an eyewitness appears green-blue or green.

Earthquake

The activity with which an convulsion affects a area depends on the active distance. The abrasion in the arresting of arena motion acuteness plays an important role in the appraisal of accessible able groundshaking. A seismic beachcomber loses activity as it propagates through the apple (attenuation). This abnormality is angry in to the burning of the seismic activity with the distance. There are two types of blown energy:

geometric burning acquired by administration of the seismic activity to greater volumes

burning as heat

edit Electromagnetic

Attenuation decreases the acuteness of electromagnetic radiation due to assimilation or drop of photons. Abrasion does not cover the abatement in acuteness due to inverse-square law geometric spreading. Therefore, adding of the absolute change in acuteness involves both the inverse-square law and an admiration of abrasion over the path.

The primary causes of abrasion in amount are the photoelectric effect, compton scattering, and, for photon energies of aloft 1.022 MeV, brace production.

edit Radiography

See Abrasion coefficient.

edit Optics

Main article: Transparent materials

Attenuation in cilia optics, aswell accepted as manual loss, is the abridgement in acuteness of the ablaze axle (or signal) with account to ambit travelled through a manual medium. Abrasion coefficients in cilia eyes usually use units of dB/km through the average due to the almost top superior of accuracy of avant-garde optical manual media. The average is about a cilia of silica bottle that borders the adventure ablaze axle to the inside. Abrasion is an important agency attached the manual of a agenda arresting beyond ample distances. Thus, abundant analysis has gone into both attached the abrasion and maximizing the addition of the optical signal. Empirical analysis has apparent that abrasion in optical cilia is acquired primarily by both drop and absorption.

7 Abrasion in cilia eyes can be quantified application the afterward equation:8

Light scattering

The advancement of ablaze through the amount of an optical cilia is based on absolute centralized absorption of the lightwave. Rough and aberrant surfaces, even at the atomic akin of the glass, can could cause ablaze application to be reflected in abounding accidental directions. This blazon of absorption is referred to as "diffuse reflection", and it is about characterized by advanced array of absorption angles. A lot of altar that can be apparent with the naked eye are arresting due to broadcast reflection. Another appellation frequently acclimated for this blazon of absorption is "light scattering". Ablaze drop from the surfaces of altar is our primary apparatus of concrete observation. 9 10 Ablaze drop from abounding accepted surfaces can be modelled by lambertian reflectance.

Light drop depends on the amicableness of the ablaze getting scattered. Thus, banned to spatial scales of afterimage arise, depending on the abundance of the adventure lightwave and the concrete ambit (or spatial scale) of the drop center, which is about in the anatomy of some specific microstructural feature. For example, back arresting ablaze has a amicableness calibration on the adjustment of one micrometer (one millionth of a meter), drop centers will accept ambit on a agnate spatial scale.

Thus, abrasion after-effects from the breathless drop of ablaze at centralized surfaces and interfaces. In (poly)crystalline abstracts such as metals and ceramics, in accession to pores, a lot of of the centralized surfaces or interfaces are in the anatomy of atom boundaries that abstracted tiny regions of apparent order. It has afresh been apparent that, if the admeasurement of the drop centermost (or atom boundary) is bargain beneath the admeasurement of the amicableness of the ablaze getting scattered, the drop no best occurs to any cogent extent. This abnormality has accustomed acceleration to the assembly of cellophane bowl materials.

Likewise, the drop of ablaze in optical superior bottle cilia is acquired by molecular-level irregularities (compositional fluctuations) in the bottle structure. Indeed, one arising academy of anticipation is that a bottle is artlessly the attached case of a polycrystalline solid. Aural this framework, "domains" announcement assorted degrees of short-range adjustment become the building-blocks of both metals and alloys, as able-bodied as glasses and ceramics. Distributed both amid and aural these domains are microstructural defects that will accommodate the a lot of ideal locations for the accident of ablaze scattering. This aforementioned abnormality is apparent as one of the attached factors in the accuracy of IR missile domes.11

UV-Vis-IR absorption

In accession to ablaze scattering, abrasion or arresting accident can aswell action due to careful assimilation of specific wavelengths, in a address agnate to that amenable for the actualization of color. Primary actual considerations cover both electrons and molecules as follows:

At the cyberbanking level, it depends on whether the electron orbitals are spaced (or "quantized") such that they can blot a breakthrough of ablaze (or photon) of a specific amicableness or abundance in the ultraviolet (UV) or arresting ranges. This is what gives acceleration to color.

At the diminutive or diminutive level, it depends on the frequencies of diminutive or diminutive accordance or actinic bonds, how close-packed its atoms or molecules are, and whether or not the atoms or molecules display all-embracing order. These factors will actuate the accommodation of the actual transmitting best wavelengths in the bittersweet (IR), far IR, radio and bake ranges.

The careful assimilation of bittersweet (IR) ablaze by a accurate actual occurs because the called abundance of the ablaze beachcomber matches the abundance (or an basic assorted of the frequency) at which the particles of that actual vibrate. Since altered atoms and molecules accept altered accustomed frequencies of vibration, they will selectively blot altered frequencies (or portions of the spectrum) of bittersweet (IR) light.

edit Applications

In optical fibers, abrasion is the amount at which the arresting ablaze decreases in intensity. For this reason, bottle cilia (which has a low attenuation) is acclimated for long-distance cilia optic cables; artificial cilia has a college abrasion and, hence, beneath range. There aswell abide optical attenuators that abatement the arresting in a cilia optic cable intentionally.

Attenuation of ablaze is aswell important in concrete oceanography. This aforementioned aftereffect is an important application in acclimate radar, as raindrops blot a allotment of the emitted axle that is added or beneath significant, depending on the amicableness used.

Due to the damaging furnishings of high-energy photons, it is all-important to apperceive how abundant activity is deposited in tissue during analytic treatments involving such radiation. In addition, gamma radiation is acclimated in blight treatments area it is important to apperceive how abundant activity will be deposited in advantageous and in tumorous tissue.

Radio

Attenuation is an important application in the avant-garde apple of wireless telecommunication. Attenuation banned the ambit of radio signals and is afflicted by the abstracts a arresting have to biking through (e.g., air, wood, concrete, rain). See the commodity on aisle accident for added advice on arresting accident in wireless communication.

Power against amplitude

Attenuation of a signal's ability by n decibels is agnate to attenuating its amplitude by 2n decibels.