In physics, attenuation is the gradual loss in intensity of any kind of flux through a
medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by
lead, and light and sound are attenuated by water.
In electrical engineering and telecommunications, attenuation affects the propagation of waves
and signals in electrical circuits, in optical fibers, and in air.
Background In many cases, attenuation is an exponential
function of the path length through the medium. In chemical spectroscopy, this is known as
the Beer-Lambert law. In engineering, attenuation is usually measured
in units of decibels per unit length of medium and is represented by the attenuation coefficient
of the medium in question.
Attenuation also occurs in earthquakes; when the seismic waves move farther away from the
epicenter, they grow smaller as they are attenuated by the ground.
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 traveling through a medium, one can adjust
the input signal amplitude to compensate for any loss of energy at the desired imaging
depth. Ultrasound attenuation measurement in heterogeneous
systems, like emulsions or colloids, yields information on particle size distribution.
There is an ISO standard on this technique. Ultrasound attenuation can be used for extensional
rheology measurement. There are acoustic rheometers that employ Stokes' law for measuring extensional
viscosity and volume viscosity. Wave equations which take acoustic attenuation
into account can be written on a fractional derivative form, see the article on acoustic
attenuation or e.g. the survey paper. 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 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:
There are two general ways of acoustic energy losses: absorption and scattering, for instance
light scattering. Ultrasound propagation through homogeneous media is associated only with
absorption and can be characterized with absorption coefficient only. Propagation through heterogeneous
media requires taking into account scattering. Fractional derivative wave equations can be
applied for modeling of lossy acoustical wave propagation, see also acoustic attenuation
and Ref. Light attenuation in water
Shortwave radiation emitted from the sun have wavelengths in the visible spectrum of light
that range from 360 nm to 750 nm. When the sun’s radiation reaches the sea-surface,
the shortwave radiation is attenuated by the water, and the intensity of light decreases
exponentially with water depth. The intensity of light at depth can be calculated using
the Beer-Lambert Law. In clear open waters, visible light is absorbed
at the longest wavelengths first. Thus, red, orange, and yellow wavelengths are absorbed
at higher water depths, and blue and violet wavelengths reach the deepest in the water
column. Because the blue and violet wavelengths are absorbed last compared to the other wavelengths,
open ocean waters appear deep-blue to the eye.
In near-shore waters, sea water contains more phytoplankton than the very clear central
ocean waters. Chlorophyll-a pigments in the phytoplankton absorb light, and the plants
themselves scatter light, making coastal waters less clear than open waters. Chlorophyll-a
absorbs light most strongly in the shortest wavelengths of the visible spectrum. In near-shore
waters where there are high concentrations of phytoplankton, the green wavelength reaches
the deepest in the water column and the color of water to an observer appears green-blue
or green. 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 groundshaking. A seismic wave loses energy as it propagates through
the earth. This phenomenon is tied in 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
dispersion as heat Electromagnetic
Attenuation decreases the intensity of electromagnetic radiation due to absorption or scattering
of photons. 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 1.022 MeV, pair production.
Radiography See Attenuation coefficient.
Optics
Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity
of the light beam with respect to distance travelled through a transmission medium. Attenuation
coefficients in fiber optics usually use units of dB/km through the medium due to the relatively
high quality of transparency of modern optical transmission media. The medium is typically
a fiber of silica glass that confines the incident light beam to the inside. Attenuation
is an important factor limiting the transmission of a digital signal across large distances.
Thus, much research has gone into both limiting the attenuation and maximizing the amplification
of the optical signal. Empirical research has shown that attenuation in optical fiber
is caused primarily by both scattering and absorption.
Attenuation in fiber optics can be quantified using the following equation:
Light scattering
The propagation of light through the core of an optical fiber is based on total internal
reflection of the lightwave. Rough and irregular surfaces, even at the molecular level of the
glass, can cause light rays to be reflected in many random directions. This type of reflection
is referred to as "diffuse reflection", and it is typically characterized by wide variety
of reflection angles. Most objects that can be seen with the naked eye are visible due
to diffuse reflection. Another term commonly used for this type of reflection is "light
scattering". Light scattering from the surfaces of objects is our primary mechanism of physical
observation. Light scattering from many common surfaces can be modelled by lambertian reflectance.
Light scattering depends on the wavelength of the light being scattered. Thus, limits
to spatial scales of visibility arise, depending on the frequency of the incident lightwave
and the physical dimension of the scattering center, which is typically in the form of
some specific microstructural feature. For example, since visible light has a wavelength
scale on the order of one micrometer, scattering centers will have dimensions on a similar
spatial scale. Thus, attenuation results from the incoherent
scattering of light at internal surfaces and interfaces. Incrystalline materials such as
metals and ceramics, in addition to pores, most of the internal surfaces or interfaces
are in the form of grain boundaries that separate tiny regions of crystalline order. It has
recently been shown that, when the size of the scattering center is reduced below the
size of the wavelength of the light being scattered, the scattering no longer occurs
to any significant extent. This phenomenon has given rise to the production of transparent
ceramic materials. Likewise, the scattering of light in optical
quality glass fiber is caused by molecular-level irregularities in the glass structure. Indeed,
one emerging school of thought is that a glass is simply the limiting case of a polycrystalline
solid. Within this framework, "domains" exhibiting various degrees of short-range order become
the building-blocks of both metals and alloys, as well as glasses and ceramics. Distributed
both between and within these domains are microstructural defects that will provide
the most ideal locations for the occurrence of light scattering. This same phenomenon
is seen as one of the limiting factors in the transparency of IR missile domes.
UV-Vis-IR absorption In addition to light scattering, attenuation
or signal loss can also occur due to selective absorption of specific wavelengths, in a manner
similar to that responsible for the appearance of color. Primary material considerations
include both electrons and molecules as follows: At the electronic level, it depends on whether
the electron orbitals are spaced such that they can absorb a quantum of light of a specific
wavelength or frequency in the ultraviolet or visible ranges. This is what gives rise
to color. At the atomic or molecular level, it depends
on the frequencies of atomic or molecular vibrations or chemical bonds, how close-packed
its atoms or molecules are, and whether or not the atoms or molecules exhibit long-range
order. These factors will determine the capacity of the material transmitting longer wavelengths
in the infrared, far IR, radio and microwave ranges.
The selective absorption of infrared light by a particular material occurs because the
selected frequency of the light wave matches the frequency at which the particles of that
material vibrate. Since different atoms and molecules have different natural frequencies
of vibration, they will selectively absorb different frequencies of infrared light.
Applications In optical fibers, attenuation is the rate
at which the signal light decreases in intensity. For this reason, glass fiber is used for long-distance
fiber optic cables; plastic fiber has a higher attenuation and, hence, shorter range. There
also exist optical attenuators that decrease the signal in a fiber optic cable intentionally.
Attenuation of light is also important in physical oceanography. This same effect is
an important consideration in weather radar, as raindrops absorb a part of the emitted
beam that is more or less significant, depending on the wavelength used.
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. In addition,
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.
Attenuation limits the range of radio signals and is affected by the materials a signal
must travel through. See the article on path loss for more information on signal loss in
wireless communication. See also
References
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 Underwater Radio Communication