In
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, attenuation (in some contexts, extinction) is the gradual loss of
flux intensity through a
medium. For instance, dark
glasses attenuate
sunlight,
lead
Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
attenuates
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s, and
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
and
air
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing f ...
attenuate both
light and
sound
In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' b ...
at variable attenuation rates.
Hearing protectors help reduce
acoustic flux from flowing into the ears. This phenomenon is called
acoustic attenuation Acoustic attenuation is a measure of the energy loss of sound propagation in media. Most media have viscosity and are therefore not ideal media. When sound propagates in such media, there is always thermal consumption of energy caused by viscosity ...
and is measured in
decibels (dBs).
In
electrical engineering and
telecommunications, attenuation affects the
propagation of waves and
signals
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
in
electrical circuits, in
optical fibers, and in air.
Electrical attenuators and
optical attenuator
An optical attenuator, or fiber optic attenuator, is a device used to reduce the power level of an optical signal, either in free space or in an optical fiber. The basic types of optical attenuators are fixed, step-wise variable, and continuously ...
s are commonly manufactured components in this field.
Background
In many cases, attenuation is an
exponential function of the path length through the medium. In optics and in chemical
spectroscopy, this is known as the
Beer–Lambert law
The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied t ...
. In engineering, 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.] Attenuation also occurs in
earthquakes; when the
seismic waves
A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic energy. ...
move farther away from the
hypocenter, they grow smaller as they are attenuated by the
ground.
Ultrasound
One area of research in which attenuation plays a prominent role, 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.
[Diagnostic Ultrasound, Stewart C. Bushong and Benjamin R. Archer, Mosby Inc., 1991.]
*''Ultrasound attenuation'' measurement in
heterogeneous systems, like
emulsions or
colloids, yields information on
particle size distribution
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 amount, typically by mass, of particles present according to size. Sig ...
. There is an ISO standard on this technique.
*''Ultrasound attenuation'' can be used for
extensional rheology measurement. There are
acoustic rheometer
An acoustic rheometer employs a piezo-electric crystal that can easily launch a successive wave of extensions and contractions into the fluid. It applies an oscillating extensional stress to the system. System response can be interpreted in ter ...
s that employ
Stokes' law for measuring
extensional viscosity
Extensional viscosity (also known as elongational viscosity) is a viscosity coefficient when applied stress is extensional stress. It is often used for characterizing polymer solutions.
Extensional viscosity can be measured using rheometers tha ...
and
volume viscosity Volume viscosity (also called bulk viscosity, or dilatational viscosity) is a material property relevant for characterizing fluid flow. Common symbols are \zeta, \mu', \mu_\mathrm, \kappa or \xi. It has dimensions (mass / (length × time)), and the ...
.
Wave equations which take
acoustic attenuation Acoustic attenuation is a measure of the energy loss of sound propagation in media. Most media have viscosity and are therefore not ideal media. When sound propagates in such media, there is always thermal consumption of energy caused by viscosity ...
into account can be written on a fractional derivative form.
[S. P. Näsholm and S. Holm, "On a Fractional Zener Elastic Wave Equation," Fract. Calc. Appl. Anal. Vol. 16, No 1 (2013), pp. 26–50,]
Link to e-print
/ref>
In homogeneous media, the main physical properties contributing to sound attenuation are viscosity and thermal conductivity.[G. Kirchhoff, "Ueber den Einfluss der Wärmeleitung in einem Gase auf die Schallbewegung", Ann. Phys. , 210: 177-193 (1868)]
Link to paper
/ref>[S. Benjelloun and J. M. Ghidaglia, "On the dispersion relation for compressible Navier-Stokes Equations,]
Link to Archiv e-printLink to Hal e-print
/ref>
Attenuation coefficient
Attenuation coefficient
The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...
s 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:
: