attenuation

(physics)

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. Various mechanisms can give rise to attenuation. Among the most important are geometrical attenuation, absorption, and scattering.

For unconfined radiation from a point source in free space, the power density (watts per square meter) decreases in proportion to the square of the distance. The power densities, I₁ and I₂, at distances r₁ and r₂ from the source, are related by Eq. (1).
1.
See also Inverse-square law.

If the signal, in a parallel beam so that there is no geometrical attenuation, passes through a lossy medium, absorption reduces the power level, I, exponentially with distance, x, according to Eq. (2), where a is the attenuation coefficient.
2.
See also Absorption; Absorption of electromagnetic radiation; Sound absorption.

Scattering is said to occur if the power is not absorbed in the medium but scattered from inhomogeneities. See also Scattering of electromagnetic radiation.

More complicated situations occur with guided waves, such as acoustic waves in pipes or electromagnetic waves in transmission lines or waveguides, where absorption may take place and irregularities may cause reflection of some power. See also Transmission lines; Waveguide.

In electric circuits, constituent elements are often described as attenuators when they reduce the level of signals passing through them. See also Attenuation (electricity).

Attenuation is usually measured in terms of the logarithm of the power ratio, the units being the neper or the decibel. See also Decibel; Neper.

Attenuation (electricity)

The exponential decrease with distance in the amplitude of an electrical signal traveling along a very long uniform transmission line, due to conductor and dielectric losses. If the peak voltage at the sending end of the transmission line is denoted by V₀, the peak voltage at a distance x from the sending end is given by the equation below, where α is the attenuation constant of the line.

Attenuators find numerous applications, typical examples being: in a signal generator, to vary the amplitude of the output signal; and in the input line to a television receiver that is very close to a television transmitter, so that overloading can be avoided. See also Signal generator.

Attenuators for the dc (steady voltage) to very high-frequency (VHF) range (frequencies from 0 to 300 MHz) often contain resistors arranged in T or π configurations.

Piston attenuators (sometimes called waveguide-beyond-cutoff attenuators) are used at both intermediate and microwave frequencies (see illustration). The attenuation is varied by altering the separation between the two coils. The circular tube acts as a waveguide beyond cutoff, and the launching system is designed so that only one mode is excited in it.

Piston attenuator.

A variable waveguide attenuator can be produced by moving a lossy vane either sideways across the waveguide or into the waveguide through a longitudinal slot.

The rotary vane attenuator is a very popular instrument. At the input end, there is a rectangular-to-circular waveguide taper containing a fixed lossy vane perpendicular to the incident electric vector. The central section contains a lossy vane diametrically across a circular waveguide that can be rotated, and the output section is a mirror image of the input section.

Many different techniques for measuring attenuation have been devised. The power-ratio method is widely used. The simplest configuration requires only a stable well-matched filtered source and a well-matched low-drift power meter. Substitution methods of attenuation measurement are also very popular.

Low values of attenuation can be determined accurately by making reflection coefficient measurements on the device under test with a sliding short behind it. Several bridge techniques for measuring attenuation have been devised.

The attenuation in a waveguide can be found by making Q measurements on resonant sections of different lengths.

When only moderate accuracy (on the order of ± 0.5 dB) is required over a wide frequency range, a leveled swept source can be connected to the device under test, and the emerging signal can be fed to a diode detector that is followed by a logarithmic amplifier and oscilloscope.

Network analyzers yield both the magnitude and phase angle of the transmission and reflection coefficients of the device under test over a wide frequency range. By using ingenious calibration and computer-correction techniques, high accuracy can be achieved. See also Transmission lines.

Frequency dependent attenuation of electromagnetic radiation in standard atmosphere.

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. ^[1]

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.^[2]

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 (α) 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:^[2]

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:

1. geometric dispersion caused by distribution of the seismic energy to greater volumes
2. 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 1.022MeV, 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 signals.

Attenuation is caused by several different factors, but primarily scattering and absorption.The scattering of light is caused due to 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.^[3] Attenuation in fibre optics can be quantified using the following equation:^[4]

Applications

In optical fibers, 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 attenuators 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

• Path Loss
• Rain Fade
• Radiation length - EM energy loss for high energy photons
• Attenuator (genetics)
• Attenuator (electronics)
• Cross section (physics)
• Mean free path
• Radiography
• Wave propagation

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

References

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