Attenuation
From AmericolaWiki
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 centimetre of medium (dB/cm) and is represented by the attenuation coefficient of the medium in question. [1]
Contents |
[edit] Attenuation and 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]
[edit] Attenuation (earthquake)
The energy, with which an earthquake affects a location, depends from the running distance. The attenuation 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 volumes as great
- dispersion as heat
[edit] Attenuation coefficient
Attenuation coefficients are used to quantify different media according to how strongly the input ultrasound amplitude decreases as a function of dB/cm. The attenuation coefficient (<math>\alpha</math>) can be used to determine total attenuation in the medium using the following formula:
<math>\mathrm{Attenuation(dB)} = \alpha\mathrm{(dB/MHz*cm)}\times\mathit{l}\mathrm{(cm)}\times\mathrm{f(MHz)}</math>
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]
| Material | <math>\alpha\mathrm{(dB/MHz*cm)}</math> |
|---|---|
| Lung | 41 |
| Bone | 20 |
| Kidney | 1.0 |
| Liver | 0.94 |
| Fat | 0.63 |
| Blood | 0.18 |
| Brain | 0.85 |
| Water | 0.0022 |
[edit] Attenuation and optics
Attenuation 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 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 much faster speed of light as compared to sound. 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]
<math>\mathrm{Attenuation(dB)} = 10\times\log_{10}\left(\frac{\mathrm{Output\ Intensity(W)}}{\mathrm{Input\ Intensity(W)}}\right)</math>
[edit] See also
- Attenuator (genetics)
- Attenuator (electronics)
- Cross section (physics)
- Mean free path
- Radiography
- Wave propagation
[edit] 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
[edit] References
de:Dämpfung it:Attenuazione nl:Demping nn:Demping pl:Atenuacja
