Versions Compared

Old Version 7

changes.mady.by.user Unknown User (tanc)

Saved on

compared with

New Version 8

changes.mady.by.user Unknown User (minhn)

Saved on

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

...

Amplifiers / optical (Minh) -> completed

frequency (Thai) -> completed

depending variables (Phat) -> completed

---------------------------------------------------------------------------------------------------------

...

  1. Reflection and the Ray Model of Light - Lesson 1 - Reflection and its Importance
    http://www.physicsclassroom.com/class/refln/Lesson-1/Specular-vs-Diffuse-Reflection
  2. B.G. Potter. Module 3 - Attenuation in optical fibers
    http://opti500.cian-erc.org/opti500/pdf/sm/Module3%20Optical%20Attenuation.pdf

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

 

Frequency


Background 

The requirement is to understand what factors and dependencies cause attenuation in fiber optic.


Initial questions

How is attenuation dependent on frequency?
 
 

Selected problem

All questions were selected to answer in detail.
 
 

My explanation

Attenuation has relation in increasing with frequency, which means the higher the frequency the higher the attenuation.

 

Critical evaluation 

More details and reading as well as reasoning that need to be provided.

 

Finding more information 

More information is obtained by reading books and searching for online information.

 

Final theory

Attenuation is a loss of intensity in an energy beam as it passes through a substance or object or the energy loss of signal transmission through a given medium. Coefficient is a quantitative measure of either an effect or a property. It is the ratio by which a change in one property will change another property. The attenuation coefficient is thus a ratio comparing the loss of intensity to the distance that the energy beam passes through the material. The units used to express the intensity will depend on the precise energy beam. 

The attenuation coefficient is also used in ultrasound. When ultrasound waves propagate in a medium, energy is removed from the ultrasound waves by two main processes, absorption and scattering.The mechanism that removes energy from the ultrasound waves is called “attenuation”. Ultrasound is absorbed by the medium if part of the wave energy is converted into other forms of energy, such as heat. The absorption is frequency dependence. When ultrasound waves propagate, they not only become smaller in amplitude but they also change shape. Absorption in the body has a major effect on the penetration depth. It would limit the detectable penetration of the ultrasound waves in the body or the maximum depth at which tissues can be imaged. The attenuation of ultrasound in a material could be described by the attenuation coefficient in the units of decibels per centimetre per megahertz (dB/cm/MHz). 

Attenuation always serves as a measurement parameter that leads to the formation of theories to explain physical or chemical phenomenon, which decreases the ultrasonic intensity. Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. The attenuation coefficient (α) can be used to determine total attenuation in dB in the medium using the following formula:

Image Added

α: attenuation coefficient
 ℓ: medium length
 ƒ: frequency of the incident ultrasound beam

The attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below:

Materialα(dB/(MHz·cm))
Air1.64 (20°C)
Blood0.2
Bone, cortical6.9
Bone, trabecular9.94
Brain0.6
Breast0.75
Cardiac0.52
Connective tissue1.57
Dentin80
Enamel120
Fat0.48
Liver0.5
Marrow0.5
Muscle1.09
Tendon4.7
Soft tissue (average)0.54
Water0.0022

Figure 1. Diffuse reflection. Copied from [1]


 
Reference
 
  1. Culjat, Martin O.; Goldenberg, David; Tewari, Priyamvada; Singh, Rahul S. (2010). "A Review of Tissue Substitutes for Ultrasound Imaging". Ultrasound in Medicine & Biology 36 (6): 861–873.
  2. Tole, Nimrod M. (2005). Basic physics of ultrasonographic imaging. 
    Chapter 3: http://www.isradiology.org/isr/docs_books/basic/Chapter3.pdf