representation of Various Noises

[Pardeep-singh]

Can any body tell me how to represent various noises in equation form

 

[jiripolivka]

As the electrical noise is a random process, it is not possible to express it as an equation. You should read a good textbook on noise (Van der Ziel: Noise) where you will see that a noise can be mathematically expressed as "representation" or "realization", "distribution" or otherwise.

 

[E Kafeman]

Noise related to electro magnetic waves is called thermal noise. It have an simple voltage equation, Et = 2√(KTBRs).
This noise is also known as Johnsson noise or Nyqvist noise and the formula is often reformatted as temperature, current or resistive noise.
That is all there is to know about noise...almost.

Many experts have done their own noise definitions & expressions for special types of noise relative frequency/temperature/medium.
Two main types of expression are statistical distribution and power spectral density.
Many special cases of noise have their own names. Common names in RF-area are white noise, pink noise, dark current, gaussian noise, channel noise.
What differ these types of noise and respective mathematical model is easy to find on internet.
Diffusion noise (Ziel) and shot noise are mainly of interest when designing diodes and transistors.
NF (noise figure) is especially important to keep low at input stages for all analog sources (radio waves, sound waves, light, pressure..).


It is seldom the noise in it self that we are interested to know, it is resulting SNR & CNR that tells us in what amount noise is going to affect final signal.
In digital signal world do they have a similar signal quality-index, BER.
Most of these types of noises can be represented in several different ways in mathematical equations or statistical models.
Depending on type of noise can it be of more interest to describe it in linear or logarithmic(dB) and in time domain or frequency domain.
Information about these different kinds of noises and several more types are already described in other, easy to find forums.

 

[Pardeep-singh]

Hey , thanks for it. Its good to have this information

---------- Post added at 09:20 ---------- Previous post was at 09:13 ----------

What kind of noise will occur if specificaly Acoustic wave propagtion medium is used ofr communication

 

[E Kafeman]

Observe that noise does not occur, it is there all the time and the kind of noise is still thermal related.
For acoustic waves (pressure variations) in air as medium is it thermal noise that limits how low levels of sound that can be measured.
This noise is mainly due to random movement in air molecules. Air movements, different temperature layers and humidity adds distortion and variations in attenuation for a specific wave but that is more related to distortion then noise.
Acoustic waves in other kind of media such as fluids or other gases then air, and it is still thermal noise but source is then of course molecules of actual media.
Solid metals as medium behaves much the same as for electric waves regarding signal transmission and reflection. Also thermal noise is the same.
Empty space -> not much media that can transfer acoustical sound waves (pressure variations) but also very low thermal noise level.

 

[Pardeep-singh]

Hey Thanks Kafeman, i am specific about wireless communication in Oil well using acoustic wave communication and need to know what all noises can occur because need made a model of this process. so help if u can

 

[E Kafeman]

If you are using good low noise transducers do I not think thermal noise is the limiting factor in this case, at least not below 10-20 kHz but will also depend on oil temperature viscosity, pressure and well location. It is probably more like in sea water, less relevant sound sources adds an relative high background sound level which will limit your wireless function.
Typical loud sound sources can be a mixture of turbulent streaming oil/gas in a crushed rocky ground, distant drilling/pumping or movements in earth.
My only real experience about sound-waves in oil is that I have used silicon oil to protect underwater electronics.

 

[Pardeep-singh]

Thanks for this, really helpful,

 

[FvM]

I guess, you are talking about noise generated by fluid flow, particularly turbulences. An exact modelling of turbulent flows is mostly beyond the capabilities of computational fluid dynamics, so I think, aquiring empirical noise spectra for typical flow situations is the best you can do.

 

[Pardeep-singh]

Ok, this is helpful. Ya again how to plot this Empirical Noise Spectra in Matlab...

 

[FvM]

FFT (power spectrum) plot of aquired data is easy. The question is, what kind of noise source should be used in a simulation of the communication channel. If you want to simulate it in time domain, you can e.g. replay the aquired noise data. Or design a noise source from a pseudo random generator and a filter representing the spectral shape.