What is Filters: Definition and 150 Discussions

A particulate air filter is a device composed of fibrous, or porous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. Filters containing an adsorbent or catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants such as volatile organic compounds or ozone. Air filters are used in applications where air quality is important, notably in building ventilation systems and in engines.
Some buildings, as well as aircraft and other human-made environments (e.g., satellites, and space shuttles) use foam, pleated paper, or spun fiberglass filter elements. Another method, air ionizers, use fibers or elements with a static electric charge, which attract dust particles. The air intakes of internal combustion engines and air compressors tend to use either paper, foam, or cotton filters. Oil bath filters have fallen out of favour aside from niche uses. The technology of air intake filters of gas turbines has improved significantly in recent years, due to improvements in the aerodynamics and fluid dynamics of the air-compressor part of the gas turbines.

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  1. D

    What kind of filters to use for audio LED spectrum analyzer?

    Hi, all I've been reading these forums for a while, and now that I have a question, I've finally decided to join. The question I have is school-related, but not really direct coursework since it's very open-ended. I am part of a group (in electronics class) that is building an LED...
  2. M

    Digital Signal Processing - signals, reconstruction, filters

    Homework Statement No problem statement - just the following questions. Homework Equations 1.Display on a CRT is most similar to what reconstruction filter? 2.How many samples are required to represent a given signal without loss of information? 3.What signals can be reconstructed...
  3. S

    Stargazing Amateur telescope eyepiece filters

    So, I have a relatively small (6") Orion Dobsonian with a 25mm and 7.5mm Plossl eyepieces and live in a somewhat light polluted area. Because of this, I am considering purchasing an eyepiece filter to reduce light pollution and enhance nebula/planetary viewing. I was wondering how this would...
  4. N

    Why aren't interference filters also polarizers?

    After all, the un-passed light reflects off a transparent medium with a definite index of refraction, right? HELP!
  5. C

    Can filters remove chlorine and fluoride from drinking water?

    I am not sure I want to debunk something here - but I would like some more info on something a co-worker spends money upon. 'Bill" spends several hundred dollars a month on various things in health food stores... echinecia just being one. He recently said he has a 'filter' home that takes...
  6. N

    Tabulation of the Standard Astronomical Filters

    Hi all, Does anyone know where I can find a tabulation of the standard Sloan/Johnson/etc astronomical filters? Any help would be greatly appreciated! :-) Natski
  7. P

    Optical band-pass filters (radome coatings)

    Hi, I am reading about a new field that I am not familiar with at all, and I was hoping that someone could give me some info on that. The general idea is that if you have a conductive layer, with a periodic array of apertures perforated in it (such as crosses), you can design a band-pass...
  8. U

    Exploring the Math of Kalman Filters: A College Calculus Student's Guide

    I have been doing some reading on Kalman filters trying to figure where to start. I have done some college level calculus, but clearly I don't currently know enough to understand the math involved. Where is a good place for me to start? I downloaded a copy of the linear algebra book by Jim...
  9. B

    Filters Explained: What Are DSP Filters?

    Filters what are they?? Can someone explain to me what are filters (DSP) in plain english? Fir filter, IIR filter,... Chebyshev, Butterworth,... Bands (stop, pass, lowpass, highpass).
  10. P

    Finding the Cutoff Frequency for RL High-Pass/Low-Pass Filters

    Does anyone know the formula to find the cutoff frequency for an RL High-Pass/ Low-Pass Filter? I know with RC filters it's 1/ (2ΩRC) ,but this does not work for RL filters.
  11. T

    Kalman Filters & accelerometers - linear vs extended, what do I need?

    Hello, I have a problem for work where I am attempting to integrate (a single) accelerometer readout in order to gain insight into the resulting velocity and position curves with respect to time. From experience I know this is a tricky task due to drift and error being amplified when...
  12. 0

    AR coatings and IR filters

    can anybody tell me the difference (terms of specific application) b/w the AR coatings and Interference filters?
  13. B

    How Does Rotating Polaroid Filters Affect Light Intensity?

    Homework Statement Two sheets of polaroid are aranged as polarized and analyzer. Suppose that the preferential direction of the second sheet is rotated by an angle \phi about the direction of incidence and then rotated by an angle \alpha about the vertical direction. If unpolarized light of...
  14. S

    Cascaded low pass filters problem

    Homework Statement Given the coupled RC network shown below (see attachment), show that the voltage transfer function is \frac{Vout}{Vin}=\frac{1}{(1-\omega^2C^2R^2)+3j\omega CR} Hint: \frac{Vout}{Vin}=\frac{V1}{Vin}\frac{Vout}{V1} Homework Equations For Capacitor, Z=\frac{1}{j\omega C}...
  15. A

    Carbon filters? part soloution of global warming?

    When air goes through a carbon filter it separates the carbon and turns it into water and lithium carbonate. Could the lithium carbonate be sold and that money used to buy parts for hydro electricity plants that use the water, or the water be used for drinking as it is already pure?
  16. I

    Communication Problems (Filters, BW, Bit rate, FM)

    Homework Statement A 1kHz analogue signal is to be encoded using 8-level Pulse Code Modulator. This specification results in a bit rate of 5.5K bits/sec. The signal is to be transmitted over a 1 km distance with a 10kHz bandwidth on a Frequency Modulated, F.M., Carrier. The carrier frequency...
  17. G

    High Pass filter & Low Pass Filters features

    Hi I want your help to know what's the features of HPF and LPF, and their applications area? Thanks
  18. G

    Low pass & High pass filters

    Hi I want to know the features of Low pass and High pass filters, and their application areas. Thanks
  19. I

    Kalman filters, gyros and accelerometers in 3DOF, Oh my

    Can someone please explain to me how kalman filters would help a system with accelerometers and Gyros work better together? Thanks
  20. T

    % lost to liquid crystal filters

    Hi, I've got a random question. To my knowledge a liquid crystal filter bends incoming light coming in from various angles towards one particular angle as light leaves the filter. Does anybody know the % loss of light caused by this action?
  21. S

    Most Light Through Polarizing Filters: 0, 45, 90

    Which combo of polarizing filters allows most light to pass through? Each orientation is given with respect to vertical unit of following values given are angles: 1. 0, 45, 90 2. 30, 60, 90 3. 180, 0, 180 4. 90, 135, 180 5. 10,20,100 I tried by using the formulas: I1=0.5Inought...
  22. S

    Are All Filters Both Finer and Coarser Than Themselves?

    I am confused by the definition of fineness on filters. Are all filters both finer and coarser than themselves?
  23. Z

    Polaroid filters question

    Homework Statement A light source is viewed through 2 polaroid filters, A and B (B is infront of A) a) describe carefully what is seen when polaroid B is rotated slowly in its own plane through 180 degrees. b) describe how you would use one of the polaroid filters to determine whether...
  24. C

    Light through polarizing filters

    Since a long time I am intrigued by the following experiment: Polarizing filters remove light polarized at 90° to the filter's polarization axis. If a horizontal polarizer is placed on top of a vertical polarizer, there is minimal light transmission. So far "understandable". Now put a third...
  25. J

    How to design for Fourier filters?

    I am working on an experiment on Fourier optics-optical image processing. In order to get edge enhancement, how do we actually go about high pass filtering. How to design filters? How to decide the filter dimensions? Can anyone please help me?
  26. G

    MATLAB Matlab: How to apply filters to and ECG signal using matlab?

    i need to apply a low pass and high pass filter, as well as a band pass filter, to a plot I've made using MATLAB does anyone know how i can do this?
  27. R

    Understanding Resolution of 852x600 Display with RGB Filters

    Hello, I have somewhat of a technical question and was hoping you would be able to help me. I have a display that is 852 (x3) x 600. This means that that there are 852 x 600 of the red, blue and green filtered pixels. Say however that I am looking at a completely red object, thus only red...
  28. E

    Searching for Intelligent Life: Understanding Radio Wave Filters

    It seems that Earth is constantly being bombarded by radio waves that have been created by natural occurrences. Concerning the search for intelligent life, what sort of filter is used to disseminate between a naturally occurring radio wave and a radio wave created by intelligent life.
  29. E

    Help On My Project ( I Need Circuit Diagram For Active Band Stop Filters)

    I Have A Project To Design And Construct An Active Band Stop Filters I Need Circuit Design Please Thanks
  30. E

    I Need Circuit Diagrams For Active Band Stop Filters Please. Can Anyboby Help Me

    I Have A Project To Design And Construct An Active Band Stop Filters I Need Circuit Design Please Thanks
  31. C

    Modeling Filters in Airflow CFD: How to Simplify and Analyze

    If you wanted to model a filter in an airflow using CFD, how would you go about it? I imagine that an exact model of a filter would be way too complex to model and take vast amounts of time to solve, so there must be some kind of simplification? I was thinking that maybe some known properties of...
  32. A

    Lowpass Butterworth & Bandpass Filters

    Please help me find proofs of the Butterworth lowpass filter and the bandpass filter. for the butterworth. i need to prove (Vo/Vin) = 1 / [1 + (f/fc)^4]^(1/2) for the bandpass: (Vo/Vin) = 1/[1 + {[(w^2) / (wo^2)] - 1} / wRC] my professor said these proofs were on the internet, but i...
  33. U

    Polarization-polarizing filters

    Can someone possibly help me understand what polarizing filters do? My teacher gave us a website to go to but I don't quite get what they are saying. Here is the link to the website: http://www.colorado.edu/physics/2000/polarization/" Any help at all on this topic would be greatly...
  34. H

    Optimizing Air Conditioner Filter Placement for Maximum Efficiency

    I have 2 places to put air filters on my home system. One is a 16x20x1 at the wall intake and the other is 4ft up the same duct at the unit (16x25x1). The filter in the wall is a pain in the ass to change in comparison to the one at the unit. But, they are both usually dirty at 3 months so I...
  35. D

    How Do You Design a Low Pass Filter Using a Butterworth Transfer Function?

    I want to design LPF using Butterworth transfer function & I have the following information Minimum gain attenuation Amin , Maximum gain attenuation Amax , Frequency of passband edge wp, Frequency of stopband edge ws, Amin,Amax,wp & ws can take any value (Using only Butterworth table...
  36. electronic engineer

    Disadvantages of simple filters

    I was wondering about the disadvantages of the simple filters consisting of resistors and capacitors and use active filters instead of it? what are the advantages of active filters over simple filters that make us use it? what about using inductors for such purposes , i found that is...
  37. C

    Polarizing Filters: Explaining the 45 Degrees Effect

    Try this if you're not familiar with the effect: Position two polarizing filters with their polarizing axis' at right angles to one another so they block the light passing through them? Okay, now, keeping the positions of those two, position a third polarizing filter between them with its axis...
  38. L

    Finding the Phase Angle & Understanding Bandstop Filters

    Hi Does anyone know the equation for the phase angle by which v0 leads v1 in a bandstop (parallel resonant) circuit? also, can anyone tell me how (in terms of the equation for the transfer function) a bandstop filter works? Thanks a lot, any help appreciated. :redface:
  39. C

    Designing Bandreject filters

    Is there a way I can design a broadband reject filter using a lowpass filter with cutoff frequency f_1 and a highpass filter with a cutoff frequency f_2,\ (f_1 \ll f_2) , combined in parallel with a summing amplifier? My current design yields a response that isn't a broadband, there is a...
  40. zoobyshoe

    Focus, Attention, And Mental Filters

    Since I got interested enough in Neanderthal man to start reading about him, and because this leads to questions about whether or not modern man could have bred with Neanderthal, I have found myself getting into the habit of constantly checking out the slope of the foreheads of every person I...
  41. CarlB

    Complex phases in Stern-Gerlach filters.

    I found an interesting way of calculating the complex phase changes induced by a series of Stern-Gerlach filters applied to a beam of spin-1/2 fermions. It's obvious enough that I'm sure someone else has seen it before, and I was hoping someone would give me a reference. A Stern-Gerlach...
  42. O

    How do Kalman filters work in robotics?

    Hi, can anyone enlighten me on kalman filter? the maths is overwhelming for me. Thanks in advance!
  43. M

    Incubating the filters two overnights with primary antibody., WESTERN BLOTTING

    Is that okey that I incubate the filter with primary antibody two overnights before adding secondary antibody for detection? This is because I do not have the equipment to detect the signals in the weekend so I have to detect it on a Monday. Due to it I incubate the filter with primary antibody...
  44. matthyaouw

    Can Inserting Tags into Words Bypass Language Filters and Web Blockers?

    On a message board that I'm moderating, we have a language filter installed. We frankly don't mind bad language, but the problem is many of our users acess from colleges, libraries etc, which block pages with bad language in them. One of our users has figured out that inserting tags into a word...
  45. M

    Circuits and low pass filters

    You are given a low pass filter of 50nF and a resistor of 100 KOhm. a) Draw the circuit diagram; b) Calculate the 3dB frequency; c) Calculate the impedance of the series circuit of R and C and draw it in the complex plane at about 2 decades around the 3dB point and at the 3dB point. d)...
  46. B

    Band Pass Filters: Formula for Calculating

    does anyone know the formula for band pass filters. thanks
  47. F

    Why is the frequency cut-off 0.707 in filters

    Hi Guys, There is a formula which shows that the frequency cut-off (0.707) is found from 1/(2*Pi*RC). But why? Could someone show me the proof, so that I could understand? Thx in advance
  48. D

    What is the best and safest way to view the sun through a telescope?

    I have been thinking about picking-up two things for my telescope. A camera adapter and solar filter. How much does a good solar filter usually cost (one that I can buy at a store, not on the internet)? Where can I purchase one? I also hear that one should never buy one that goes over the...
  49. F

    Polarized Filters: The Surprising Effect of Red Light on Highway Headlights

    I was playing around with two polarized filters the other day. I was actually on the highway at the time. Something that I noticed when I had them aligned to block out all light, the only light that seemd to make it through was red. It was pretty cool looking to look at all the oncoming car's...
  50. K

    What are the differences between the filter and net approaches in topology?

    Greetings, Having decided on a physics/math double major next year, I decided to get a head start this summer. After tackling some classical mechanics, my next target is topology. My problem is the following: I have been informed that there are two approaches to the subject, one...
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