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. carllacan

    Difference between prediction and update on Bayesian Filters

    Hi. I have a couple ofsimple question about Bayesian Filters, just to check that I'm correctly grasping everything. I've been thinking on the difference between the prediction and update steps. I understand the "Physical" difference: in the first one we calculate the probability of the world...
  2. O

    Electronics Lab Prep: Filters & Transfer Functions

    Homework Statement Figures 1 (a), (b), and (c) show low-pass, bandpass, and high-pass filters. Write the transfer function H(ω) for each of these filters, showing the ratio Vout/Vin as a function of the angular frequency ω of the input voltage. -The low-pass filter calculations...
  3. D

    How Do Polarizing Filters Work?

    I've looked into polarizing filters a bit but I'm still not clear on what's actually happening and I was wondering if someone could help clear it up. Does a polarizing filter (such as on a camera or in sunglasses) block a specific mode of polarization and let all others through... or does it...
  4. JJBladester

    Geometric Mean vs. Arithmetic Mean in Bandpass Filters

    Why is the geometric mean used to define the center frequency of a bandpass filter instead of the arithmetic mean? I read in this book that 1. All the lowpass elements yield LC pairs that resonate at ω = 1. 2. Any point of the lowpass response is transformed into a pair of points of the...
  5. M

    Drag Lab Using Coffee Filters

    Homework Statement We are given 3 coffee filters. Using a stopwatch and a meter stick, make measurements to predict how long it would take for those filters to fall down a stairwell. Come up with an experimental method to predict this time. During the lab, we will be told the number of...
  6. G

    Snell's law and optical filters.

    Hi, I recently did a experiment in which I measured the intensity of light out of a optical filter at different angles of incidence. The optical filter was designed such that only light of wavelength 405 nm is transmitted. The wavelength of incident light (the laser I used) was 402 nm. My...
  7. Runei

    Filters and DC/DC Converters in parallel

    Hello there people. Hopefully someone will have a bit of insight into how a possible solution to my problem could look. Im designing a power distribution unit for a piece of equipment, and there has to be galvanic isolation between the primary power input and the loads. I have a DC/DC...
  8. S

    Are My ECG Filters Meeting the Industry Standard for Cuttoff Frequencies?

    Hi guys i built an ECG system(preamplifier and filters), but i faced a problem in the filters: the standard of ECG filters is 0.1 Hz cuttoff frequency for highpass filter and 100 Hz for lowpass filter but i only obtain a good ECG signal when i make the cuttoff frequency of highpass filter...
  9. D

    Why do welders use filters to block out UV light?

    When welder are working on pipelines, they use filters that block out UV light. Why is this precaution necessary? Does the question mean that they wear some type of glasses to help protect their eyes? or does the filter block UV light from going into the pipeline? This is what I...
  10. I

    Passive Filters: Deriving Cutoff Formula

    Hi, Can someone explain me how the cutoff formula for passive filters is derived. Why is it 1/2*pi*R*C? Thanks
  11. J

    FIR filters, impulse responses and unit step function

    Hey everyone, the question I am faced with is this: Which of the following expressions involving δ[n] is incorrect? where "m" is a non zero integer and u[n] is the unit step function. A. u[n-m] = δ[n] + u[n-m+1] B. x[n]δ[n-m] = x[n-m] C. δ[n] = u[n] - u[n-1] D. δ[n]δ[n-m] = 0...
  12. C

    Filters for Real World Accelerations

    Hi, I'm having trouble with the application of filtering to real acceleration data. I have looked at a lot of recommendations, filtering through fft, butterworth high and low pass filters, and I'm not finding something that's working for my data and I'm looking for recommendations. I...
  13. T

    Polarizing Light through Two Filters

    Hello, all. I have a general question related to something I read in a book right now. Not taking a course so I have no professor to ask, so I come to you all! To speed things up, I have this image I took from the book: My question is if the analyzer's polarizing axis was an arbitrary theta...
  14. mishima

    What can I buy as an alternative to lab filters?

    I have tried using coffee filters in the past but they seem to be too thick. The ones I have are 300 g / m^2. Just looking on wikipedia they say filters actually made for chemistry are 80 g / m^2. What other paper can I buy in a rush, in a pinch, from a place like Wal-mart that will be a...
  15. Runei

    DSP - FIR Filters, linear phase conditions

    Hello there I'd like to know if anyone has a proof of why the condition of symmetry or antisymmetry must be followed by a FIR filter, in order for it to have a linear phase response? I've been pouring over this for an exam, and my initial question was what constitutes a linear phase...
  16. O

    Modeling Lowpass and Highpass Filters with MATLAB

    Homework Statement use MATLAB to model a lowpass and highpass filter Homework Equations the high-pass filter: V(sin(ωt)) = 1/C ∫i dt + iR V(ω/(s^2 + ω^2)) = 1/C ∫i dt + iR ⇔LAPLACE⇔ L(i) = (Vωs/R)(1/(s^2 + ω^2)(s + 1/RC)) ⇔ partial fractions... ⇔LAPLACE^-1⇔ V/2 (cos(t) + sin(t)) -...
  17. K

    Side Stream Filters - preferential flow through side stream without using a pump

    Hi, I am trying to understand side stream filtration. Iv been finding it really hard to find out anything on preferential flow (multiphase) through a side stream using a valve to create a pressure drop (general info for my understanding) Im trying to do this without the need for a...
  18. H

    Controls, Filters, Bandpass, Gain

    What is effect on closed loop when one has unity gain at the center of the pass band for an open loop ? Is this desirable? If not how could situation be improved? Are there any consequences for the suggested improvement?
  19. L

    2 high-pass filters with different R and C values, discuss the differences

    You have two high-pass filters with different R and C values discuss the difference in the response. Filter a) R=5kΩ C=250μf Filter b) R=25kΩ C=50μf I calculated that they have the same cut-off frequency but without actually testing the circuit on Multisim and connecting a bode plotter...
  20. Pengwuino

    Do water filters remove fluoride?

    I noticed on the news a thing on tooth care and one thing they said was that drinking tap water is better than bottled water because of the fluoride in the tap water. That made me realize that a good portion of the water I drink is filtered using one of those Brita filters! So do filters, such...
  21. N

    Unlocking the Mystery of Low-Pass Filters in Lock-in Amplifiers

    Hi I am reading about lock-in amplifiers, and I read that the low-pass filter in a lock-in amplifier acts as an "integrator", since much of the noise that ends up in the DC output of the multiplier will integrate to zero. How is it that the low-pass filter can do that? I would appreicate...
  22. P

    Purging Carcinogens from used Cig Filters

    Hi all I am posting this question because i have NO prior experience in chemistry and thought this would be the best place to start for my question regarding carcinogens. I am a horticulture student at my university and have been looking into different substrate materials to implement in...
  23. J

    Effect of filters on frequency response

    I’m interested in audio design and was messing around with NI Multisim building some basic filters. I created a 2nd order low pass filter with capacitors and found that the 2nd stage of the filter impacts the frequency response of 1st filter as illustrated below. This would seem to indicate...
  24. P

    Difference between RF and Signal Processing Filters?

    I've designed both types of filters for years and can't really explain the difference in clear simple terms. I'm hoping somebody here can help me out with that. First, some working terminology: RF filter - a filter with a controlled input and output impedance, usually 50 ohms, to match the...
  25. A

    Understanding the Use of Kalman Filters and Gyros in Aircraft Design

    Hi, I am starting a project that involve building a aircraft . I am just not very clear about using kalman filters, gyros and Magnetometer. From what I read , the kalman filter is used to filter the drift in gyros reading and combine it with the accelerometer reads . In our project we will...
  26. B

    Low-pass filters, frequency selection and H(jw)

    Hello, Recently I've encountered with this topic. Low pass filters and such. But, generally I am not comfortable with this new idea. How does this frequency selection works? I mean, I know the math behind it, like integrals H(jw) etc. But how does a, let's say capacitor in a RC...
  27. S

    Filters for Co K radiation are usually made of iron oxide power

    "filters for Co K radiation are usually made of iron oxide power. If a filter contains 5 mg Fe2O3, what is the transmission factor for the Co K(alpa) line? What is the intensity ratio of Co K(alpha) to Co K(beta) in the filtered beam?" I was wondering if you could expand further as I'm stuck...
  28. D

    Biomedical applications for frequency filters

    Homework Statement Hi what are some biomedical applications for - low pass filter - high pass filter - band pass filter - band stop filter Homework Equations -nil- The Attempt at a Solution filtering out high frequency noise? eg. handphone signals
  29. A

    Do carbon block water filters actually remove some fluoride?

    Hi! This is my first post on physicsforums. I wasn't sure if it belonged here or in the Chemistry forum, but just picked here. I am trying to figure out if my drinking water filter system removes some fluoride from my water. (I know it doesn't remove all as it's not specifically a fluoride...
  30. R

    Stargazing Telescope Filters: Is Something Missing?

    I have a question about the colored filters used for the telescopes. Here I will give an example for the filters of the big telescopes, but I assume that it applies also to the smaller personal telescopes. The range of the visible light goes from about 400nm to about 750 nm. Where the "blue"...
  31. R

    Can You Print Polarizing Filters on Glass or Plastic Substrates?

    Is there any printing technology that can print small enough parallel lines on, say a glass or clear plastic substrate, that would make it a polarizing filter?
  32. R

    Particle size in polarizing filters

    I know that light can be polarized by passing through a medium that has linear particles all oriented in the same direction. But what size particles are necessary to do that?
  33. QuarkCharmer

    Polarized Light and Filters: Understanding the Basics

    Homework Statement Not really a problem, but a question that my professor could not answer (Well the TA). Homework Equations The Attempt at a Solution In an image such as this, illustrating polarized light: I have 2 questions. 1.) Are the filters illustrated with slats as an...
  34. F

    Active Filters in Multiple Order

    Hi Guys. I have a question on the behavior of active filters (low/high/band) that are connected serially in multiple order magnitude. For instance given 2 low pass filters. Now I understand there exists a -6dB/octave cutoff, and generally f_o or the -3dB value is situated at the...
  35. T

    Calculating Gain in Current Mode Filters: Current Gain vs. Voltage Gain

    How to you calculate the gain in a current mode filter (of any current mode circuit in general) For a voltage filter you have G= Vout/vin Do you also use the Current gain (G=Iout/Iin) to simulate the behavior of a current mode filter or should you convert the currents to voltages and...
  36. R

    Are Stern-Gerlach Filters Modeled by Hermitian Operators in Quantum Mechanics?

    Can anyone help me with this simple question based on Feynman's Vol 3? Suppose I have a sequence of filters designed to show the suprising result that extra filters allow previously blocked photons, polarised light, spin one and spin one half particles etc. to get through. Then matrix...
  37. Kawakaze

    Red shift and narrowband filters

    Im planning on trying out some narrowband filters to hopefully dodge the light pollution here and get some nice photos, I was just wondering if red shift ever comes into play, could a distant object be shifted enough to be actually blocked by the filter? I know it depends on the filter and so...
  38. 1

    Signal Processing and Mathematica (filters)

    Mathematica problem: Filter: H(jw)=(3.84+5 (j w))/(5+j (1.11 w^2)) Bandwidth where: |H|>1/Sqrt[2]*Hmax Calculate the bandwidht Tips or solution would be much appriciated Thanks
  39. E

    Why is there a 90 degree phase shift in RC filters?

    Hi! Bear with me: When the frequency is low in an RC series circuit and we take the voltage across the capacitor, the capacitors reactance is high and thus most voltage is across it. But when the frequency is high, the reactance goes down, and little to no voltage is dropped across the...
  40. A

    Band-reject and Low-pass filters (Butterworth)

    Hi friends, I have just started to work on active filters and designed the following: 1. 1st Order HighPass BW (cut off @ 1kHz) 2. 1st Order LowPass BW (cut off @ 2Khz) 3. Band pass using above two (1kHz to 2kHz) 4. 2nd Order HighPass BW (cut off @ 1kHz) 5. 3rd Order HighPass BW (cut...
  41. R

    Filters in closed-loop control

    Hi friends, I have a question regarding the use of filters in closed-loop control systems. Very often in a system like that there is noise from the sensor, and we have to put a filter after the sensor to eliminate the noise. However, all filters produce a phase shift as well as an amplitude...
  42. C

    Determining Decoupling Capacitor Value for EMI Reduction

    Hello: How does one determine the decoupling capacitor value in a circuit that is prone to EMI? For example, I need to remove the noise at 400 MHz. I use the resonant frequency formula f = 1/ (2pi*sqrt(L*C)) What should be the value of inductance L in the equation? how to determine the...
  43. F

    Engineering Mastering Circuits: Filters Homework Statement & Equations

    Homework Statement Homework Equations *in picture The Attempt at a Solution Not sure where to start. This is a review but I have not covered it yet or if we did, it was very brief?
  44. mnb96

    IIR Filters: Questions & Answers

    Hello, I know that Finite Impulse Response (FIR) filters can be equivalently expressed as a convolution. The effect of convolution in frequency domain is well known. In conclusion it is easy to make sense of FIR filters. My questions are: - can also Infinite Impulse Response (IIR) filters...
  45. H

    Why Do Cascaded Low Pass Filters Not Simply Multiply Transfer Functions?

    I do not understand section 8.6 of my lecture notes (see the attachment). 1. "If two low pass filters are cascaded the output voltage is not simply the product of two transfer functions." : Why not? Why would we even think of multiplying two transfer functions? Which transfer functions...
  46. J

    Simple RC filters phase question

    Homework Statement I'm a bit confused about how to find the phase equation of a lossy low pass filter eg. Homework Equations For a normal low pass filter it's just arctan(wRC) The Attempt at a Solution So I am comparing this by subbing in values and comparing to a bode...
  47. W

    Delay operator in feedfoward filters

    Homework Statement The symbol z = e^(iw) is called the delay operator. Explain why (in terms of the phasor) z^(-k) corresponds to a delay of k sampling intervals. The Attempt at a Solution We really have only discussed how to manipulate the filter equations, not really waht teh delay...
  48. H

    How do polarizing filters work

    If I'm not mistaken polarized light consists of light waves vibrating only in a specific direction because the waves vibrating in other directions have been filtered out. I watched a video on youtube and the guy showed how putting 2 polarizing filters together in a certain alignment so as they...
  49. T

    What Are the Best Optical Filters for Diverging Light?

    Hi, Is there a particular type of optical filter which is suited for use with diverging light? Most of the filters which I have encountered; dichroic, interference ... are suited to collimated light. I have an application where I would like to long pass filter a diverging beam of light...
  50. G

    Life of Light/Color filters

    Hello, I would like to ask someone to help me figure out on what the life-span of the color filters depends. I have information that some of the monochromatic filters of an instrument will "break" after around 5 years when working with the instrument 3-4 hours a day. The filters are illuminated...
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