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. S

    A few questions about airflow and filters...

    Just a few quick questions regarding airflow, mostly in connection with motors creating the airflow, like pc fans, vacuum motor, air going into a car engine through air filter, etc. 1) Adding more filters increases the availably surface through which the air can flow, therefor reducing the...
  2. P

    I Expose a Hidden Message w/ Polarisation Filters

    I want to expose a very fine grain film where an image disguises a message but when viewing the through polarising filter allows the message to be revealeded. I envisage exposing the B & W image that is light by light polarised through a positive mask of the message and simultaneously a negative...
  3. M

    Digital Filters: why is sampling frequency equal to 2*pi unit circle

    Hi, I was working through a filter design problem and got stuck on a concept. Scenario: Let us say we have the following pulse transfer function and the sampling frequency is ## f_s = 50 \text{Hz} ##. G(z) = \frac{1}{3} \left( 1 + z^{-1} + z^{-2} \right) The zeros of the transfer function...
  4. M

    Engineering Linear Filters for Removing Impulsive noise

    Hi, I was working on the following homework problem and just wanted to check whether my thoughts were along the right lines: "Explain briefly why a linear filter would not be appropriate for removing impulsive noise in a signal, and suggest an alternative filter." Attempt: When we use a linear...
  5. P

    Engineering Bode plot for active low pass filters

    Hi Can anyone help with this? I understand what the bode plots should look like but I am not sure how to produce a plot in excel for them without any values The transfer functions are: Vout/Vin = 1/(SQRT(1+(ω/ωc)^2) and Vout/Vin = -G/(1+j(ω/ωc)) As both are first order filters they will have...
  6. jim hardy

    When Mozilla Thunderbird message filters quit....

    I set message filters in Thunderbird to recognize and automatically forward spam to: the ISP from which they were sent Spamcop.org and my ISP's support desk then move the spam to a folder named Spamwars . Worked fine for a few weeks then stopped for no apparent reason filters won't even run...
  7. PavanKumar

    A Photon Spin and Polarization filters

    I understand how polarization can be explained using EM waves. However, I am unable to understand how to explain how polarization filters work when we use the concept of photon spins. Can someone help me with that?
  8. N

    Stargazing Is there any material I can use for an aperture solar filter?

    I live in a small town and would like to make some observations today. However i don't have a solar aperture filter, i only have an eyepiece filter of which i do not trust. So is there any household items, materials i could use safely to save me from from ordering on the net and waiting...
  9. J

    Do Blue Light Filters Actually Work?

    I have both my iPhone and desktop computer set to a "night light" mode, and obviously my eyes appreciate the warmer colors at night. But what I'm wondering is this: Do they actually work? Here's what got me thinking: I have a pair of Elvex glasses rated to block all blue wavelengths, and I...
  10. A

    Efficient DC Estimator Algorithms for Half-Cycle Sampled Signals

    I wanted to remove the dc component from a signal but i can't use the average method because i have only a half cycle sampled not a full cycle . Is there any algorithm or method that can remove the dc component using only a half cycle? (The signal is sin(x)+2*sin (2x) +3*cos (3x)+4*cos (4x) +5...
  11. berkeman

    HEPA filters and air purifiers for allergy relief?

    My wife and some of my friends have been experiencing a spike in moderate allergy symptoms (Sx) this past week or two, and I was wondering if adding a HEPA air purifier at home in our bedroom would help. This recent PF thread discussed air purifiers, but not really from an allergy perspective...
  12. flyguille

    Why adding more polarized filters results in more light passing?

    I was watching this video ... And basically the video show how adding in middle angle polarized filters it makes more light to pass. Then comments about quantics and blah blah blah. Why cientifics thinks lot weird and how was discard the following line of a more natural thinking: My...
  13. dRic2

    Charcoal/carbon activated filters

    Hi PF, can someone suggest me any books/sources to learn how carbon activated filters work. I'm curious about the absorption process: is it physical or chemical? where can I find some experimental data? Thanks Ric
  14. J

    Understanding Normalized Filters and Their Impact on Waveform Distortion

    Hello, I am learning about filters but I'm having trouble understanding what 'normalised' means and it's significance. I am using a LPF filter in a project and whilst there is software that will design it for you, I would also like to learn to calculate the values for myself and learn the...
  15. A

    Iref extraction in active power filters using active filters

    i was doing a research on active power filters and i read about Iref ( the cancelation waveform) extraction algorithm, and they are using complicated algorithms like FFT or DFT in frequency domain or the park transform and clark transform in time domain, i was wondering, why isn't there any...
  16. isukatphysics69

    Need help understanding air resistance lab (coffee filters)

    Homework Statement Homework Equations Equations posted in pictures attached The Attempt at a Solution I haven't attempted it yet because I am confused about something. If the-cv^2 case is for objects that hit terminal velocity at the fastest rate, why are my graphs showing that I should use...
  17. brainbaby

    RF Tuner: Adjacent Channel Interference Rejection Filters

    Hi guys The IF section of both monochrome and colour receivers must include rejection filters to suppress adjacent channel interference... Why exactly so, since the work of rf tuner in television receiver is to select a particular channel and as the required channel is selected it then passes...
  18. adamaero

    OpAmp general rules-of-thumb (educated guess) on filter type

    Homework Statement For example, what type of filter is this: How can one tell, as a general guideline, without finding the transfer function? I think finding the transfer function is the only real way to tell for sure, but what is a quick way to make an educated guess? The Attempt at a...
  19. heavystray

    B Why Do We Use Different Pairs of Filters to Determine the Temperature of Stars?

    Hi, in determining the temperature of stars using colour index (U-B,B-V,V-R, etc), why do we need to use the appropriate pairs of filters based on their range of temperature? (this is what i read from wiki) For cool stars, we use R-I, and for hotter stars, we use B-V. I don't understand how...
  20. Z

    Magnitude of the transfer function

    Hi all, In one of my problems there is a question with a plot of the transfer function magnitude versus frequency. In the graph I notice that the magnitude of the transfer function is greater than 1. This is confusing as I thought that filters are not able to increase the magnitude of the output...
  21. J

    Passive and Active Low Pass Filters

    Hello, I have been doing some research into the above filters and ran into a question. I saw a video that covered the pros and cons of each type of filter and it was recommended that a reactive filter was used at lower frequencies, the reason given was that the inductor that would be required...
  22. P

    Why Do Experimental Values Differ from Ideal in Filter Circuits?

    For my lab I need to explain why my ideal values are different compared to my experimental values. besides things like resistor and capacitor tolerances, what would cause a slightly different output?
  23. S

    Fraunhofer diffraction experiment- neural density filters

    I'm currently carrying out an experiment with Fraunhofer diffraction. It involves shining a laser beam through neural density filters, a lens and a diffraction grating, to create a diffraction pattern which is then picked up with a CCD camera, to find the intensity of the maximal peaks. However...
  24. W

    Combining High and Low Pass Filters

    Homework Statement Suppose we have a noisy signal (superposition of many frequencies), but we are only concerned with a particular part of the signal within the noise at a frequency f(desired) = 1 GHz. In a sentence or two, explain how you could isolate the desired part of the signal by using...
  25. E

    Passive Harmonic Filters (Power System)

    Hi again, got a few questions (marked with numbers) for passive filters used to filter harmonic currents in the power system. Look at my one-line diagram below. Let's say I have a passive filter connected in shunt with respect to the load and the passive filter "diverts" the harmonic currents...
  26. U

    Microwave Based Photonic Filters

    Hi ! I am actually seeking help regarding working principle of microwave based photonic filters from basics, I have tried a lot I did find allot of material but non of them were describing the basic working principle. Helping in this regard will be highly appreciable. Regards
  27. Addez123

    How does image matrix filters work in matlab?

    I got an image called img and I want to sharpen the vertical spatial lines. I created this matrix: hp = [-3 -2 -1 12 -1 -2 -3]; hp= rot90(h_lowp); Then I do this to get the new image: newimg = uint8(conv2(single(hp), single(img))); It SORTA works, but I'm not so sure why it works. I mean all...
  28. khurram usman

    Upsampled input to an Adaptive filter?

    I will try to explain the issue I am having as clearly as possible without going into my coding or maths. I have my own and a MATLAB Central implementation pf standard LMS in MATLAB. Fixed step size. No normalization or other stuff. I am trying to use it in a system identification setup. I...
  29. T

    Transfer functions of active filters with Amplification

    Homework Statement Derive the transfer function for both circuits \frac{V_{out}}{V_{in}} sketch Bode plots for each circuit (amplitude and phase) Homework Equations Z_c=\frac{1}{j{\omega}C}~and~{\omega}_C=\frac{1}{RC} The Attempt at a Solution We can treat this as a potential divider using the...
  30. G

    Understanding High-Pass Filters and CE Amplifiers

    So I wanted to make something clear. I really posted too much threads this week, but that's for the purpose of learning. So a CE Amplifier has a blocking capacitor which together with the input impedance forms a high pass filter. But what I think is this filter is a high pass one only when the...
  31. J

    Low Pass Filters and Laplace Transform

    Homework Statement Given that r(t) = L^-1 (Inverse laplace) *H(S) and by making the link between the time-domain and frequency-domain responses of a network, explain in detail why the ideal “brick-wall” lowpass filter is not realisable in practice. [/B]Homework EquationsThe Attempt at...
  32. M

    Optical and electrical bandwidth "outside" air

    My questions is about the conversion of a range of wavelengths in Hz, such as for an optical filter, in a medium different from air. Since the refractive index n is higher than 1, (let's say n=1.5) and than the value of c is almost 2x108 m/s, also the equation in the link below has to be...
  33. U

    Questioning RF Frequency & Filters

    My question is about RF frequency... The RFs are electromegnetics waves...That means they are one type of Light waves... right?? The other question is about perticular frequency of RF signals...When we want to transmit or receive some perticular frequency... we tune our radio at that perticular...
  34. Jeprox

    Light, wavelength and optical filters

    Hi, I have read from previous topic regarding color of light vs wavelength. It is a bit confusing for on my self as I do not have any background on this. Hopefully someone could chime in and educate myself. Here it goes, it has been lingering in my head that the color of light emitted by a...
  35. Z

    Need help with question regarding polarization filters

    Un-polarized light is incident upon two polarization filters and do not have their transmission axes aligned. If 18% of the light passes through this combination of filters, what is the angle between the transmission axes of the filters. I got 64 degrees using I = Imax(cosΦ)^2 but the correct...
  36. F

    Transfer function of cascaded filters

    1. Problem statement Find the transfer function , H(s) by two methods. Use Nodal analysis with Cramer's method, and decompose into simpler sections and use the property, H(s) = H1(s) * H2(s).Homework Equations H(s) = H1(s) * H2(s) H(s) = Vout/Vin Cramer's Rule. X = Dx/D : where D is the...
  37. ranju

    Analysis of Filters: Calculating Characteristic Impedance Z0T

    Homework Statement In the given filter network , we have to find the characteristic impedance Z0T And the author found it as Z0T= √Z12/4 + Z1Z2 But I didn't get this , how this expression came up..! I found Z0 by considering Z1/2 in parallel with Z2 & the resultant is in series with Z1/2... & a...
  38. A

    Electromagnetic filters (color filters)

    Hello all I am searching for electromagnetic filters(color filters) for visible region of EM spectrum with narrow bandpass. I want to filter the white beam with continuous spectrom (400 to 800 nm) into red,blue,green and ...with FWHM about 1 nm around each line. Is there such EM filters? How...
  39. F

    Microwave cavity resonators as filters

    I'm not sure if I should be asking this in the EE section, but here goes. I'm trying to understand more about resonant cavities and how they act to filter out frequencies other than the resonant frequency. In the case where a cavity resonator is coupled between two transmission lines, how...
  40. C

    Why would someone choose a high or low Q factor for a high or low pass filter?

    Hi, Can someone please explain why you might choose a high or low Q factor high (or low) pass filter for a particular application? I can understand why it's useful for band pass and notch filters, for selectivity, but I cannot see any reason for it in a low or high pass. Thanks, Chris
  41. fluidistic

    Problem with gmail and its filters

    I'm having a problem with gmail. I am subscribed to a mailing list (of Maxima software) and I've set that every time I receive an email from that mailing list, the email should appear under the label Maxima and nowhere else. However it also appears under the category "forums", even though I've...
  42. pretty_fly

    Designing and Simulating Shaping Amplifiers for Detector Applications

    Hello everybody, nowadays I am working on shaping amlifiers and need guidiance about how to design one and do its simulations before pcb design and manufacturing. When googling about shaping amplifiers i found documents mostly about filters (active and passive), which i know works for rejecting...
  43. G

    The Secrets of Snapchat Filters: How Do Apps Track Speed?

    I don't know if many of you have snapchat, but it's basically an app for taking pictures and sending them to others. Something you can do is add a filter that shows what speed your phone was going while the picture was taken.. How is this done? Or, more generally, how do things like running apps...
  44. I

    RC vs RL Filters: Power Loss & Which is Better?

    this guy on youtube said "if you try pushing more than a few miliamps through an RC filter then you get significant power loss" and that therefore the RL filter was more preferable. @ 2:40: i thought L & C components don't dissipate any power, so what's the difference between running the same...
  45. Captain1024

    Help Finding Transfer Function Vo/Vs

    Homework Statement Find ##a)\ H(s)=\frac{V_o}{V_s}## of the filter in the circuit below. ##b)\ ##The center frequency ##\omega_0## ##c)\ ##The band-width B Correct answers: ##a)\ H(s)=\frac{6s}{12s^2+11s+1}## ##b)\ \omega_0=0.28\frac{rad}{s}## ##c)\ B=0.71\frac{rad}{s}##...
  46. C

    Polarizing Filters Fraction of Light Passing Through

    Homework Statement Unpolarized light passes through 4 polarizing filters. The polarizing direction of each filter is offset by 25 degrees from the filter in front of it. Find the fraction of light that gets through all four filters. Homework Equations I=I0Cos2θ The Attempt at a Solution From...
  47. L

    Bootstrapping Amplifiers, and Signal Bypasses

    Hello, I am reading The Art of Electronics, and I have two specific questions regarding examples it has introduced. I am dealing with bipolar transistors the current chapter. I think my questions mostly stem from not understanding concepts. The text has introduced the technique of...
  48. P

    Understanding Poles & Filters: Counting Capacitors & dB/decade

    Hello, I was reading over poles and filters, but I guess since I don't learn calculus, I don't learn transforms. With that said, the definition of Poles in my text is a bit iffy... It talks about how the term poles means how many bypass capacitors are in a circuit. I can't find ANY definitions...
  49. C

    Bernoulli's with Resistances - model flow between oil filters

    Bernoulli's with Resistances -- model flow between oil filters Hey guys, It's been a while since I was in school and practicing this stuff daily, so bear with me please! I'm a BSME and I want to model up flow between oil filters. Currently we use a single oil filter with the same pump...
  50. andre_teprom

    Infra-Red optical filters: What are made of ?

    Dear friends, I wish know what is/are the chemical elements commonly used for build filter lens applied on moderns night vision cameras. Thanks in advance. +++