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Case Fans: Air Flow And Noise Level

How To: Properly Plan And Pick Parts For An Air-Cooled PC, Part 2
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What Does Air Flow Really Mean?

The simple answer is that air flow corresponds to the volume of air moved through the fan in a set interval. Thus, it is a very important parameter for us. Unfortunately, it's often conveyed using different units, making it potentially confusing for the beginner. In English (and in most data sheets), the typical parameter is CFM, or cubic feet per minute, while in Europe, m³/h (cubic meters per hour) is more common. Yes, it'd be easy to mix the two up, but here's a handy conversion chart to keep your comparisons straight.

Input Value
Conversion Factor
Conversion Result
1 m³/h
x 0.58867
0.58867 cfm
1 cfm
/ 0.58867
1.69874 m³/h


Example Conversion:

At an end-of-summer clearance, we bought a desk fan sporting 840 m³/h at an appliance store in Germany. What would be the corresponding CFM value for the same fan purchased in the U.S.? We apply the conversion factor and get:

840 x 0.58867 = 494.4828 CFM

See? That wasn’t too hard, after all.

What Is Fan Noise, And How Does It Come About?

Typically, noise is a mix of several frequencies, which makes it hard to define and compare. Looking at the data sheets of fans, noise levels are either specified in dB (decibel), dB(A), or in Sone (loudness). The drawback of merely looking at a specified acoustic spec is that it's hard to imagine how unbearable that number is in the real world.

Whether noise is considered annoying is a multi-faceted issue affected by several factors. Is the disturbance slight, is it a humming motor, or a squeaky bearing? Unfortunately, a fan that boasts a low noise level on its data sheet can be more annoying than one with a less attractive spec as a simple consequence of its tone.

This looks like it could potentially be pretty noisy. First and foremost, a fan's blades impact its acoustic profile through more or less audible flow separation, which depends on build quality and rotational speed. To that end, surface quality, the blade's angle, and the number of blades all directly influence whether or not a fan is noisy. Many manufacturers claim to minimize acoustics with curved blades and golf ball dimples. But more than anything, you can help mitigate noise by not placing fans too close to case openings and meshes, which can result in the dreaded siren effect.

Sleeve, Ball Bearing, Or Something Entirely Different?

A fan's motor can contribute to noise output as well. Many times, motor noise is most annoying at low speeds, and is manifested as humming. Grinding or clanking sounds from the motor bearings are tough to quantify in a specification, but those are super annoying as well. Ball bearings endure less friction than sleeve bearings, and they typically last longer as well (depending on lubrication and design). However, some cheap ball-bearing fans only employ a single bearing, and the rotational forces (plus air pressure) may cause torsion, leading to premature bearing failure. Fans with double ball bearings do not exhibit this problem.

Sleeve bearings are typically quieter than ball bearings, and a ceramic sleeve bearing has the longest life span of all bearing types, since ceramics with a high-quality surface finish offer the least amount of friction. In general, however, double-ball-bearing fans present the best compromise between noise and longevity. Other design features fall into the category of personal preferences; in general, most high-quality fans perform equally well.

Keep in mind that even experts have a tough time predicting how a given fan will perform in the real world.

Interim Conclusion

Regardless of marketing proclamations about innovations like liquid bearings, the best product isn't always the most expensive one in the shiniest box. The true quality of a fan is determined by the ratio of its air flow to measured and perceived noise level. The lower the noise level for a given (high) air flow, the better we consider it for our purposes.

Display all 60 comments.
  • 8 Hide
    Ragnar-Kon , November 16, 2011 4:13 AM
    Helpful even for myself (who has been building PCs for years). Granted all it really told me is that I was doing it right all along, but it did give me some useful bits of information that I didn't know before.

    Kind of disappointed that the process of aftermarket GPU cooling wasn't explained more. That is something I have never done before, but always wanted to try out on one of my older cards.
  • 0 Hide
    drutort , November 16, 2011 4:21 AM
    gloves? who uses gloves :p  I use my finger :)  I have been doing it before there was even aftermarket paste :p 

    the trick is, that i did and do twist, BUT i double check how much paste there is on the heatsink and the cpu, if there is enough I then fix it up on both ends and press the 2 together and do small twist... in fact if you have it lapped, you can twist a bit and it starts to give some resistance... IMO this tells me that access paste has distributed even and any access is gone...

    This works so well that you cant separate the heatsink and cpu if you try to pull it straight off most of the time, but have to switch and move it.

    How do I know it works? wouldnt be getting the results i did.. my old 720 be unlocked 4 cores and running at 3.3ghz .. I recently replaced it recently with Noctura D14 and phenom II X6, that I dialed into 4.13ghz

    but then again I would say don't do this at home :p  (the way I do it works good enough when using thicker pastes, such as the arctic silver 5)

    ***I forgot to mention that this works for those heatsinks were you have a strap and not screws... I agree that those that have high pressure through screws you probably should not move or twist... but all those heatsinks that can create a lopsided effect due to the strap, this way works very well, especially when having thermal paste on both ends, and best way is to strap it down and take it apart, examine it and then fix it up add/remove any paste and place it back on.***

    For the noob this is a good enough article...
  • 1 Hide
    fancygoods , November 16, 2011 6:10 AM
    On page 5 the second picture the top case fan points down yet hot air comes up.

    Im curious although it wont have a large affect, if the CPU cooler fan would act like a puller if put on the left side whether it would hurt or help the memory temps
  • 8 Hide
    ojas , November 16, 2011 11:05 AM
    Really good article, and a real help. Just thought i'd chip in with what i know/have to ask....

    I remember reading that sleeve bearing fans should not be used horizontally, that shortens the life span of the fan and makes it noisier over time. Something to do with the fluid in the bearing. Other bearings don't have this problem.

    I also remember watching a program on NatGeo on submarines. Apparently modern day subs use propellors with a large number of thin blades. This basically avoids pressure diferences between water in front of the blade (in the direction it's spinning), thus prevents excessive low pressure in the region through which a propellor has passed. This avoids small bubbles of steam from building up and then popping, causing noise.

    Seeing that air is a fluid too, i wonder if a similar principal will apply to it. Of course, there wont be any air bubbles popping in air, but i'm wondering if turbulence has any role to play, since i assume more blades=less turbulence. not sure though.

    dB(A) is the A-curve...what about the C-curve? That's got more bass, which may be audible as a hum...C-cure measurements seem to ba a decible or three higher than A...

    If the fans are connected to the mobo, you could use SpeedFan too...

    With respect to the positive pressure section:
    You say it goes against natural convection. What if i set the front and side panel as intake and the rear and top fans as the exhaust? I'm assuming the PSU is bottom mounted. Would it still go against convection?

    What about neutral air pressure?

    If, for example, the cooler needs to be re-seated following improper installation, is it a good idea to use isopropyl alcohol (Doctor's Alcohol) to clean the CPU and heatsink?

    Cooler Master fans are pretty good, their higher quality fans 120mm fans move 79 to 90 CFM, their 200mm megaflow moves about 110 CFM. In both cases, rated noise is 19 dB(A). however, no rubber screws or frame so a that's a con.

    BTW there's no prob with exposed heatpipes and the Arctic Silver, is there?
  • 4 Hide
    gio2vanni86 , November 16, 2011 11:55 AM
    I've been doing the whole spread the paste evenly flat onto the CPU/Cooler for almost 8 years now. I'm not a professional when it comes to this solution, nor do i have eyes of a hawk to know if i do it right all the time, the temps seem to be right. I wish you guys did a temp difference on how the paste was spread in a lot different ways to give me a point of view of if there is any difference in temps, obviously the one where you can see the writing more then likely is not effective. Great article as always, cheers!
  • 3 Hide
    Augray37 , November 16, 2011 12:06 PM
    @ ojas

    I'm not sure about all the other questions, but i know 90% isopropyl alcohol will work to remove thermal paste. "Goof Off" or similar products are even better. Also, i'd suggest using a coffee filter to apply the alcohol or goof off and wipe away the paste. that's what i do, works every time.
  • 1 Hide
    icepick314 , November 16, 2011 12:28 PM
    I read years ago that thinner the layer of thermal paste, the better.

    Well...not THIN as in barely there thin but just enough to make lettering opaque, not completely covered.

    I thought the paste is just to fill in cracks and imperfections on the heat sink and the surface of the processor...

    also any word on Zalman's heat paste that comes in liquidy paste? it comes in a small bottle with a brush like nail polish...been using that all this time...
  • 1 Hide
    Branden , November 16, 2011 1:13 PM
    i've sometimes moved a fan's molex wires from 12v to 5v for silence's sake, but sometime it was too much. never knew i could get 7v by crossing the 12v and 5v lines like that (and wouldn't have dared try if i'd even thought of it anyway), glad to know there's another option there!
  • 2 Hide
    mjmjpfaff , November 16, 2011 1:13 PM
    I would have liked to seen silverstone's air penetrater fans in here. Just to see how they stack up against other fans.
  • 0 Hide
    chesteracorgi , November 16, 2011 1:40 PM
    I think that you may have proven my case for GPU cooling being superior as a case exhaust design over the shroud design. The shroud design produces temperatures from 10-30 C above the case exhaust designs.

    My own experience with a GTX 470 (from PNY) with a shroud showed an idle temperature abour 45-48 C. When I modded the card with a Zalman 2000F cooler the temperatures dropped to 30 C at idle. This (delta) temperature reduction was consistant for all uses: from idle to load. Similar loads produced temperatures that were 15-20 degrees C cooler with the case exhaust method over the shroud.

    Just two weeks ago I added a second GTX 470 (from Galaxy) in SLI. This model has, what is touted to be, superior cooling to the reference design. It consistantly runs about 10 C hotter than the PNY cooled by thge Zalman aftermarket cooler at idle. Because of space considerations the shroud design had to be mounted above the case exhaust GPU. When the GPU load is increased the shroud design temperatures show a increase (delta) of 15- 20 C over the case exhaust. This is expected, as the top mounted GPU is the one with the major work load.

    I ordered the mesh sided side door from Corsair (600T) to provide extra ventillation from a side panel fan blowing on the GPUs. If this configuration works, I expect a 5-7 degree C in the top mounted
    GPU. I will be using the 200 mm fan (from Corsair) that was modded out from the top of the case when I replaced it with two 120 mm fans (from Xigmatek).
  • 2 Hide
    triculious , November 16, 2011 3:06 PM
    The Tcase Temperatures - Thermal Compund table states it's measured in Celsius but the comments below mention Kelvin.

    Just to give you an idea of how different Celsius are from Kelvin, if the table states 58 degrees using the Nanoxia Nano TF1000 and they are Kelvin and not Celsius, that would be -272.15C (or -355F). That thermal paste would be hella better than liquid hidrogen!

    I still agree that "differences of 1 kelvin can barely be measured consistently".

    All in all, I like this articles. Keep the simple and efficient articles coming. Also the "advanced techie" ones... explained for us illiterate ones.
  • 7 Hide
    JPForums , November 16, 2011 3:17 PM
    There are a few things I dispute.

    For the negative pressure case:

    Quote:
    Amplification of natural convection


    While the top fan blowing upward does indeed support (rather than oppose) natural convection, this is not a trait of a negative pressure setup. This is a trait of proper fan orientation (cover in the last article). Negative pressure means fewer particles available to transfer heat to. Thus negative pressure opposes natural convection. Conversely, positive pressure with a more particles available to transfer heat to supports natural convection. A good example of a properly oriented positive pressure system is the Silverstone Raven 2/ Fortress FT02.


    Quote:
    Compatible with all direct heat exhaust graphics cards (the ones that blow dissipated heat out of a vent built into the shroud)


    Again, the more of a vacuum you create, the fewer the particles available, the worse the heat transfer. As a side note, fans are noisier in a vacuum as well, but I'll let you look into that if you're interested. In the setup your have, orienting the top fan downward would suck in some of the exhausted heat back into the case, but that again is a fan orientation issue. Again, the cases mentioned above post some of the best video card temps in the industry (especially for external exhaust style cards).


    Quote:
    Augments the cooling performance of downward-facing CPU heat sink fans


    You'll have to explain this one to me, but I suspect this is another fan orientation issue rather than a positive vs negative airflow issue.


    Quote:
    Without a dust filter, dust gathers inside the case


    Even with a dust filter, you have to understand that most cases aren't air tight. With negative pressure air will be drawn in through every crack, bringing dust with it.


    For the positive pressure case:

    Quote:
    Only average cooling performance in enthusiast-oriented configurations


    There are many counter examples to this statement. I'll stick with the Silverstone Raven 2/Fortress FT02. Though I do agree that if you orient the top fan down in the example case to achieve positive pressure, the improper orientation with drop your cooling effectiveness.


    Quote:
    Better support for graphics cards without the ability to exhaust dissipated heat


    Not sure this is true either. While you do have more particles to work with, negative pressure systems tend to be better at directing airflow. This makes it easier to direct the heat away from the video card and out of the case. Of course, their are positive pressure systems that can achieve this as well.


    Quote:
    Counters convection


    Only with an inappropriately oriented fan such as in the example you show. Not an inherent flaw of positive pressure systems.


    Quote:
    Air flow is determined by size/position of case openings


    Having a hard time seeing this as a negative to positive pressure systems. This allows you to put the air where it is needed. I tend to think of a negative pressure systems more intuitive ability to direct airflow as a positive. If anything, it just requires more thought. At the opposite extreme, highly negative pressure systems can suffer from air coming in holes in the case and going straight out without doing any effective cooling.


    Quote:
    Counteracts the cooling performance of downward-facing processor coolers


    Again, you'll have to explain this one, though I suspect it is an orientation rather than pressure issue.


    Quote:
    Graphics cards with DHE (direct heat exhaust) can partly counter the overpressure


    This is true, but can't be considered a downside to cooling, especially when you already think the positive pressure systems don't cool as well. Rather it is something to consider (along with the PSU) as you design your cooling system.


    A true downside to positive pressure systems is that you can get areas where air simply stagnates and doesn't get drawn into the flow. This leaves you only marginally better off than straight convection cooling. A good positive pressure case will make sure that air is still flowing around the components that generate heat.

    In my findings, the best results come from a slightly positive pressure case. These still benefit from being able to direct the airflow for optimum cooling, while preventing dust build up and removing the vacuum effect. Though I must admit, the Silverstone Raven series/Fortress FT02 cases seem to get around these issues while remaining highly positive pressure. That said, fan orientation is of far greater concern, and should be setup to direct heat out of the case smoothly as shown in your first article. Improperly oriented fans will far outweigh the benefits of positive/negative pressure systems. In other words, make sure the case is designed for the type of system you want to implement rather than forcing the system you want on a case that isn't designed for it.
  • 0 Hide
    gnfishin , November 16, 2011 3:22 PM
    Where, in the U.S., can you get Xilence X5?
  • 2 Hide
    Zeh , November 16, 2011 4:26 PM
    Another excellent article.

    Also, good post from JPF on 1st page, altough I disagree that the (very) small variation in pressure would do any measurable harm to heat transfer. Smaller density has a bigger impact in the heat transfer through conduction, not much in convection.

    From my experience, I could see that negative pressure inside the case did generate better temperaturas (most likely because you're able to direct the air by positioning fans acordingly).
    Also, to help explain why negative pressure seems to bring in less dust, notice that the air is moving slower the farthest you go away from the fans, and slower air brings less dust in.
    More fans bringing air in = more speed = more dust.
    Less fans (and negative pressure) don't automatically mean less air will go in, but it does mean the air will enter slower (but through every possible little crack).

    Ofc, please feel free to correct me if I'm wrong.

    JPForumsThere are a few things I dispute.For the negative pressure case:While the top fan blowing upward does indeed support (rather than oppose) natural convection, this is not a trait of a negative pressure setup. This is a trait of proper fan orientation (cover in the last article). Negative pressure means fewer particles available to transfer heat to. Thus negative pressure opposes natural convection. Conversely, positive pressure with a more particles available to transfer heat to supports natural convection. A good example of a properly oriented positive pressure system is the Silverstone Raven 2/ Fortress FT02.Again, the more of a vacuum you create, the fewer the particles available, the worse the heat transfer. As a side note, fans are noisier in a vacuum as well, but I'll let you look into that if you're interested. In the setup your have, orienting the top fan downward would suck in some of the exhausted heat back into the case, but that again is a fan orientation issue. Again, the cases mentioned above post some of the best video card temps in the industry (especially for external exhaust style cards).You'll have to explain this one to me, but I suspect this is another fan orientation issue rather than a positive vs negative airflow issue.Even with a dust filter, you have to understand that most cases aren't air tight. With negative pressure air will be drawn in through every crack, bringing dust with it.For the positive pressure case:There are many counter examples to this statement. I'll stick with the Silverstone Raven 2/Fortress FT02. Though I do agree that if you orient the top fan down in the example case to achieve positive pressure, the improper orientation with drop your cooling effectiveness.Not sure this is true either. While you do have more particles to work with, negative pressure systems tend to be better at directing airflow. This makes it easier to direct the heat away from the video card and out of the case. Of course, their are positive pressure systems that can achieve this as well.Only with an inappropriately oriented fan such as in the example you show. Not an inherent flaw of positive pressure systems.Having a hard time seeing this as a negative to positive pressure systems. This allows you to put the air where it is needed. I tend to think of a negative pressure systems more intuitive ability to direct airflow as a positive. If anything, it just requires more thought. At the opposite extreme, highly negative pressure systems can suffer from air coming in holes in the case and going straight out without doing any effective cooling.Again, you'll have to explain this one, though I suspect it is an orientation rather than pressure issue.This is true, but can't be considered a downside to cooling, especially when you already think the positive pressure systems don't cool as well. Rather it is something to consider (along with the PSU) as you design your cooling system.A true downside to positive pressure systems is that you can get areas where air simply stagnates and doesn't get drawn into the flow. This leaves you only marginally better off than straight convection cooling. A good positive pressure case will make sure that air is still flowing around the components that generate heat.In my findings, the best results come from a slightly positive pressure case. These still benefit from being able to direct the airflow for optimum cooling, while preventing dust build up and removing the vacuum effect. Though I must admit, the Silverstone Raven series/Fortress FT02 cases seem to get around these issues while remaining highly positive pressure. That said, fan orientation is of far greater concern, and should be setup to direct heat out of the case smoothly as shown in your first article. Improperly oriented fans will far outweigh the benefits of positive/negative pressure systems. In other words, make sure the case is designed for the type of system you want to implement rather than forcing the system you want on a case that isn't designed for it.

  • 0 Hide
    wolfram23 , November 16, 2011 4:47 PM
    Great article except for one thing:

    "Be sure to set the T Junction correctly..."

    Thanks, but how do we do that?

    Anyway about case pressure, I tend to believe positive pressure is much better than negative. I was running negative for a while and it's so bad that the top fan on my Antec 900 couldn't push a small piece of paper in the air. I then turned off one of the exhaust fans and the paper flew off... so basically you definitely aren't going to help remove heat by having negative airflow if your fan can't even push enough air to move a small paper.

    I'm currently running a front mounted 220 rad in push/pull, plus a 5th fan (Silverstone Air Penetrator) blowing at my 5850s (it's mounted using the "rear mount bracket" so it is spaced off the radiator by 2 fan widths). 5850s I switched coolers from stock (blower fans) to Scythe Setsugens (low profile with Scythe Slim fans) which reduced the noise. I also use a Scythe Slim on the side intake blowing at the gpus. With all that the cooler performance also increased over stock (with lower noise) - although without those 2 extra fans the cooling as a bit worse than stock.

    Anyway I have temp probes setup (Scythe fan controller) for intake and exhaust temps. The intake is placed just in front of the radiator and the exhaust is just inside of the rear exhaust fan. Under normal usage the delta is about 3C, under gaming it's around 5C so not bad. I don't know if the probes are very well calibrated though, I haven't bothered to test that yet.

    I'm also a little surprised about in Part 1 with the PSU bottom mount. Having an Antec 900 II the PSU has to act as an exhaust - you can't flip it without cutting the case. I assumed it was ok as it would help bring airflow along the bottom, although it does go opposite to the airflow of every other component. I guess I might have to try cutting a nice hole and flipping it over. That'll help cable management a little bit too haha.
  • -3 Hide
    erelyes , November 16, 2011 6:21 PM
    An OK article. I was surprised the "line" method of applying paste is not explained, as this is what is recommended by many manufacturers (http://www.arcticsilver.com/pdf/appmeth/int/vl/intel_app_method_vertical_line_v1.1.pdf).

    I also think you are giving a little too much credit to convection. A 50rpm change in fan speed achieves more than convection every could. Convection helps passively cooled systems; in anything with a fan, convection is overshadowed.
  • 0 Hide
    gerchokas , November 16, 2011 8:16 PM
    Great article! And LOLed so hard at the end!!! :D 
    I remember once when i had my old Athlon 64 3200+, i was checking why it was heating so much, I took the cpu cooler out and just then found I didnt have any thermal paste to put it back in... And it was some kind of holiday so it would take days for the shops to open again.. So i used oil (cooking oil) :p  I monitored temps constantly, fearing i had made a huge mistake, but to my surprise, it worked great..! It even kept temps even lower than before! :p  Played games and everything. A couple of days later i bought the paste and cleaned the mess, but temps didnt go any lower than with my "culinary fix"
  • 0 Hide
    buzznut , November 16, 2011 8:26 PM
    I found out the hard way that the drop method (or any spreading technique as well) doesn't work for the newer heatsinks with exposed heatpipes. Here the best method is putting thin stripes of TIM on the heatpipes as they tend to be a little recessed to the plate.
    It is the best way to achieve an even spread of the material with HDT type heatsinks, as recommended by Frostytech and many other hardware sites. Take my advice please, don't use the drop method with these type of HSF or you'll end up with an inefficient mess.
  • 2 Hide
    freddy782 , November 16, 2011 8:42 PM
    triculiousThe Tcase Temperatures - Thermal Compund table states it's measured in Celsius but the comments below mention Kelvin.Just to give you an idea of how different Celsius are from Kelvin, if the table states 58 degrees using the Nanoxia Nano TF1000 and they are Kelvin and not Celsius, that would be -272.15C (or -355F). That thermal paste would be hella better than liquid hidrogen!I still agree that "differences of 1 kelvin can barely be measured consistently". All in all, I like this articles. Keep the simple and efficient articles coming. Also the "advanced techie" ones... explained for us illiterate ones.


    A difference of 1 Kelvin = A difference of 1 Celcius, the article said "differences of 1 kelvin" which is correct, there was only a difference of 1 Kelvin. Although I agree it is a bit weird changing between Celcius and Kelvin like that without any need.
  • 0 Hide
    tacoslave , November 16, 2011 10:39 PM
    The one thing that No one can seem to agree on is how to spread thermal paste. I remember when i was starting out i tried to find out the proper way some say grain of rice, some say enough to make it transparent, and yet others say none to just lap both the cpu and cooler and thats best. At the end i just gave up and used my finger to spread a little paste and just lapped my cooler not my cpu.
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