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 Post subject: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 7:36 am 
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5dBA 120mm Fan. Sounds incredible but may actually be true.

Reader beware: This post may be received like the never ending conflict between baseball fans who seem to hate or love statistics! I am a little tone deaf, and cannot really distinguish the subtleties of fan noises when they are loud enough to hear, so in some sense I do not understand this game. But I can appreciate a beautiful fan that is super quiet, and I want to have a tool to use to help me keep multiple sound sources quite. I tried to do this post so it is understandable for those with some science background, but if you role your eyes at this type of thing just read the next paragraph and then skip down to the last few paragraphs!

I think this post may be more readable if you go to a window display, full height of your screen, half width of screen, rather that a full screen window.

Actual noise of a low RPM fan as if it was all by itself in the universe cannot be measured accurately by SPCR with a single measurement because of the ~11dBA background level and meter and electronics sensitivity. However it is possible to use standard data reduction techniques to estimate the noise level of the fans as if there was no other noise source around. This is partly a fun exercise, but I did it so I can use better estimates of noise resulting from multiple fans when they are operated at RPMs that are "noiseless" for any one by themselves.

As an example, consider the SPCR data for Noctua NH-D14 Cooler fans, 120mm and 140mm, data at http://www.silentpcreview.com/article1020-page5.html . See TABLE ONE. Both fans were tested at lots of different voltages (more than normal), and are quiet at low RPM. There is also data for both fans combined so the analysis method can be checked out by comparing the calculated pair noise with measured pair values.

Image

For this analysis, there actually is a little problem with using the SPCR data. From physics point of view, the noise likely a function of the fan rotational speed, and only indirectly a function of the voltage on the fan. When you raise the voltage, the noise does not go up because you changed the voltage; it goes up because the fan is rotating faster. Unfortunately for this analysis, SPCR controlled variable in their measurements was the voltage on the fan, not the fan RPM. I am not saying they did it wrong, their method is an appropriate, practical approach: obviously voltage is what we can control easily in our own PCs. This issue can be overcome though. Fortunately these fans are not PWM fans so the RPM is actually a direct function of voltage: double the voltage and you double the fan RPM. See Graph One. The best line fit to the data is actually a second order polynomial and not a straight line through the origin, but the directly proportional model is close enough for the purposes of this discussion. SO, I will look at sound levels as a function of voltage, not RPM.

Image

Now look at the data for the 120mm fan, Graph Two. It is clear that the sound level has a linear relationship with fan voltage above the measurement floor (greater than 6V, 700 RPM). Sound engineers can probably explain the physics here, but not me! I only need the practical result that we can fit the data to a simple model: the measured noise (Nm) is a linear function of voltage unless the voltage is reduced so much that we have to consider the background noise (Nb). So, if the trend continues, the noise of the fan alone will be a straight line at the low voltages.

Image

Hopefully the background level Nb is constant so this model can work. The noise levels as defined are in dBA. So we need to fit the data to the classical model of how to add noises together. You have to use the definition of decibels: convert the dBA to the square of their power level, add these together, and then re-convert to dBA:

Nm(f) = 10 X Log10 ( 10^(Nf(f)/10) + 10^ ( Nb/10) )

Using least squares fitting, the data fits a background level of 8.4 dBA. (Sorry for the bad formating of the graph titles, they seem to change when I go from Excel to pictures, and they should say "CALCULATED AND ACTUAL NOISE"

Image

This seems lower than anticipated from the comment in most of the SPCR noise measurements that the noise floor is 11dBA, but I suspect that they really mean that the noise floor is definitely less than 11dBA and fluctuates between fan noise analysis sessions and probably even during a session. But consider this: the estimated fan noise is equal to the background noise at about 5.5V. Add two noise sources of equal value you get 3dBA increase, in this case to 11.4dBA. Since this is the actual reported value of background plus fan, this low value makes sense despite the note on background by SPCR.

The calculated dBA for the fan alone after subtracting out the 8.4 dBA calculated background noise is shown in Graph Three. The fan at low RPM, according to this model, is REALLY quiet, effectively it goes down to 7dBA! As a quick check, the straight line noise curve for the fan alone can be added to the fitted background noise level. As you can see the dashed red line follows SPCR total noise measurement quite well.

Now look at the dBA for the 140mm NF-P14 fan as function of voltage, Graph Four. The dBA/Voltage seems linear 6-11V, very nice straight line. The highest RPM data point seems too far off a straight line compared to the other points, so I ignored it for the rest of the analysis. Do the least squares fit and you get Graph Five, noise floor value of 10.8 dBA. This is more like the expected value per SPCR comments. Again, at low RPM, the calculated fan noise is REALLY low around the same 7dBA, but at a higher RPM than the smaller fan. And again the model seems to hold up as indicated by the dashed red line that fits the SPCR data quite well.

Image


Image

Now what to do with the calculated noise levels? Predict what the noise level of the two together would be, and check the calculated values against SPCR measured values. The noise measurement should be the noise of the two fans plus a background noise, but what is the background noise? We can calculate the expected combined noise for a range of background levels, see Graph Six. Not bad fit, eh? The calculated dBA are typically within o.5dBA of the SPCR values for 8dBA background level, except at the highest voltages. So it seems the drop in noise compared to the linear model at large voltages may be due to something happening with the fans…that is, the model fails at large voltage/noise levels. Also, the assumption that RPMs are directly proportional to voltage may be hurting the model since at larger voltages the actual RPM is lower than a linear estimate.. But then this fall off is not at noise levels we have any real interest in.

Image

Minor point I need to raise…the article is not totally clear if the two fan noise level is measured at 6 or 6.5V (see the asterisk in Table One). However the data is more consistent with 6V, not 6.5V, so I used 6.5V.

So in summary, one can use the fan’s noise level as function of fan voltage measurements to subtract out the background noise and get the noise of the fan itself. Now one can add noise levels together to get estimate of noise level of multiple fans. There will be problems for scenarios different that this set of SPCR data..the fans did not move from their position during the measurements. But in your PC, they will be put at different distances from the listener, and have different sound paths to the listener. So this noise estimation process can only be used as a guide.

OK, but where did my title come from? SPCR’s reference Nexus fan of course! It calculates to less than 5dBA for RPMs less than 625cps!

Image

According to this model, I can put up to 4 fans in my pc build (P/S, heat sink, one or two case fans through a low air resistance filter, positive pressure, overclocked 2600K) with expectations of max noise of 11dBA from the fans if operated at <650rpm, less if I have a good case than does not let the noise go directly to the user. I am going for computing power, not gaming, so I will not have a video card, just use on board graphics.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 9:39 am 
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I will admit that I don't understand any of this.... But it still doesn't make sense to me.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 9:55 am 
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ces wrote:
I will admit that I don't understand any of this.... But it still doesn't make sense to me.

Let me summarize then:
1) SPCR does not measure the sound of a fan when it is operating at low RPM, they measure the sound of the room on top of limitations of their sound measuring system plus the noise from the fan.
2) At higher RPM they DO measure (mostly) the the sound of the fan because the room sound is so low and their measurement system is so good. Other popular sites usually cannot even come close to SPCR measurement capability.
3) You can use standard physics data analysis techniques, with the noise addition model, to show that the true fan dBM for really good fans at low RPM is in the 5-7dBM range. You cannot measure it directly, but that is what the numbers are saying.
4) This is why two fans that measure 11dBA in their room do not add up to 14dBA when you combine them. The fans themselves are actually much lower than the 11dBA.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 3:20 pm 
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photonblaster wrote:
It is clear that the sound level has a linear relationship with fan voltage above the measurement floor (greater than 6V, 700 RPM). Sound engineers can probably explain the physics here, but not me! I only need the practical result that we can fit the data to a simple model: the measured noise (Nm) is a linear function of voltage unless the voltage is reduced so much that we have to consider the background noise (Nb). So, if the trend continues, the noise of the fan alone will be a straight line at the low voltages.

Are you sure about this? Seems to me that we should get diminishing returns when lowering the RPM and that this is not really related to the noise floor. I mean, at a certain point the fan will stop. It has a limit on how slow it can spin, right? At that lowest setting we get a little bit of sound, and more than 5dBa I would think.
But then again, I cannot explain the physics either. :p

photonblaster wrote:
According to this model, I can put up to 4 fans in my pc build (P/S, heat sink, one or two case fans through a low air resistance filter, positive pressure, overclocked 2600K) with expectations of max noise of 11dBA from the fans if operated at <650rpm, less if I have a good case than does not let the noise go directly to the user. I am going for computing power, not gaming, so I will not have a video card, just use on board graphics.

How can you expect 11dBa from four fans when SPCR tests in their chamber get a higher reading than that from a single fan? Even if the noise floor matters, your ambient will be louder than in an anechoic chamber.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 3:50 pm 
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1. "This is why two fans that measure 11dBA in their room do not add up to 14dBA when you combine them. The fans themselves are actually much lower than the 11dBA." This part I get.

2. Dbs are logarithmic. So 15db plus 15 db doesn't equal 30 db. Are you taking that into account? (my recollection is that doubling the volume of a sound increases its db by 6.7 db. That means two 30db sound sources together will produce 36.7 db... and the psycho acoustics of hearing makes things even quirkier)

3. So what is the point of all this? A rational for using more fans? Use as many as you want until it gets too loud for your ears. That level of volume varies substantially from person to person and even for the same person it varies substantially from environment to environment.

Generally, more slow fans will move more air with less noise than fewer faster fans.... all things being equal (which they never are).

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Last edited by ces on Sat Feb 19, 2011 4:23 pm, edited 1 time in total.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 4:07 pm 
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photonblaster wrote:
It is clear that the sound level has a linear relationship with fan voltage above the measurement floor (greater than 6V, 700 RPM).
This is not true. It is so not true at so many levels, it diminishes the credibility of everything else you say.

photonblaster

There is no relationship between fan voltage and sound level that is linear. It's sort of a crazy thing to say. For any particular model fan I guess you could experimentally graph it out... but think about it.... do you really believe it will be linear except by accident? There are too many intervening variables. Think hard.

Do a thought experiment with a piece of graph paper. And look at SPCRs non db fan data. And then remember when you drive your car.... at early low speeds.... there is no wind noise.... neither wind noise nor aerodynamic friction increase linearly from 7mph, to 70mph to 700mph.

Then I propose you return with a little more humility and make your statements more in terms of questions than with assertions of authority.

If you can explain where I am wrong I will back down... but this just isn't right... at least for purposes of extrapolation.

EDIT: I came back and look at what I wrote. I think I was too sharp and apologize for being so... but I still sort of mean what I am saying. It's just not making sense to me.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 7:48 pm 
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Vicotnik wrote:
photonblaster wrote:
It is clear that the sound level has a linear relationship with fan voltage above the measurement floor (greater than 6V, 700 RPM). Sound engineers can probably explain the physics here, but not me! I only need the practical result that we can fit the data to a simple model: the measured noise (Nm) is a linear function of voltage unless the voltage is reduced so much that we have to consider the background noise (Nb). So, if the trend continues, the noise of the fan alone will be a straight line at the low voltages.

Are you sure about this? Seems to me that we should get diminishing returns when lowering the RPM and that this is not really related to the noise floor. I mean, at a certain point the fan will stop. It has a limit on how slow it can spin, right? At that lowest setting we get a little bit of sound, and more than 5dBa I would think.
But then again, I cannot explain the physics either. :p

photonblaster wrote:
According to this model, I can put up to 4 fans in my pc build (P/S, heat sink, one or two case fans through a low air resistance filter, positive pressure, overclocked 2600K) with expectations of max noise of 11dBA from the fans if operated at <650rpm, less if I have a good case than does not let the noise go directly to the user. I am going for computing power, not gaming, so I will not have a video card, just use on board graphics.

How can you expect 11dBa from four fans when SPCR tests in their chamber get a higher reading than that from a single fan? Even if the noise floor matters, your ambient will be louder than in an anechoic chamber.


Re the first point:
This is one of the reasons why I did the analysis. Obviously when the fan noise is large enough for you to hear, as you lower the RPM the noise reduces. The garphs show that this is linear reduction. But why wouldn't the noise continue to reduce as RPM is reduced? The data fully supports the hypothesis that it does continue to decrease linearly. Cetainly a hypothesis that the noise stops to reduce as you slow the fan down past, say 700 rpm, really does not make much sense to me personally. Then the only question is how much does it drop? answer is, according to the data, linearly. If it does not drop linearly, then the logarithmic addition of noise levels model fails.

Also think about the perfect fan. Then ONLY noise it would make is the noise of the air flow. I think you would agree, for a fan in free air so there is no pressure difference outlet to inlet, the air flow intuitively should be linear with RPM. So if the best fans are close to perfect, the the noise should continuously drop with RPM, not reach a point of "diminishing returns".

Also consider conservation of energy..less and less energy is used to keep the fan rotating as the voltage is reduced, so there is less and less energy availble to make noise.

SPCR could confirm the model by repeating the Nexus fan measurements at a closer distance. The hypothesis remains the same, but the noise of the fan at smaller distance would be high enough that the measured values would be well above the noise floor.

Re second point: the FANS have 11dba, the room and measurement system have noise floor around the same value, so the measured value will be higher. But again, this measured value is the room noise plus the fan noise, not the fan noise alone. Of course I have to operate Nexus fans at 625rpmm to get this.

And note that the hypothesis is tested when the two fans are measured together by SPCR. There is only one way that the two fans together can give ~11-12dBA noise measurments is for the hypothesis to be working and the fans themselves be as low as caluculated.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 8:23 pm 
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ces wrote:
photonblaster wrote:
It is clear that the sound level has a linear relationship with fan voltage above the measurement floor (greater than 6V, 700 RPM).
This is not true. It is so not true at so many levels, it diminishes the credibility of everything else you say.

photonblaster

There is no relationship between fan voltage and sound level that is linear. It's sort of a crazy thing to say. For any particular model fan I guess you could experimentally graph it out... but think about it.... do you really believe it will be linear except by accident? There are too many intervening variables. Think hard.

Do a thought experiment with a piece of graph paper. And look at SPCRs non db fan data. And then remember when you drive your car.... at early low speeds.... there is no wind noise.... neither wind noise nor aerodynamic friction increase linearly from 7mph, to 70mph to 700mph.

Then I propose you return with a little more humility and make your statements more in terms of questions than with assertions of authority.

If you can explain where I am wrong I will back down... but this just isn't right... at least for purposes of extrapolation.

EDIT: I came back and look at what I wrote. I think I was too sharp and apologize for being so... but I still sort of mean what I am saying. It's just not making sense to me.


When you say something is linear, this is a mathematical statement. If you graph the data, and that data is a straight line, it is a linear relationship between the variables. This is a methematical fact. You do not have to know why it is linear, it is linear. The lines I showed are linear. They have a coeficient of correlation in the 4 nines region. Also, note that I stated that the model appears to start to fall apart at higher RPM (your fast car).

Also note that even if two variables have a linear relationship, this is does not even "prove" cause and effect. I implied tis when I said that only indirectly linear because it intuitively should be linear to something like RPM, but fortunately for my analysis RPM is also linear with voltage. I was able to just use RPM for the nexus fan since I did not have to try to predict the nose of the two fans together.

And you are wrong about no noise level at low speeds for a car...the noise is just not loud enough compared to all the other noises around for you to notice. So the noise from a car may indeed be linear with speed...at low speeds, who knows? You just saying it can't be is not enough.

And everythig I said is based on the mathematical fact that the noise varies linearly with RPM (voltage) in the range stated. What do you think I did...I did exactly as you stated.."do a thought experiment with a piece of graph paper", except my graph paper was Excel.

I assume the phrase "it is clear that" turned your tirade button on. Please be aware that this is classical language when doing mathematical proof, or supporting a physical hyphothesis.

You also mention extrapolation. Extrapolation is vey risky in science. You are right that one cannot take what happens to a car at 7 or 70 mph and extrapolate to 700mph...the physics changes. But I am not extrapolating, I am interpolating. I hope you will agree with me that at zero rpm the fan makes no noise, so from the range of no noies to small amounts of noise the hypothesis holds up. Above 25dBA or so, the hypothesis indeed seems to break down.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 8:30 pm 
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ces wrote:
1. "This is why two fans that measure 11dBA in their room do not add up to 14dBA when you combine them. The fans themselves are actually much lower than the 11dBA." This part I get.

2. Dbs are logarithmic. So 15db plus 15 db doesn't equal 30 db. Are you taking that into account? (my recollection is that doubling the volume of a sound increases its db by 6.7 db. That means two 30db sound sources together will produce 36.7 db... and the psycho acoustics of hearing makes things even quirkier)

3. So what is the point of all this? A rational for using more fans? Use as many as you want until it gets too loud for your ears. That level of volume varies substantially from person to person and even for the same person it varies substantially from environment to environment.

Generally, more slow fans will move more air with less noise than fewer faster fans.... all things being equal (which they never are).



Re 2), yes I did. Operate two Nexus at 625rpm, and you get about 5dBA for the fans adding up to 8dBA, two more and it goes to 11dBA.
re 3), I quote from my post:
. This is partly a fun exercise, but I did it so I can use better estimates of noise resulting from multiple fans when they are operated at RPMs that are "noiseless" for any one by themselves.

But in your PC, they will be put at different distances from the listener, and have different sound paths to the listener. So this noise estimation process can only be used as a guide.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 10:11 pm 
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photonblaster wrote:
Reader beware: This post may be received like the never ending conflict between baseball fans who seem to hate or love statistics! I am a little tone deaf, and cannot really distinguish the subtleties of fan noises when they are loud enough to hear, so in some sense I do not understand this game. But I can appreciate a beautiful fan that is super quiet, and I want to have a tool to use to help me keep multiple sound sources quite. I tried to do this post so it is understandable for those with some science background, but if you role your eyes at this type of thing just read the next paragraph and then skip down to the last few paragraphs!

WOW! :!: I wrote the first sentence somewhat in jest, but also in anticipation of mixed responses to my post. I figured, just like in baseball, there are people who like the numbers behind what they are experiencing, and those who have much less interest or understanding of those numbers. I happen to be a PhD out of a unviversity in SPCR's back yard with decades of R&D experience, and love the numbers. For me it is not just good enough to say "that fan is running so quiet I cannot hear it" but I want to put a number on how quiet it is. It is not silent from a physics perspective, even though from a user's perspective it may well be.

And being able to put a number on it also helps decide which fan really is the quietist even when the noise gets so low not even SPCR's fancy measurement system can differentiate between fans using a single low RPM measurement. I would hope that everyone reading SPCR's forums would like to be able to know what fan should be crowned "quietest fan, bar none" regardless of their level of understanding of the physics or the technical analysis. (Note I (humbly :) ) said I did not understand the physics, but I will say I am pretty darn good at technical analysis) And of course, as I stated differently before, it can help you estimate how much noise you would get if you put two or more "noiseless" fans together.

I tried to thwart the negative response a little by my suggestion to readers to not bother reading the details if they are not interested in this sort of thing, but I guess I should have had close to zero expectations that that would happen instead of minimal expectations. But so far there has been little from the "statisticians" side. Just from "you show no humility" to "I don't believe it" to "5dBA seems too low". Some polite, some not so polite. But no valid technical resonse.

Just in case it did not come across in my analysis, for a listener, the 5dBA number is meaningless. It is way below hearing limits, is could be 1dBA or 9dBA and still be "silent". For a number junky, it has value even if no meaning to a listener.

I call on my technical bretheren to do a "peer review" of my analysis. :D
Is there something wrong with my hypothesis? My analysis methods? My final conclusions as to what the analysis is telling us?


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sat Feb 19, 2011 11:07 pm 
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Hmm your graphical analysis seems reasonably sound from what I can see assuming things work as you'd expect, but unfortunately I'm nowhere near an expert in how to measure sound and what actually is the main cause of noise in the first place. I'm also wondering if there's some sort of relative minimum db caused by the fan bearings and air being moved.

I'd think it certainly is possible to get a much lower db than the floor of the spcr chamber, but it's also pretty worthless as well. Either way it's really diminishing returns when your fans are far lower noise than other things such as your actual room background noise level. I'd rather have the added effect of keeping my system cool at that point.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 2:45 am 
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photonblaster wrote:
Also think about the perfect fan. Then ONLY noise it would make is the noise of the air flow. I think you would agree, for a fan in free air so there is no pressure difference outlet to inlet, the air flow intuitively should be linear with RPM. So if the best fans are close to perfect, the the noise should continuously drop with RPM, not reach a point of "diminishing returns".

If we're talking about some kind of theoretical perfect fan, then I can follow your reasoning. But I don't think that any real world fan is that close to perfect so that we can draw any conclusions about how much noise it would emit based on your theoretical numbers.
I have not yet come across a fan that's silent for real, no matter how low the RPM. The only moving part in my system is a single fan spinning at <600RPM and I can hear it above my ambient (only faint though). The 5dBA 120mm fan doesn't exist, I guess that's my point.

photonblaster wrote:
Also consider conservation of energy..less and less energy is used to keep the fan rotating as the voltage is reduced, so there is less and less energy availble to make noise.

Well, again we have a floor on that as well and it's not zero. I theory you could perhaps have a 1 RPM 120mm fan that would emit 5dBa or less but you cannot as far as I know create that scenario in real life.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 5:51 am 
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Vicotnik wrote:
I have not yet come across a fan that's silent for real, no matter how low the RPM. The only moving part in my system is a single fan spinning at <600RPM and I can hear it above my ambient (only faint though). The 5dBA 120mm fan doesn't exist, I guess that's my point.

I agree with you re silent fan, provided you modify your statement to "If a 120mm fan is operating above 300RPM or so, you can always get close enough to it to hear a noise". The 300rpm is just a low number, nothing scientific. And even if you stop the fan blades from rotating, stick it right up against your ear you might even hear the wires humming in the electromagnet. Ie it may not be zero noise even at zero rpm. But how does this negagate my calculations?

An extension of the wording in my agreement is that I do not think you are comparing apples with apples. First, if you get closer to a fan that had a calculated value of 5dBA in SPCR chamber, you would eventually be able to hear it. So the distance is critical. There is a very strong dependence of noise level on distance. Second, and I think this may be more important, you are hearinge noise from a system...a fan in a resonator. You are not looking at an isolated fan in SPCR chamber at exactly the same distance and the same angle from the fan. So to say "because I can hear my fan in my system the fan in SPCR's chamber must not be at 5dBA" I believe to be an invalid qualitative argurement, even if you were listening with the same geometries as SPCR.
Vicotnik wrote:
Well, again we have a floor on that as well and it's not zero. I theory you could perhaps have a 1 RPM 120mm fan that would emit 5dBa or less but you cannot as far as I know create that scenario in real life.

So you agree you could have a 5dBA fan in SPCR chamber, great. But at what RPM? you say 1 RPM, do you have any data to back that up? I have SPCR's data that says it is at 600rpm and less for the nexus, under their exact measurment conditions. What did I do wrong?

And the comment about real life conditions,
1) I stated you cannot go from the SPCR chamber to real life, other things happen. This discussion is about the fundamental performance of the fans all by themselves as if there was no other noise source in the world and you had a perfect measuring system. One can argue therefor that these numbers are useless, and if all you care about is if you can hear noise from your computer and that that noise is "benign", then ya, it is useless. Just crank up your fan voltage untill you are happy. But if you are interested the physics and numbers behind the sound, then it is not useless.
2) Putting a number on the fan noise level at very low RPM is fun to me. For me, even if I could not use the result in the real worlld, this would be enough for me to run the calculations, it is like a crossword puzzle to others. But I would not post the results if it was totally useless in the real world.
3) It can be used as a guide (which is why I did this post)...putting two or more fans together at low RPM that measure 11dBA in SPCR chamber will not measure +3dBA or more. What will they measure then? Use the calculated dBA of fans, the logarithmic addition of those levels, couple with SPCR chamber background level and you can predict what those two fans will measure. And guess what, for the two fans in the article, the prediction is pretty darn good compared to the SPCR measured values. Put them in a case, and things change. Well baffled, little direct path from the fan to the listener, low resonance cases probably will result in reduced noise compared to the measured values in SPCR chamber but for a case with resonance effects etc the noise level can go up.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 6:13 am 
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photonblaster wrote:
Quote:
According to this model, I can put up to 4 fans in my pc build (P/S, heat sink, one or two case fans through a low air resistance filter, positive pressure, overclocked 2600K) with expectations of max noise of 11dBA from the fans if operated at <650rpm, less if I have a good case than does not let the noise go directly to the user. I am going for computing power, not gaming, so I will not have a video card, just use on board graphics.


This is not yet possible, as H67 can't OC. Wait until Z68 if you want to OC and use integrated GPU.


As for the fan issues. one thing I know is 2 fans with the same noise level don't add up (at 1m) mathematically like was suggested. Assuming they both have the exact same noise profile (and even the exact same waveform in sync with each other) there are phase cancellations to consider since the sound is emitted from 2 different locations- that are actually an area and not even a spot. 1+1 does not equal 2 in this case. so don't expect ~6db increase in real life anytime soon. Take 2 identical signals and add them up in a mixer or audio software in exact phase sync -that is the only way to get that kind of math to work.

Also you need to consider sound behaves in an inverse square to distance measured. So while its possible to have silent fan at 1m no way its gonna be silent when getting closer. There is actually no such thing as a silent fan since fans move air and moving air IS sound. The only silent fan is one that has no voltage.

DUH ... :lol: IMO the point is to find a practical use for these SPL measurements. Fans under 600 RPM are quiet enough for a silent system. That has been shown and proven a few times in the unnaturally quiet SPCR's test chamber. The less fans like this you use the better.

When you use 4 fans its not just the total sound they produce individually added up in an audio calculator. they are also moving more air, so turbulence from grills is increased and case resonance affect the total sound of a system in unpredictable ways.

I don't think there is a way to truly know what the level of a sound is beneath the noise floor. Its like trying to estimate the sound of a product from a review at overclockers.net (or tom's ....add your favorite bad audio reviewer's name here). If their admitted noise floor is 40DB nobody knows what goes on below it. It is impossible to calculate.
Same thing happens in SPCR's room, though we do have the advantage of freq analysis and a very low noise floor plus, and maybe the most important, subjective observation of Mike C. If a sound is lower in level than that floor and the freq spectrum seems unaffected by the sound there is no way to know what level it has 4-5-6-8db? can anyone know for sure? (I remind you this is typically measured@1m). Even if 2 fans together seem to have some effect on levels you can't assume that 2-1=1, or it's db logarithmic equivalent. It might be that a certain frequency it stronger on both fans leading to an increase in SPL on that particular freq but on 2 other fans in slightly different speeds it would not happen. Its all hypothesis. You can get a better prediction from the subjective observation that from this kind of math.

This is why its so important that we have SPCR chamber to measure things. On pre-chamber days there was much more speculation going on. Some of it was proven right, some wrong. But could only be proven after the noise floor was lowered and reflections minimized.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 6:18 am 
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merlin wrote:
I'd think it certainly is possible to get a much lower db than the floor of the spcr chamber, but it's also pretty worthless as well. Either way it's really diminishing returns when your fans are far lower noise than other things such as your actual room background noise level. I'd rather have the added effect of keeping my system cool at that point.
Merlin, thank you for the concession that the fans themselves could have lower noise than the noise level of the noise floor of the SPCR chamber.

In some sense this fact is worthless, just like doing crossword puzzles can be considered worthless. But I think you missed my point that says it is not worthless. The model can be used to predict measured noise levels in the SPCR chamber when you add fans together. When you read the referenced article, did you say to yourself "hmm, that's strange, the two fans going together do not seem loud enough if I casually apply the logarithmic summing of noise levels equation". I did, always have when I see noise values asymptotically approaching the background noise level, but that review had enough data in it to do a more in depth analysis than just casual thinking about noise floor levels. And now I believe I have a sound model to explain the measured combo fan numbers, and can use it to help me get a general feel for what will happen if I put in enough fans to cool an O/C 2600K.

If you personally do not have any inclination to mathematically predict noise levels to help you with your builds, then indeed this article is worthless..to YOU.

If you have no incling to wonder which fan is truely the quietist when they are running so low that the SPCR system cannot differentiate between them under their measurement protocol, then indeed, this post is useless..to YOU.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 6:42 am 
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ame wrote:
photonblaster wrote:
This is not yet possible, as H67 can't OC. Wait until Z68 if you want to OC and use integrated GPU.
I stand corrected, I was imprecise. Pls tell me if this is a better statement: It does not really O/C, but you can increase the frequency bump step size when the CPU senses heavy calculation load. If you run CPU stres software, it stays at high clock rates so it looks like it is overclocked on a screen shot of performance parameters, but it is only stepped up." I just googled one screen shot that showed 5.1GHz on air! Would be far too loud for us of course, I am hoping for the mid-4's with my fans running below 700rpm.
Quote:
This is why its so important that we have SPCR chamber to measure things. On pre-chamber days there was much more speculation going on. Some of it was proven right, some wrong. But could only be proven after the noise floor was lowered and reflections minimized.
I agree with you, but is it not also really important to understand the limitations and effects of the SPCR system? Will this understanding not allow you to look at data such as the two fans together in the referenced article and not just be able to say "My experience says these measured results are reasonable, but rather be able to do some calculations that support your experience? Don't you want to be able to say, "Man this seems to be the quietist 120mm fan I have ever seen tested, and the calculated noise level at 600rpm suports this statement" rather than just the first part of this sentence?


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 7:15 am 
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For the first Q I have to say NO. Overclocking is only possible with P67 chipset. With H67 the maximum possible freq is 3.8 for when a single core is active and down to 3.5 for when all cores are active. Thats it. 5.1 on air was done using P67 chipset.

As for the second Q, and I really am serious, The most quiet fan ever tested was not connected at the time of testing, and yes it still measured the same as the fan that was on and had a level just below the noise floor. There is now way to know for a fact what a something sounds like if you can't hear or measure it. this is all speculation. How can you prove that Nexus @ 400 RPM is quieter than Arctic Cooling @ 400 RPM if you can't hear or measure either of them.

If I wanted to speculate I could ask: What would it mean if both of them together measured at 12db? who knows? You can't tell me if one of them is 8db and the second is 10db or the other way around, nor if they would measure 11db when both are @450 RPM (lower than 12db @ 400 = yes its possible that certain frequencies will not match and overlap causing them not to add up at certain speeds)

Like I said on the previous post some parts that SPCR measured before the chamber were considered as good as others. Then came the chamber and clearly showed how one is superior to the other. Only measuring can show this. not math. If you put them both in the same bad room and measure you can make no conclusion if one of them or both are under the measurable noise floor. Only if both are measurable.

In my system I have Noctua 120@650 and AC120@520. I think the Noctua is louder because when I put my ear right next to them I think the faint sound is coming from the Noctua. But HDD sound drowns them both. I could have had 2 more and still HDD would be louder - and this is from subjective observation.

Your chart is not linear, but even if a certain section appears to be linear you cannot conclude its all linear, nor can you calculate how a bearing noise would sound to a human when at a specific speed. a clicking bearing may not even register as DB value but it might be annoyingly noticeable.

Regardless I commend you on the attempt to bring some logic to this. Theory of sound is very interesting IMO

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 8:06 am 
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It is so interesting in fact, I went back to re read the entire first post.

I must say there are some major factors you did not consider.

First is the one fact is these measurements are made with the fans installed on the heatsink. not just the fans in free air. This alone may tilt the findings to an un calculative point.
Second - its also clear that 2 fans add up differently at different speeds suggesting specific frequencies are overlapping at various speeds causing non linear additive qualities.


In one place, where you said "not a bad fit", there were miss fits of up to 3 db. this alone puts the entire calculation at question. graph six has the most noticeable tilt.

Please don't get me wrong the calculated graphs look good and make alot of theoretical sense. But they offer no accurate way of calculating anything if they can be off by 2-3 db here and there. Certainly not when we are trying to calculate a number that is that is very low. this could lead to a 50-100% error...

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 9:13 am 
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ame wrote:
It is so interesting in fact, I went back to re read the entire first post.

I must say there are some major factors you did not consider.

First is the one fact is these measurements are made with the fans installed on the heatsink. not just the fans in free air. This alone may tilt the findings to an un calculative point.
Second - its also clear that 2 fans add up differently at different speeds suggesting specific frequencies are overlapping at various speeds causing non linear additive qualities.


In one place, where you said "not a bad fit", there were miss fits of up to 3 db. this alone puts the entire calculation at question. graph six has the most noticeable tilt.

Please don't get me wrong the calculated graphs look good and make alot of theoretical sense. But they offer no accurate way of calculating anything if they can be off by 2-3 db here and there. Certainly not when we are trying to calculate a number that is that is very low. this could lead to a 50-100% error...


Hi Ame: First let me thank you for your kind comment in your previous post.

Actually I did consider everything you said. It actually is critical that the fans be mounted on the HS. That way, the combined readings are taken with the fans in exactly the same position and environment as they were when individual readings taken. If the fans were measured un-mounted, one would have to take into account distance changes, not that difficult, but certainly less clean than having them mounted the same way throughout the measurements. And of course the mounting could change the result, maybe even more than the change in distance, and that cannot be accounted for in the calculations.

Re the quality of the fit, I guess you are refering to graph Six since that is the graph I made the good fit comment about. Indeed if you push the fans hard enough, the measured values fall short of the predicted values. I commented on this...part of the reason is that the fan's rpm is over-estimated because the rpm-voltage relationship is really not a straight line, just close enogh for calculations in the region of interest. You may notice I switched to RPM for the last graph since i did not have to related two fans together and could use the more logical RPM. I also stated that there may indeed be a change in the physics at these very high noise levels. EG, pure speculation, maybe there is a transition form laminar to turbulent flow in this region. BUT in the stated voltage ranges the fit is typically withing a half dBA. And look at Graph seven: the points and lines touch each other. There is also another mathematical test to look at whether the fits are good...correlation analysis. The correlation is in the 3-4 9's range (0.999 to 0.9999), which is phenomonally good.

If you look carefully at the RPM-Volts graphs, you will see that the points are NOT randomly above and below the lines. That is a good clue that the best fit is not linear. As stated earlier, it is however good enough to proceed with the calculations based on volts rather than RPM. The measurement error of the sound level swamps the error introduced by this simplification, at least at the lower RPMs.

Is there a data point that that I did not discuss that you saw as being 3dBA off in refernce to my good fit comment? If so, can you give me exactly what point(s) you are refering to?

You may also be refering to the points being somwhat randomly above and below the lines. This is normal in experiments, the individual values have a measurement error that moves them off the calculated lines in a random fashion, the distances vary depending on how much experimental errro there is. No measurement is ever totally repeatable. To do things proper, I would have to repeat the tests to evaluate the experimental error to see if the data points hit the line within that experimental error, but obviously unless SPCR lets me spend some vacation time in their lab I do not have that luxury. Do you remember "error bars" from a physics course you might have taken? That's what we need to properly estimate the error.

Looking the statistics of how far off the line the points are, I estimate the error in the 5dBA number as being lesss than 1dBA. This is not as good an estimate as re-doing the tests several times, but it usually gives a reasonable estimate of the error.

In your previous post, you mentioned one cannot resolve which fan would have the lower noise. Actually you can. Expermental physics is filled with examples where good values of a parameter of interest can be pulled out of noise and interacting signals. For instance, with proper sensors and signal analysis, one can identify single photons coming from a fast pulsed light source even if there is millions of photons comming from other places every second. And if you do not accept that arguement, it could be proven by SPCR by remeasureing the fans at closer distance where the measured values are much higher than the noise level at the same RPM.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 10:04 am 
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Why this is pure theory and has no implication on calculating what your next system will sound like (I just repeating the main points in my argument) -

A. sound is frequency dependent. Adding 10db of 100 Hz tone and 10db of 1000 Hz tone will result in a complex waveform 10 db loud. adding to that 10000 Hz tone will still stay at 10 db of loudness. there is no way to linearly predict what the adding of complex waveforms will yield.
B. Masking - how do these calculations explain the fact that a fan that measures 11 db in the lab is equivalent in loudness to the same lab without an operating fan (or the fan operating at 0 volts?)
C. air molecules colliding in a completely sealed and isolated room will still have some loudness. There is absolutely no meaning to a value of 5db for 'the fan alone' as there is no such thing.

I agree that the only way to measure fan loudness in these low ranges is placing the mic closer to the source. This is the ONLY way to really know what happens below the current noise floor. But then we will start seeing issues like proximity effect tilting the readings. Its also non standard technique. making results incomparable to standard testing methodology. I don't think its very practical either to know what level of sound a fan has at 5-10 cm since no one sits that far from a fan.

I think your math is right, I just don't expect your numbers to mean anything or be accurate at all. Since SPCR's room is so close to minimum possible we are lucky to be able to speculate in these ranges. Like I said in a previous post, before we had the chamber there was much more speculation going on by many users and some reviews. A good deal of it was proven wrong when the 'curtain was pulled' exposing the 11-21 db range.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 10:35 am 
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ame wrote:
There is absolutely no meaning to a value of 5db for 'the fan alone' as there is no such thing.
If SPCR does a two fans on a HS test agian, give me the numbers before two fans are tested together. I will predict what the two fans toghether will measure. The fans should be good ones. like the ones already tested. They will show numbers for the fan alone down in the 5-7dBA range for low RPM.

If my prediciton matches the SPCR combined measurements, surely you would agree that the calculated values for the fans has meaning? At least in a mathematical sense?

This is essentially what I did, except for timing. I did not cheat and try to make the final sum match what I calculated for the fans alone.

If you still say the values have no meaning, that is like saying the value for PI has no meaning. If I haven't convinced you by now, I probably never will. Our worlds are just too far apart.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 12:06 pm 
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photonblaster wrote:
ame wrote:
There is absolutely no meaning to a value of 5db for 'the fan alone' as there is no such thing.


If SPCR does a two fans on a HS test agian, give me the numbers before two fans are tested together. I will predict what the two fans toghether will measure. The fans should be good ones. like the ones already tested. They will show numbers for the fan alone down in the 5-7dBA range for low RPM.


Ok
Lest put this to the test
This is a heatsink with one fan

12V =16 dBA@1m =1100 RPM
9V = 13 dBA@1m = 890 RPM
7V = 12 dBA@1m = 720 RPM
5V = 11 dBA@1m = 530 RPM

The result of this heatsink with 2 fans like this are already printed so no cheating... I actually want this to work and so should you.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 1:42 pm 
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man, these statistics reminds me of how one of my friend works.

if you want to be that anal, you need to consider that any testing isn't done in a vacuum. so beside motor noise, you have wind resistance and impedance.

the fact of the matter, is that even if you did, your actual pc doesn't run in a vacuum - so it won't matter anyhow.

also since it is actually subjective, and the fact that you are tone deaf as you stated to some level, means that these statistics won't matter.

as with any statistics - it isn't an exact science by itself, so trying to prove something with stats is kinda redundant.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 1:58 pm 
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ame wrote:
photonblaster wrote:
ame wrote:
There is absolutely no meaning to a value of 5db for 'the fan alone' as there is no such thing.

If SPCR does a two fans on a HS test agian, give me the numbers before two fans are tested together. I will predict what the two fans toghether will measure. The fans should be good ones. like the ones already tested. They will show numbers for the fan alone down in the 5-7dBA range for low RPM.
Ok
Lest put this to the test
This is a heatsink with one fan

12V =16 dBA@1m =1100 RPM
9V = 13 dBA@1m = 890 RPM
7V = 12 dBA@1m = 720 RPM
5V = 11 dBA@1m = 530 RPM

The result of this heatsink with 2 fans like this are already printed so no cheating... I actually want this to work and so should you.
Ok, I will try to find some time to redo the analysis, hope to post next weekend. First I need to establish some more facts.
1) is this fan mounted on the HS when tested?
2) I assume the second fan is identical to this one?
3) what is the distance to the microphone, from fan one and from fan two? (Remember I said the fans either had to be at the same distance as when the two individual measurements when mounted, or one had to adjust for the changes in distance). If the measurements are not "as mounted", there will be more chance of disparity, but let's see what we get.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 2:20 pm 
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Lt_Dan wrote:
man, these statistics reminds me of how one of my friend works.

if you want to be that anal, etc.
AH Ha, I knew there would be at least one really "right winger" in the woodworks. See my first sentence in my post. Just a quick rebuttal...this is physics, not statistics. Statistics plays a role in analysing how good the physics is. And you mentioned subjective...pls read Ames posts which says that the SPCR sound lab was set up to get better quantitative results instead of subjective backed by relatively poor quantitative data, so no, it is not subjective when you talk about measured values. If it is anal to want to put numbers to help understand wonderous everyday phenomena, then I am glad to be cosidered amongst the ranks of many great physiscists who do so. Thank you for the compliment.

(I consider it wonderous to see desktop CPUs with hundreds of more computing power that a mainframe back in the 70's, being cooled by fans so effiecient that the noise they make at low RPM is almost exclusively from the air flow through them. I wonder if that's anal too.)


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 2:50 pm 
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I read this and enjoyed it. I didn't comment. I don't understand how this has become so contentious. I haven't really rad the back and forth. I'm, interested in the DB to CFM ratios at different speeds. Is one fan producing 14db moving more or less air than two fans producing 11 DB each? Do 4 fans producing 8DB each make a more pleasant sound than one fan producing 14DB? This data is really interesting, I'm just not really sure what to do with it.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Sun Feb 20, 2011 3:39 pm 
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goatsandmonkeys wrote:
I read this and enjoyed it. I didn't comment. I don't understand how this has become so contentious. I haven't really rad the back and forth. I'm, interested in the DB to CFM ratios at different speeds. Is one fan producing 14db moving more or less air than two fans producing 11 DB each? Do 4 fans producing 8DB each make a more pleasant sound than one fan producing 14DB? This data is really interesting, I'm just not really sure what to do with it.
Thanks for your post. Re your first question, see post http://www.silentpcreview.com/forums/viewtopic.php?f=9&t=19297 From my point of view, the percieved sound from a fan drops faster than the air flow when you lower the RPM. So the more the merrier within limit of course. Re "pleasant sound", that seems to be quite personal, and as I mentioned in my opening paragraph, I do not have a good ear for sound nuances, (nor the training, I would think the reviewers on SPCR have spent years developing their ability to distinguish subtle sound differences). But I did build a custom case with 4 of 120mm quiet fans running down at 500-600 RPM range and I could not hear them when sitting at the desk (the fans were on the other side of the case, away from me). But I need a new configuration for my new system with different design goals.

This is what I am doing with it....I am estimating how much dBA I will be comfortable with, put in the maximum number of fans that should be equal to or less than that number when combined at the right RPM and will fit comfortably in a small case, strategically locate them in this custom case, play with the voltages until I am comfortable with the trade off between the noise level and my computing power with an O/C CPU. I want my cake and eat it too...high clock rate at low noise level. At least what I consider low noise. If you just want a "silent" pc to read emails and run your home theatre, you do not have to worry about all this, just build one following the "silent pc build guides". For those, one fan can sometimes be all you need.


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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Mon Feb 21, 2011 2:34 am 
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Thanks for linking to possibly the best thread ever posted on SPCR! this was the one that initially convinced me this is a very serious site. I'm posting the link again just o be clear.

viewtopic.php?f=9&t=19297


If you've read the first test Mike C did you realize how difficult it is to estimate what will happen below a noise floor (in that case 16-17 db)

Knowing what we know today after that floor has been lowered to 11db it seems still some of the conclusions from that first 2005 post were, while having solid theory backing them up, not db accurate. Mike estimated that certain rules apply within certain dbA levels (over 20db). but really this conclusion was affected by his own noise floor. today you can make the same assumption with 11db. That is more accurate as the range above that floor is higher enabling you to calculate above it. but you cannot accurately calculate below it based on testing done in that room.

What Mike realized even then when he posted the equal loudness curves (and is the point of everything I've been saying), was that human perception and subjective observation of sound is more important that db values.

You cannot add 1+1=2 if you don't realize 1+1+x=? where x is the noise floor. (I'm of course simplifying the logarithmic formula to make a point)

Another reason why you don't wanna jump to conclusions - Some of the things Mike noticed where that sound behavior is frequency dependent. So you cant say 1 db is 1 db and call it a day. If you add 2 different frequencies @ X db you will not get X+6 db like in a perfect circuit. These rules apply to electrical or software emulated electrical (eg: DAW) implementation of audio. Math logarithmic calculations are just that.

A fan has broadband sound signature. Its sound is spread over a wide range of frequencies and its speed determines the range and overall tone (or what freq is most dominant). Adding a fan sound that is eg: most dominant @ 400hz with a sound that is most dominant @ 1000 Hz will not yield a 1+1=2 formula but rather create a new sound that has 2 dominant frequencies. And while is will likely be louder it will not comply to just adding db formula. not 3db and not 6db. it is completely dependent on what source sounds you used. tweak it by a few hz (or a few RPM for that matter) and the outcome is again unique.

The idea of a single logarithmic formula that can calculate sound levels of a fan given for any voltage based on 4 measurements done in an isolated room is reaching...

+ again the notion of 'a fan alone' is physically flawed and has absolutely no meaning. Why? because setting a range of 5-7 db for silent fans is like saying the tallest man in the world is 5-7 feet tall. That is a huge range when your theoretical or calculated noise floor is 0 dbA.

Sorry for being repetitive in this post. I just figure if you've read the 2005 post you would have known all of this.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Mon Feb 21, 2011 6:29 am 
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goatsandmonkeys wrote:
I'm just not really sure what to do with it..
Me too. But I think the discussion is getting more and more refined and by the time it is done things will be clearer.

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 Post subject: Re: 5dBA 120mm Fan.
PostPosted: Mon Feb 21, 2011 7:03 am 
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MikeC, even though the content of your post no doubt continues to be basically correct. presumably over the 6 years (almost to the day) since you last posted it, your knowledge has no doubt likely become more refined. Would you consider reposting an updated version of this message?

Also if two slower fans blow the same amount of air at the same noise level... do you have any thoughts about a third variable... static pressure... that is the extend to which a high static pressure affects the CFM rate of the two slower fans vs. the one faster fan. All things equal, perhaps there might be one result in a high static pressure testing environment and another in a low static pressure testing environment. (boy, there seems to be no end of non-intuitive and counter-intuitive complexity when dealing with this subject)

MikeC wrote:
Are 2 fans noisier than 1 blowing the same CFM?

This question came up again in the context of a long discussion about case airflow techniiques, strangely entitled, question on an oddity ive noticed here. I decided to do some empirical testing to answer the question with 2 Nexus fans versus one. The following, slightly edited from the orignal, was posted deep in the above thread where it will never be found.

---------------

NOTE: A basic rule of acoustic addition states that 2 identical sound sources will result in a 3 dB increase in overall noise. Four identical sound sources will result in +6 dB.

2 Nexus 120 fans, powered by a fanless PSU, in "the quiet room", in which the B&K SLM was seeing an ambient reading of 15~16 dBA (at dawn). Very hard to stop the meter from moving -- even a car 2 blocks away can have an impact when it's this quiet.

EDIT: A Nexus 120 @ 12V blows 37~40cfm; at 6V, it blows around 18~20 cfm.

from 1 meter:
1 fan at 6V (about the setting in my lab PC): 16~17 dBA.
2 fans at 6V (just like in my lab PC): 16~17 dBA.

I could not hear the change from one meter. But from a foot, I could hear it... so I put the meter closer.

from 1/2 meter:
1 fan at 6V (about the setting in my lab PC): ~17 dBA.
2 fans at 6V (just like in my lab PC): ~18 dBA.

OK, so that shatters the idea that 2 identical noise sources means a 3 dB increase. Obviously not a universal rule! I learned something new here!

This also confirms my impression that the lab PC, which has a very slow Nexus 120 for exhaust and a Seasonic Super Tornado with Yate Loon 120 fan, is ~18 dBA/1m. I should really measure it.... so I did.... and got 18~19 dBA.

Time to try the Nexus fans at 12V.

from 1 meter:
1 fan at 12V: 21 dBA.
2 fans at 12V: 22 dBA. :shock:

This is actually kind of amazing. It is not what I expected. So maybe the +3dBA rule only holds for louder noises? Or maybe it's the low freq balance and absence of midband of the noise that makes this rule not apply? Perhaps the rule is only valid without A weighting.

What did I hear at the 12V drive level? At 1m, the difference was very marginal, but I could hear it. From less than ~1/2m, the increased noise was more obvious but not big.

I tried the meter w/o any weighting. Levels jumped into the mid-30s and higher, and the needle was jumping up/down ~10 dB! I stood there and listend for a while to see if I could hear what the meter was picking up. 2 blocks away is Main St, by now starting to get busy. There was some distant rumbing that was audble but at a very low level. I am guessing that the meter was picking up on the low freq traffic rumbling <30Hz that was not so audible to me. In any case it was not usable w/o the low filter of the A weighting.

Time to try something more drastic: A much louder 120mm fan, which shall remain nameless for now.

from 1 meter:
1 fan at 12V: 30 dBA.
2 fans at 12V: 33 dBA. :shock:

Yup, +3dB "rule" confirmed at this level -- with this fan. Tried the test both ways -- either of the fans read 30 dBA by itself. Adding the other always brought the reading up to 33 dBA. And there was no question of the audibility at 1 meter. Totally obvious. This fan also had a whole lot more noise in the midband.

My preliminary conclusions:

1) The +3dB rule is valid but only when not weighted by any compensation curve. (Like the "A" weighting we use.)
2) The +3dB rule does NOT always reflect human hearing perception.
3) The exceptions are when the noise is mostly in the low freq, or probably when it is very low in level, say under 20 dB in the midband.

Secondary conclusions:

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

This jibes with my experience of Panaflo 80Ls... which I found puzzling for a while. My wife's PC has 3 80mm Panaflos -- one in the PSU, one on the HS and another at the front to keep the suspended B-IV drive cool. The fans all run at 5V or less. It measures 17~18 dBA/1m and sounds quieter than any PC I've built -- except for experiments with notebooks drives. I remember taking out the front fan; heard no difference so stuck it back in to keep the drive cool.

Again, I think it's a question of freq balance. When you slow any decent fan down enough, the sound shifts to a lower freq balance. This is true of every size of fan, I can vouch from years of experimentation. Both the whistling caused by blade edges and midband whooshing at high rpm drops dramatically at low speed. So the whole sound moves down in freq to where our hearing is less sensitive.

I think it is safe to say that when you combine low speed fans, that +3dB rule gets moot because...

1) your hearing is not sensitive to that noise because it's mostly low freqs. Certainly you don't hear volume changes as linearly as in the midband. So you just can't hear the increased acoustic energy / SPL even though it's there.
2) the audibility may be borderline because it's at or near ambient level, so again, you can't hear it or not well.

BTW, AFAIK, low freq sensitivity in human hearing does not change much with age -- except maybe for those who really elderly or with abnormal hearing loss/damage.

--------------------------

In the original thread, NeilBlanchard followed up with this post:

Here is a link to an explaination of the Fletcher Munson curves. Huh? What does this have to do with this discussion?

Image

Just about everything...

Other links:
http://www.allchurchsound.com/ACS/edart/fmelc.html
http://www.sfu.ca/sonic-studio/handbook/Equal_Loudness_Contours.html
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html#c1
EDIT: A note about the above curves: These are the inverse of the frequence response of human hearing at different loudness levels. The top curve tell us that when subjected to a broadband sound of 110 dB SPL, we hear the frequencies between 1000 and 7000 HZ most acoutely, with the rest being fairly linear. When the sound is 50 dB, the 30Hz portion of the sound is heard as ~30 dB quieter than the signal at 1000 Hz. Ditto the extremely high frequency sounds beyond 10KHz. When we get down to the 20 dB level, the 30Hz portion of the signal is perceived to be ~50 dB quieter than the midband portion.

This phenomenon of rapidly decreased ability to hear low frequency sounds as loudness level drops is precisely what is reflected in my experiments here. The shrewd PC silencer will want to exploit this aspect of human hearing to maximize cooling airflow while still keeping audibility to a minimum.

-------------------------------

After which I added:

I see the +3dB rule as an interesting physical phenomenon that can have a bearing on what we hear. By itself, +3dB rule does not dictate what we hear; it is simply a relationship between physical phenomenon and the meaurement standards/tools we have. The acoustic energy in the space is much less important than what you hear -- the 2 are not the same at all, far from it.

The A weighed dB scale does not try to read the acoustic energy (SPL is not reallly an energy measurement anyway...); rather, A weighting is an attempt to simulate the human hearing frequency response documented by Fletcher Munson (thank you NeilBlanchard ;))and others. It's a bit rough, but much better than unweighted SPL readings, which have almost no correlation to what we hear when levels / freqs are low.

So when I measured 15~16 dBA ambient, then got 16~17 dBA with one or both fans turned on a meter away, this is very very close to what I heard. IE, I could not tell whether one fan was on or two fans from a meter away. I had to get within ~a foot to hear that difference clearly. The dBA readings did a very good job of simulating what I heard, and this is what really matters: the measurements jibe with what I hear. If the measurement don't reflect with what I hear, then, for our purposes, the measuring tool/system has to be made better -- NOT the other way around.

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