Question:
If you have some hypothetical 80mm fan and from this you design a 120mm fan using the same materials for the bearings, blades, etc. the only difference being that it is a scaled up version of the 80mm fan then, for the same amount of air flow (set the speed of each fan to produce equal air flow), which is quieter?
Can this question be answered objectively by making measurements using a sound pressure meter or spectrum analyzer or is it totally subjective so that there will never be agreement.
Which is quieter - 120mm or 80mm?
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NeilBlanchard
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Re: Which is quieter - 120mm or 80mm?
Hello Cy & welcome to SPCR!
The swept area of the blades on the 120mm fan is roughly 2.33 times greater than that of the 80mm fan. (I used a hub size on the 80mm fan of 25mm, and 30mm for the 120mm fan -- this is something that will not scale proportionally.)
So, going with all else being equal (which we know it is not -- the larger fan will be a lot more efficient, since less air is "near" the tips of the blades and hence less "slips off" the blade tips...), then if the 80mm fan is spinning at 2000 RPM, the 120mm fan will be spinning at just 858 RPM. The resulting pitch of the sound is less than 1 octave lower.
Another advantage that the larger fan has is that the blade design can have a greater length of the arc in the forward sweep of the blades. This allows the blade's leading edge to have a gentler angle as it slices through the air and a longer time to accelerate the air to a higher velocity, since there is more distance to steepen the angle of the blade.
Have you ever wondered why the blades are swept forward? I think it is because the air is better kept on the blades and it moves toward the hub. If they were swept back, then a lot of air would be lost off the ends of the blades.
I have a pet theory that I tried to test, but I was limited by the methods I had available -- that if the blades were swept back and the tips of the blades were connected to a cylinder, that spun with the fan, that this design would move more air at a given RPM than a more typical design. The reason I think this is that the tips are moving faster (through the air) and also since the air could not "slip off" the ends of the blades, this would be more efficient!
Another advantage might be that since the tips could be broader and not have to be tapered (as they are on typical fans, to reduce noise for the turbulence that would result from sharp corners moving through the air), they would have a greater blade area right where it could do the most work.
Since the 120mm fan is bigger, we know that it won't have to spin as fast to move the same amount of air, we can deduce that the frequency of the sound produced will be lower in pitch.Cy wrote:Question:
If you have some hypothetical 80mm fan and from this you design a 120mm fan using the same materials for the bearings, blades, etc. the only difference being that it is a scaled up version of the 80mm fan then, for the same amount of air flow (set the speed of each fan to produce equal air flow), which is quieter?
Can this question be answered objectively by making measurements using a sound pressure meter or spectrum analyzer or is it totally subjective so that there will never be agreement.
The swept area of the blades on the 120mm fan is roughly 2.33 times greater than that of the 80mm fan. (I used a hub size on the 80mm fan of 25mm, and 30mm for the 120mm fan -- this is something that will not scale proportionally.)
So, going with all else being equal (which we know it is not -- the larger fan will be a lot more efficient, since less air is "near" the tips of the blades and hence less "slips off" the blade tips...), then if the 80mm fan is spinning at 2000 RPM, the 120mm fan will be spinning at just 858 RPM. The resulting pitch of the sound is less than 1 octave lower.
Another advantage that the larger fan has is that the blade design can have a greater length of the arc in the forward sweep of the blades. This allows the blade's leading edge to have a gentler angle as it slices through the air and a longer time to accelerate the air to a higher velocity, since there is more distance to steepen the angle of the blade.
Have you ever wondered why the blades are swept forward? I think it is because the air is better kept on the blades and it moves toward the hub. If they were swept back, then a lot of air would be lost off the ends of the blades.
I have a pet theory that I tried to test, but I was limited by the methods I had available -- that if the blades were swept back and the tips of the blades were connected to a cylinder, that spun with the fan, that this design would move more air at a given RPM than a more typical design. The reason I think this is that the tips are moving faster (through the air) and also since the air could not "slip off" the ends of the blades, this would be more efficient!
Another advantage might be that since the tips could be broader and not have to be tapered (as they are on typical fans, to reduce noise for the turbulence that would result from sharp corners moving through the air), they would have a greater blade area right where it could do the most work.
Another question with slightly different emphasis: if I need a fan simply to create a small amount of airflow (actual amount not important), but I want to do it in the quietest way possible, is say, a Panaflo 120L @ 5V quieter than a Panaflo 80L @ 5V? Or is it because motor, bearings etc are larger that more noise would be created?
Well, the question was at the same RPM, so the answer becomes a bit more complicated.mrzed wrote:At the same RPM, a smaller fan of the same type will be quieter than the bigger one, no question. So if a slow 80 does the job, then that's fine.
Neil's post outlines the theoretical base of why a 120mm is better than an 80mm. However, there are other factors that come into play. Please remember that the motor on the 120mm has to be more powerful to move a larger mass. Even if the efficiency level is kept constant, a powerful motor always dissipates more energy due to inefficiency. Energy that has to get away as heat and noise. Furthermore, most fans are designed to operate at a specific voltage level. Unless you design your fan to specificaly rotate at a certain speed (rpm), you'll be running the motor at a rate than it was designed for. This results usually the infamous click. All this adds up to why we have an established favorite 80mm fan but the discussion over which 120mm is better is still going on.
Neil, if I understand your cylinder+blade, you mean the design on the TMD fan. I've tried to examine the TMD fan in open air and the conclusion I came to was that it did not push more air but it did so at a greater pressure level and with greater consistancy across the blowhole. And I don't think this was due just to the reduced hub crosssection, the blade tips really helped both the pressure and the noise level.
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NeilBlanchard
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Hello:
The cylinder is there to trap the air and keep it from slipping off the ends of the blades, and take as much advantage as possible of the higher speed of the blade tips.
Well...sorta. I would leave the motor in the center hub, and it would be a 25mm thick fan. The main point is that the blades could be swept back (rather than forward) thereby gaining air flow because the tips could be more fully used. Forward swept blades seem to force the air to "slip" toward the hub, where there is much less blade speed.burcakb wrote: Neil, if I understand your cylinder+blade, you mean the design on the TMD fan. I've tried to examine the TMD fan in open air and the conclusion I came to was that it did not push more air but it did so at a greater pressure level and with greater consistancy across the blowhole. And I don't think this was due just to the reduced hub crosssection, the blade tips really helped both the pressure and the noise level.
The cylinder is there to trap the air and keep it from slipping off the ends of the blades, and take as much advantage as possible of the higher speed of the blade tips.
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silvervarg
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Well, you have got some of the theory here, presented in a quite nice way.
Practical test with a single fan, 80mm vs 120mm of the same type and model usually shows that you get the same airflow from the 80mm fan at 7-8V as you get with the 120mm fan at 5V. This depends a little bit on what fans you pick.
So if you need less airflow than the 80mm fan gives you at 7V then the 80mm fan is usually a better choise. If you want more airflow than the 80mm fan gives at 8V then the 120mm fan is a better choise.
In the 7-8V range they will be fairly equal, so you might be better of with a 92mm fan in this range.
With the same thinking an 80mm fan fed with less than 5V might be better to replace with a 60mm fan. However we have very little testing in this area, so results are a bit uncertain. Noice measurement at this low noise level is also very hard to do.
Comparisons at same RPM is very unfair, both airflow and noise level will be totally different. Comparisons with same blade-tip-speed is at least semi-fair since that will often be rather close in noise level.
Practical test with a single fan, 80mm vs 120mm of the same type and model usually shows that you get the same airflow from the 80mm fan at 7-8V as you get with the 120mm fan at 5V. This depends a little bit on what fans you pick.
So if you need less airflow than the 80mm fan gives you at 7V then the 80mm fan is usually a better choise. If you want more airflow than the 80mm fan gives at 8V then the 120mm fan is a better choise.
In the 7-8V range they will be fairly equal, so you might be better of with a 92mm fan in this range.
With the same thinking an 80mm fan fed with less than 5V might be better to replace with a 60mm fan. However we have very little testing in this area, so results are a bit uncertain. Noice measurement at this low noise level is also very hard to do.
Comparisons at same RPM is very unfair, both airflow and noise level will be totally different. Comparisons with same blade-tip-speed is at least semi-fair since that will often be rather close in noise level.