Bigger Fan w/Lower RPM = Less Noise, Same Airflow?

[Trackr]

Just curious if this is a rule.

If I have a 200mm fan running at 2000RPM will it deliver the same airflow as a 120mm fan at a higher RPM and cause less noise?

Also, undervolting a 12v fan is easy enough, but what about AC fans? I've been reading around and people say you shouldn't do it. Is there credibility in that?

Thanks.

 

[X72]

You can't just assume that. There are other factors to consider such as the blade type and in some cases, air pressure which can be useful.

 

[Trackr]

You can't just assume that. There are other factors to consider such as the blade type and in some cases, air pressure which can be useful.

Well, okay. Assuming we're talking about fans with very high air pressure such as inline fans..

An 8" inline fan rated at 800M3/h undervolted to 400M3/h vs. a 6" inline fan rated at 400M3/h.

Assuming the same make and model, wouldn't the 8" fan be more quiet while achieving the same airflow/air pressure?

 

[Papaj]

Larger blades that spin slower typcially result in reduced noise, as there is less turbulence being generated to move the same amount of air as smaller blades. However, in order to get the full story with real numbers, fan curves for votlage vs CFM vs SP would need to be looked at. Some fans have poor voltage control and a lower operating range of voltages or can experience slightly more non-linear CFM or SP curves than one would expect. Additionally, blade angle and aerodynamics need to be taken into account - but generally fan makers market their fans as either high CFM or high SP and have appropriate blade designs for such applications.

 

[Trackr]

So, I should, in theory, get the 200mm fan and undervolt it instead of the 150mm fan?

 

[Archmage]

Pressure is force/area.

Ignoring blade aerodynamics (shape / angle), high RPM is still needed to generate high pressure. Think of an automotive turbocharger producing pounds of "boost" - the turbines are actually about the diameter of medium-sized computer fans, but the RPM is staggeringly high (130K+ RPM).

That 200mm fan may move a similar total CFM when compared to a higher RPM smaller fan, but it will be pushing much lower velocity airflow, and likely won't be as useful for directing airflow to (or through) a target, or pushing airflow from one end of an enclosure to another. A 200mm fan may provide a good bulk intake or exhaust, but the computer case may need to rely on more powerful fans to direct that airflow.

Additionally, our fans produce axial flow. I think having sufficient pressure compensates for this because our cases are somewhat like large directional ducts anyway.

Silverstone thought differently: The concept behind Silverstone's "air-penetrator" fans was to straighten the airflow and provide enough pressure to create an optimized vertical wind-tunnel in their cases.

I'm not suggesting those fans, I just think the design illustrates targeted airflow, ducting, pressure.

High RPM will generally produce higher frequency noise... but you may actually get more usable CFM depending on the scenario.

For a case fan: pressure can still be useful.

Heatsink / Radiator fan: pressure will be very important, especially when pushing through high fins-per-inch.

If one can approximate the restriction of the heatsink / radiator / case, and you have the PQ curve (volumetric flow vs pressure) of the fan, then you may be able to approximate the performance of the fan in that specific usage case.
This is similar to approximating the flow of a water-cooling loop given the pump's PQ curve and the pressure drop of each component in the loop (I think we actually have better data for this).

It's more practical to just make some quick estimates and test it out...

Edit: To answer your noise question... I think it'll depend on the quality of fans. A good 120mm fan running at below about 1000 RPM is very quiet. 4 of these fans in a 2x2 config would yield a square "240mm fan." If the 120mm fans have no real mechanical noise, then they may not actually be much louder than the 200-250mm fan they replace. Their bearings may also last longer (fans deteriorate). Unfortunately, I don't have experience with quality fans larger than 140mm. It's difficult to evaluate these things. Adding to that: Fans often work optimally in certain voltage ranges, and exhibit noise / resonance in other ranges. This can even vary within the same line of fans due to manufacturing differences.

I think a lot of us have just tried a lot of things out over the years and drawn our own conclusions.

 

[Trackr]

That 200mm fan may move a similar total CFM when compared to a higher RPM smaller fan, but it will be pushing much lower velocity airflow, and likely won't be as useful for directing airflow to (or through) a target, or pushing airflow from one end of an enclosure to another. A 200mm fan may provide a good bulk intake or exhaust, but the computer case may need to rely on more powerful fans to direct that airflow.

Okay, but what if we're talking about inline fans that have very high air pressure.

If there is an 8" inline fan that does 800M3/h and a 6" inline fan that does 400M3/h, can I undervolt the 8" so that it is equivalent to the 6" in both air flow and air pressure while retaining lower dBa?

 

[M76]

Okay, but what if we're talking about inline fans that have very high air pressure.

If there is an 8" inline fan that does 800M3/h and a 6" inline fan that does 400M3/h, can I undervolt the 8" so that it is equivalent to the 6" in both air flow and air pressure while retaining lower dBa?

If you undervolt the large fan to have the same cfm, it will also loose air pressure, so you won't have equal cooling efficiency. You can't set up the larger fan to have both the same air pressure and same cfm at the same time. You can't have the best of both worlds. It depends on the usage, what you need more, high airflow or high air pressure. But high air pressure always comes with higher noise. As you need higher RPM for higher pressure. There is some advantage in better blade design, and number of blades, but there are no miracles.

As for inline fans I think they create much more noise than they worth. But again it depends what you want to use the fan for. Myself I'd never consider inline fans for cooling my PC: When I was looking at fans, the CM JetFlo came up with the best air pressure, but they generate equally much noise.

 

[Nenu]

I use larger fans to reduce noise and maintain or even improve cooling.
ie my external radiator is silent using an undervolted 250mm fan and works very well.
I chose a case with a 200mm top fan for the same reason.

 

[M76]

I use larger fans to reduce noise and maintain or even improve cooling.
ie my external radiator is silent using an undervolted 250mm fan and works very well.
I chose a case with a 200mm top fan for the same reason.

What kind of radiator you're using? It's important too, because the layout and clearance between radiator fins determines what fans will work better.

 

[Nenu]

Its a large car heatercore.
Its longer than 250mm but not as wide as the fan so the fan sticks out of the sides a bit.

I managed to push the 250mm fan right up to the radiator with no noise, it cools very well.
(for convenience, its a lot easier to lean the fan on the radiator than create a mount and is quick and easy to clean both the fan and radiator)
120mm fans could be heard when performing well enough and and significantly more noise was generated when pushed to the radiator.

It saved me having to make a mount and mini tunnel for the 120mm fans so they could be held apart from the rad.
And its dead silent.
My PC fans on lowest speed are the only things I can hear and they are quiet.

 

[Trackr]

If you undervolt the large fan to have the same cfm, it will also loose air pressure, so you won't have equal cooling efficiency. You can't set up the larger fan to have both the same air pressure and same cfm at the same time. You can't have the best of both worlds. It depends on the usage, what you need more, high airflow or high air pressure. But high air pressure always comes with higher noise. As you need higher RPM for higher pressure. There is some advantage in better blade design, and number of blades, but there are no miracles.

As for inline fans I think they create much more noise than they worth. But again it depends what you want to use the fan for. Myself I'd never consider inline fans for cooling my PC: When I was looking at fans, the CM JetFlo came up with the best air pressure, but they generate equally much noise.

Okay, so let's say an 8" fan has 800M3/h and X air pressure.

If I undervolt it to 400M3/h would it have the same air pressure as a 6" 400M3/h fan?

 

[M76]

Okay, so let's say an 8" fan has 800M3/h and X air pressure.

If I undervolt it to 400M3/h would it have the same air pressure as a 6" 400M3/h fan?

There is a possibility, but it would have to have different design. There are lots of fans being made for every use, industrial fan manufacturers give an exact graph on air pressure/air flow. You might want to look around.

For example sunon