Hardware Insights Forum

I've tried to keep the answers as short as practical while still providing the required information. If you have anything to add, tell me about it. For more detail, refer to this thread.

Q1. How do you choose a fan, basically?
A1. As a compromise between high airflow, small size, and low noise.

Q2. What part of the fan produces the most noise?
A2. At moderate to high speeds, the tone from the impeller usually dominates. Bearings in good condition are only responsible for a small part of the overall fan noise (except with ball bearings at very low speeds).

Q3. How do I determine the pitch of the tone?
A3. Frequency in Hz = (RPM/60)*N where N is the number of blades

Q4. What effect will obstructions have on the fan noise?
A4. Usually a small one if they're placed "behind" the fan (by which I mean on the side it blows at). But if they're in "front" of it (the side it sucks from), the noise will increase dramatically, and you may end up reducing the speed to compensate. Mounting a safety grille to the "front" of the fan will (not might) cause that.

Q5. How do I know which way the fan blows air?
A5. There should be a pair of arrows on the fan frame. One indicates the rotation direction and the other indicates the airflow direction.

Q6. What is the effect of changing the rotation speed?
A6. The flow in free air is correlated directly to rotation speed, and the pressure is correlated to the square of rotation speed. As for noise…let's just say you don't want to run the fan faster than truly necessary.

Q7. Are there big differences between the performance characteristics of different manufacturers' fans or are they all largely similar?
A7. Similar fans usually provide similar performance. Reliability is the primary basis on which manufacturer (and model) to choose.

Q8. Are the big top-mounted PSU fans really a better configuration? It looks like a lot of compromises they have to make…
A8. There are indeed a lot of compromises, and given similar ventilation in both units (there are many 80mm-fan units with awful ventilation), it is ultimately worse than the traditional rear-mounted 80mm fan.

Q9. Can you change the fan speed by altering the input voltage?
A9. Yes, to a degree, but if you go too low it won't start, and if you go too high you'll destroy it.

Q10. How do sleeve bearings compare with ball bearings?
A10. Sleeve bearings in good condition are quieter, but should only be mounted with the shaft horizontal and the lubrication is critical. Ball bearings can run in any orientation but are more vulnerable to shock. As for fans with one ball bearing and a sleeve bearing, they would seem to be merely the worst of both worlds (if not even worse!).

Q11. What are the maximum rotation speeds you'd recommend to stay at an acceptable sound level?
A11. Outside of PSUs (where wind noise often becomes the elephant in the room) — 6000RPM for 40mm; 4800RPM for 50mm; 3600RPM for 60mm; 2700RPM for 80mm; 2100RPM for 92mm; 1600RPM for 120mm — given no obstructions in front of the fan. I wouldn't really call those quiet, though — that will require significantly lower speeds again.

Q12. Are thicker fans quieter than thinner ones?
A12. Sometimes they are, sometimes they aren't. My best advice would be to look at the specs. However, a notable example of the thicker fans not being quieter is when comparing 120mm fans, where the 38mm thick models are usually (if not always) designed to be powerful, so whenever quietness is desired, use the 25mm thick versions (or the 30mm thick Phanteks PH‑F120T30).

Q13. How much power do these fans draw?
A13. At rotation speeds providing an acceptable noise level in PCs, they usually draw less than 2W each.

Q14. What if you use two fans in series or parallel? (airflow-wise)
A14. If the fans are identical, running them in parallel will double the flow in free air, but the static pressure will remain unchanged. Running them in series will not change the free-air flow, but will double the static pressure. The parallel fans will be up to 6dB louder than an individual (so a single large fan would be preferable), but far quieter than a similar fan at double the speed would be.

Q15. Is there any benefit in changing the original thermal pads out for thermal paste?
A15. Not if you're dealing with the modern phase-change pads.

Q16. How does copper compare to aluminium as a heatsink material?
A16. Copper has close to double the thermal conductivity of aluminium, but is more expensive and much heavier; so is justified mainly in places with high heat density. (Integral heatspreaders are usually plated copper.) Ultimately, though, it's not about what material is used — it's how the heatsink is designed.

Q17. Do heatsinks perform better with the fan blowing down or sucking upward?
A17. It's a bit hard to say, but consider first that fans with obstructions directly in front make much more noise. That probably explains why it's standard to have the fan blowing down.

LongRunner wrote:IQ17. Do heatsinks perform better with the fan blowing down or sucking upward???
A17. It's hard to say, but consider first that fans with obstructions directly in front make much more noise. That probably explains why it's standard to have the fan blowing down.

This link reveals something interesting.... assuming the fan is within very close proximity to the heatsink in question, that an intake fan may be more effective.

LongRunner wrote:Q8. Are the big top-mounted PSU fans really a better configuration??? It looks like a lot of compromises they have to make...
A8. There are indeed a lot of compromises, and it is ultimately worse than the traditional rear-mounted 80mm fan.

I ran an experiment recently with a Besta PT-500 Power supply. First, with its stock fan configuration, and then I transplanted it into another case with a 120mm fan (with the back half baffled, as usual). Under 250W Load in the stock configuration, the Secondary and Primary heat sinks hit about 55 and 75*C respectively. In the other case with the 120mm fan, about 40 and 50*C respectively. At least in that case, the 120mm fan did a much better job cooling than the 80m fan did. That said, however, the ventilation in the stock case wasn't the best, with only slots on the intake side. I want to repeat this experiment on a PSU with a better intake grille.

I know many PSUs with 80mm fans have terrible ventilation. I'll note that in the update. But, of course, an unfair comparison like that isn't very indicative of how the fans would compare under similar conditions.

Also, what were the (approximate) rotation speeds of those fans and how loud are they??? You did say in the review that the stock fan in that unit is a low speed model, and I imagine very low speed given all that wind noise that results inside a PSU (outside of the PSU it seems to be much less of a problem). I should add that the "maximum acceptable RPMs" I listed earlier are not recommended for PSU fans.

I still think the rear-fan setup is preferable, given comparable ventilation, simply because it doesn't amplify the tonal noise.

^
I reaslise that, which is why I want to repeat the experiment. But it does go to show that a well implemented 120mm fan design is by far preferable over a badly implemented 80mm fan design. The two fans were both generic, so I don't know what speeds they were rated for. My best guess is maybe 2000-2200RPM for the 80mm fan and 1600-1800RPM for the 120mm. This particular unit did seem somewhat louder than I remember the last PT-500 being.