There are blade design differences bearing differences motor differences etc. There are 40mm fans ranging from 10 dB to 100 dB. Again if someone wants to bother and look at what fan and what specs. they currently are, they can then source their own "better" or "quieter" equivalent.
A fan spinning at 6000~9000 rpm can never be quiet, no matter the bearing or blade design. It's impossible. At this level, you don't hear any noise from the ball bearing, you just hear the airflow. At this fan speeds, you will NEVER achieve laminar flow.
The bearing
does matter if you look for fans spinning at 300~500 rpm. Then you even prefer a sleeve bearing over ball bearing, because sleeve bearing is more quiet. Again, at this level you only hear the mechanics of the fan, not the actual airflow. But for fans with many thousand rpm, ball bearing is the only option and turbulent airflow will always be noisy.
If you want a quiet SP10, go for watercooling.
I agree with the watercooling, a cold plate could be placed on the bottom but it will only draw heat from the one blade, although it could pull out enough heat to turn the fans down depending if a chiller is used and pump, etc.
The thing is that with more than a few of these the noise is an issue too, also the heat, with the concentration in such a small area I think that it is not able to pull all the heat out with the slimmer heat sinks and the smaller fans, even if it does pull 150cfm. On a 42u Rack they have to be spaced about 4u apart or the heat from the miner below will affect the top. These are sitting on rack trays and a lot of heat is actually dissipated from the bottom, it's hot enough to cook an egg
well with my contact thermometer it shows that the front of the heat sinks get to over 100c and is about the same on the bottom of the case. The rack trays that I'm using has some vent holes and it helps a little bit but the entire tray gets hot as well so there is a large amount of heat transfer. It's hot enough to burn my hand if I try to lift the miner from the bottom where the heat sinks are touching the case. Over 100c as well.
From what I've looked at it may only cost a couple hundred at most for a single machine, cheaper of course with more since the same pumps and chiller would be able to be used for many miners. Still for one miner it may be even less as I've seen some pumps that are less than $20 and would be able to move more than enough water though a cold plate...
I think I'm going to give this a shot and if it works I'll post some pics as I think that with one SP10 I may not even need a chiller as long as I have a large enough reservoir of water, I may hook it to a radiator that they sell on Amazon for about $40 and a 12" radiator fan which can also be found for about $20 on Amazon.
Oh, also to add where I live it gets very hot, up to 120f in the summer, my central AC does just fine but now with the extra heat from about 10Th of miners in total it can get toasty and this is sitting in a 10x15 sqft living room (which can not be used as a living room due to the sound) and my AC is on 100% of the time, there is air being exhausted out a window but it is not enough, on a good day and if the temp outside is below 85-90f I may get the living room ambient temp to about 85f but when it is hot like over 90f outside then it can get close to 100f inside.
This increases my power bill from not just the miners but also the AC on 100% at full blast all the time. The bill is about $300-$400 more so it would likely be worth it to spend the one time costs to get water cooling system installed. I'm not sure about others but it's worth the money as just one or 2 months would make up for the extra hardware, especially in the hottest part where it is over 120f in the day.