stacking undervolted fans

[bobkoure]

As an experiment I just "stacked" a second Panaflo L1A on the one I've currently got set pulling air through a chevette core.
Both fans are undervolted to 5V (set up for quiet).
Idle temps went from ambient+12 to ambient+8 or +9.
Noise level with the additional fan is slightly higher (adding a second identical noise source should theoretically add 3dB) and to my ear at least, seems quieter than a single L1A at 7V, which seemed to result in ambient+10 (all readings from the CPU diode, inaccurate, 1C difference probably means nothing, 3 or 4C difference means, IMHO, something changed - even with an inaccurate diode... I am using the thermometer in a wall-mounted thermostat for "ambient" - my "motherboard" (really case temp) readings are often the same, so I think that temp's about right...

I have not remounted the CPU/wb between readings (the other thing that might alter things.
Just someone here might find this of interest - probably should also post it over in the silent-pc watercooling forum, although my PC is actually pretty noisy (probably between 16 and 20dB - by meter doesn't go this low so I'm guessing) compared to the results folks are getting there...

[Jabo]

By stacking fans you getting better air pressure and better temps. Could you try making a tapring shroud with two of your fans at one end side by side and core on the other?
Making it out of paper is the easist and cheapest way to do this test

[bobkoure]

It's all very "wedged in there" at the moment. I'm using a coolingworks shroud (with the inner opening that fan is pulling through opened up so the airflow doesn't encounter a ridge - probably not an issue for pulling air through, which is what I'm doing). Anyway, this is already somewhat tapered, and a 6" (150mm) core is not lots bigger than a 120mm fan.
I understand that what I've done is increase pressure - essentially adding pumps in a series.
Bill Adams comments in one of this radiator reviews that stacking two 7V L1A fans doesn't help (at least with the radiator he was testing with).
I encountered another data point, thought I'd add it to the general knowledge.

[MMZ_TimeLord]

If you take those two fans and make a ½" to 1" "shim-shroud" between them instead of stacking them right on each other... the blades will get a lot quieter (even at 5V) and should keep the turbulance in the airflow down to a minimum.

[krazy]

I used to have a pair of fans stacked together in my chimney fan (call it a blowhole if you prefer) primarily for the purpose of being able to shut one of them off (it was a bright blue LED fan that annoyed me at night). The second fan screwed onto the bottom would still send plenty of air through, and I was pleasantly suprised by just how much of an increase having both of them on at once would make.

I have to agree with MMZ that placing some sort of shroud between the two fans to separate them a half inch even would probably make them noticably quieter.

[bobkoure]

I put a shim shroud in (hollowed out 120x25mm fan frame).
Reported idle temps remain the same 8 or 9C over ambient.
Sound is, hmmm... sound levels at this level (particularly without being able to do a "blind" A/B test) are really hard to judge. Is it really quieter or am I just perceiving it that way because I think it should be quieter?
Bob
PS: I was aware of likely interference (if from nothing else, than from the "struts" of the first fan) but really wasn't sure whether I'd have room. Turns out I do, but the bottom right corner of my mb is now pretty occluded. I looked at all the "bits" there and there don't seem to be any heat generators (mostly just connector pins) so I think I'm OK. If I kill the mb, well, I'll be unhappy, but I have been wanting an excuse to get one of those DFI Infinity mbs...