I have a couple of 140 mm Slipstreams at 800 rpm, but they can only be undervolted downto ~600 rpm, which is not low enough for me.
For me it would be nice to go down to 500 rpm, since with 120 mm Slipstreams I am comfortable at 600 rpm ( 500 & 14 ~ 600 & 12, sort of).
I can buy 140 mm 800 rpm Glidestreams, but I don't know if they'll have the same problem as the Slipstreams. Or if you know of some
alternative 140 mm fans that have good availability and can be used at 500 rpm instead of the 140 mm Slipstreams, please tell. Thank you.

There is a PWM version of the Glide Stream series that bottoms out at 500 RPM, but I haven't tried it myself. I'm waiting to see the upcoming Slip Stream XT 140mm model at 500 RPM(SM1425XT12SL, 3-pin), but don't know when it will become available. Might not be too far away (being listed here and there) and they look interesting.

Since I couldn't find any information on 140 mm Glidestream undervolting, I had to buy one myself, and I have bad news.
It has the same problem as the 140 mm Slipstream, can't go below 650 rpm (actually this is worse).
Does anyone know of a 140 mm fan that can be undervolted until it goes downto 500 rpm?

Isn't there a 500rpm 140mm Slipstream version on the market? Called Kaze Maru, or so.

edit: how can't they go below 650rpm? Is that due to your mainboard fan control, or do they stall at lower voltages?

Looks like the Glidestream PWM version starts at 370rpm. (Embedded in the cooler review).

Looks like the Glidestream PWM version starts at 370rpm. (Embedded in the cooler review).

Kaze Maru 140 mm aren't available anymore. I don't want to use PWM fans, I like the manual control.
I use a Scythe fan controller, runs fine with 120 mm 800 rpm Slipstreams, can go downto 600 rpm or even a bit lower.
With the 140 mm ones I tested they stall under 600-650 rpm, which is still a bit loud for me.
With a 140 mm Slipstream at ~600 rpm I can cool a GTX 670 in Furmark, but for idle I want to lower the rpm without stalling.
Current options are Antec True Quiet (can buy in Bucharest) or BeQuiet Silent Wings 2 (but can't ship from Germany).

Update: In the meantime I tested a Kaze Maru 1,200 rpm, which turned out to be closer to ~1,300 rpm and only
undervolted downto ~900 rpm before stalling, and Aerocool RS12 (13 cm fan with 12 cm holes, 1,000 rpm) which
only undervolted downto ~700 rpm and had some motor noise.
Today I got a Cougar Vortex V14S (1,000 rpm) which only turned downto ~720 rpm. Also two Antec Truequiet 140 (800 rpm)
which have a high / low switch. One did ~450 @ L and ~750 @ H, the other ~420 @ L and ~720 @ H. But when undervolting
with the fan controller both couldn't go below ~630 and ~600 rpm. Puzzling, if anyone has an idea why, please tell me.
So I still can't use the fan controller as I wanted to. I also noticed that the Truequiets when in horizontal position
tend to increase rpm by 5-10%. But at ~600 rpm they were almost as quiet as a 120 mm Slipstream at ~600 rpm and
moved a decent amount of air. So I'll settle for the Truequiets, I already hoarded too many 140 mm fans that I won't use.

Different resistance for the fans themselves? As these fans are rated at a lower RPM it's fare to assume that they draw less current and therefore have a higher resistance themselves. Take a look at the current ratings of each fan to confirm the resistance via V=IR.

When you put a fan controller inline all do you is put another resistor in series. This causes a voltage drop across the fan as some voltage is acting across the resistor. The fractional difference in the voltage depends upon the resistance of the fan itself. As the resistance of the fan itself is higher, the effect of adding more resistance through your fan controller is less and therefore the volatge across the fan is greater than it is for the faster spinning, lower resistance fans that you have tested before.

Amazingly some resistor cables actually sell as a '5V adaptor cable'. Absolutely nonsensical unless they've been designed for one specific fan!

I would suggest you try running your fans on 3.3V, 5V, 6V, 7V or 8.7V by physical wiring instead. Inline resistors are not efficient or give much of a dependable result.

Can't you buy a PWM fan and do not use ist as such by connecting it to a 3 Pin header and forego all that PWM stuff?

Last time i tried, an Arctic Cooling PWM fan, it acted like a normal fan when connected to a 3 pin header.

Most PWM fans have very high RPM ranges. Run it as a DC fan and you'll get very high airflow but no good for silence.

Regarding RPM of PWM fans your completly right. But using some voltage-fu and/or some fan control could bring lower RPMs.

If the fans i want to use are cheap enough to buy and try, i'd give it a shot.