I recently finished building my computer and I checked out the fans a bit so I'll relay a little info here. I have an Antec Three Hundred case and I bought a couple Fractal Design fans including a 140mm and Nexuses. I can heartily recommend the Fractal Design fans for low noise (and low airflow) thought I had to pay a bit more than I wanted to ship from Canadian site NCIX. I'm sure it's not the 9-15 dBA as shown on the box (maybe at ? distance) but they are quiet. I put the 140mm Fractal Design in the top horizontal fan port, a Nexus 120mm in the rear, and a Fractal Design 120mm on the side panel as an intake. My CPU heatsink fan is the Scythe Mugen 2 RevB and I have the Scythe fan that came with that on. I have an extra Nexus that I might put on the other side in push-pull configuration but with the rear and top fan right there I think there is probably enough pull going on at the other side as it is.
As I tested them while building my comp, I noticed all were very quiet at 12v. This is my first experience with the Nexus. They're plenty quiet for me so I didn't experiment with undervolting them. I think my house has average ambient noise level. It was 44dB when I checked with my phone app.
I listened carefully for the ticking of either of the Fractal Design fans and there was none at all in mine unlike what SPCR staff and others experienced. The horizontal mounting doesn't have any effect that I can tell so far. The Fractal 140mm is definitely quieter than the Nexus. I get the feeling that it's pushing less air though.
The Nexus 120mm has a midrange turbulence noise when I put my ear right up to it. It's very smooth. There is a little bit of electrical buzz that is only noticeable with my ear right on it. Slightly further away (about 6 inches) mostly all I can hear is airflow noise.
The Fractal Design 120mm has a slightly deeper turbulence noise and slightly rougher and it sounds sort of like a propeller airplane when I put my ear to it. It's a bit like a buzz but not an electrical buzz. The airflow doesn't make a whisper and the faint buzz is all you can hear at about 6 inches.
The Fractal Design 140mm has a deeper smooth turbulence noise. There is a tiny buzz that is only audible with my ear right on it. At 6 inches all I hear is smooth airflow.
The Scythe (Mugen 2 comes with Slip Stream 120mm) I didn't really test since it was on the CPU heat sink but at idle the thing looks like it's barely spinning
so it's not making any noise that way. I still didn't notice it at load.
I got out a Panaflo 80mm as well and it makes a higher pitched turbulence noise because of the faster speed. It's noticeable at 6 inches.
Overal subjective sound
It's hard to compare noise levels since they make different sounds. If I had to estimate, I'd say the Fractal 140mm is quietest, then the Fractal 120mm. Louder is the Nexus and loudest the Panaflo. The Nexus sounds nicer than the Fractal 120mm though.
Overall subjective airflow
The Panaflo feels like it's pushing the hardest but it is smaller so maybe the airflow is just more focused. The Fractal 120mm actually feels like it's pushing a little more air than the Nexus. Then the Fractal 140mm brings up the rear and I can feel it but it isn't that much airflow. I don't know how much the larger exhaust area size makes up for the low airflow. The good thing with it in my system though is that I can really notice the warm air coming up through the 140mm fan when my system is under load since it's right above the CPU.
To get a better idea of airflow, I taped a small thin piece of paper I got with my case just above the fan for both the Nexus and the 140mm Fractal while they were oriented vertically (pointing horizontal). The angle of of much the paper was pushed out from the case would roughly determine the relative airflow I thought. Hopefully since the paper was wide it would account for the different fan sizes. It's definitely not scientific but maybe better than using my hand to feel the breeze. The Nexus won that test over the Fractal 140mm as you can see below. I didn't test the 120mm Fractal Design this way.
I was hoping to test all the fans individually with the dB phone app which would be pretty cool. I unplugged the hard drive, SSD, DVD and all the other fans to isolate my CPU fan but the video card fan was still the loudest. Then my wife and my rabbit came in the room so testing wasn't an option any more
Maybe if I had a lot of time on my hands I could put in a passive video card and run the tests but that's not happening right now.