With such fans as these, we simply must test them. Fans have four performance aspects that can be measured: their speed (RPM), the noise they make (approximated by measuring the Sound Pressure Level), the amount of air they push (CFM) and their static pressure (mm H2O). Since Noctua fans report their RPMs, the speed can be obtained directly from software. SPL (in dBA) and CFM must be measured. The measurement tools used in this review are the Tenma 72-942 SPL meter and the Extech AN100 Anemometer.
The Tenma is advertised as measuring sound pressure levels accurately down to 30 dBA. Not coincidentally, 30 dB is the SPL of what sounds like silence to you (so much for equating SPL with perceived noise). Outside of a room-sized anechoic chamber, a 30-dB basement is about as quiet a place as any to measure SPL. These Noctua fans had their SPL measured at 10 cm. A correction was applied to give the equivalent of the industry-standard 1 meter. While this is not a perfect system, it does give you a practical measure of how loud each fan is.
Fans have hubs, so the airflow is annular. The airflow also changes as you get further from the center of the fan. To get a single measurement of the airflow from a fan you must mix the air before you measure that flow. Special testing boxes were made to test these fans, with inlet and outlet holes fitted to each size class. Basically, the theory is that what goes in must come out. You measure what comes out. The anemometer can average measurements; each run represents an average of at least 10 readings.
In addition to open-outlet measurements, the 140mm and the 120mm fans were also measured with dust filters made from the same metal mesh. The outputs of the 120mm fans were additionally measured as they blew through a radiator with 30 fins per inch (Hardware Labs Black Ice GT Stealth). So we will be able to see how well the fans pulled through dust filters, and how well the 120mm fans pushed through a restrictive radiator.
The fans tested for this review were supplied by Noctua.