When two fans are running at almost the exact same RPM speed side-by-side or in push-pull, their acoustic interaction can lead to undesired phenomena such as periodic humming or intermittent vibrations. If you would like to learn more about the underlying acoustic interaction phenomena and beat frequency theory, please take a look at our technical background article.

To prevent this, the fans should be run at slightly different speeds. Usually, a difference of 50-100rpm is sufficient, but bigger differences such as 200rpm can be required depending on the configuration and individual acoustic preferences.

A speed offset can be achieved by setting individual fan curves, slowing down one fan with a Low-Noise Adaptor, or by using the Sx2-PP sets of our NF-A14x25(r) G2 and NF-A12x25 G2 fans, which include two fans (PPA and PPB) that are slightly offset in speed by default. The PPB fans run a bit faster than the PPA fans, with a speed offset of about 25rpm (NF-A14x25(r) G2) or 50rpm (NF-A12x25 G2).

If two fans are installed in push-pull configurations or side by side such as on 240mm and 280mm water cooling radiators or in typical case fan applications, having one fan run slightly slower than the other is sufficient to avoid the described phenomena.

If you would like to run more than two fans side by side, such as on 360mm and 420mm radiators or if your case has e.g. three front intake positions, the key is to avoid having neighbouring fans run at the same speed. For example, if three fans are operating side by side, the middle fan should run either slower or faster than the outer two.

Buying Guide for NF-A12x25 G2 and NF-A14x25(r) G2 fans

When running three NF-A12x25 G2 or NF-A14x25(r) G2 fans side by side, a single Sx2-PP set can be combined with an additional regular fan, when reducing the speed of the latter using PWM so that it runs at the same speed level as the PPA fan included in the set (NF-A12x25 G2: 1750rpm, NF-A14x25(r) G2: 1475rpm). This way, the PPB fan from the set can be put in the middle spot and the PPA and regular PWM fans can be put in the other two spots to avoid a situation where two neighbouring fans are running at almost the same speed.
Like with the dual fan set, users who want to further optimise acoustics can further reduce the speed of the outer two fans in order to increase the speed offset.

If more than three NF-A12x25 G2 or NF-A14x25(r) G2 fans are used in total (e.g. two on a radiator and four case fans, three on a radiator and three case fans, two 360 or 420 mm radiators), the same effect can be achieved by opting for multiple Sx2-PP sets and alternating between PPA and PPB. For example, a setup using three front intake fans and three fans on a radiator at the top will be perfectly covered with three Sx2-PP sets by putting the fans in a PPA-PPB-PPA sequence in the front and in a PPB-PPA-PPB sequence on the radiator.

In summary, we recommend the following setups with our NF-A12x25 G2 and NF-A14x25(r) G2 fans: