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. To prevent this, the fans should be run at slightly different speeds.
Usually, a difference of 50rpm should be sufficient, but bigger differences such as 100 or 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, in the case of the NF-A14x25(r) G2 140mm fans, by using the Sx2-PP sets that include two fans (PPA and PPB) that are slightly offset in speed by default. (The PPA fan is slightly slower offset by 25rpm running at around 1475rpm, while the PPB fan is a bit faster, running at around 1525rpm.)
If two fans are installed in push-pull configurations or side by side such as on 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 280 or 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. If four fans are running side by side, the speed should alternate from fan to fan. Again, this can either be achieved through different fan curves, Low-Noise adaptors or, in the case of the NF-A14x25(r) G2 140mm fans, by using Sx2-PP sets.
When running three NF-A14x25 G2 fans side by side, a single Sx2-PP set can be combined with an additional regular NF-A14x25 G2 PWM fan, 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. 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-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 420 mm radiators, etc.), 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 140mm front intake fans and a 420mm radiator at the top, will be perfectly covered with three Sx2-PP sets, putting the fans in a PPA-PPB-PPA sequence in the front and in a PPB-PPA-PPB sequence on the radiator.
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.