How do I avoid periodic humming noises when running fans side-by-side or in push-pull?

When two fans of the same type 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.

The easiest way to achieve a speed offset is to use 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).

Alternatively, a speed offset can be achieved by setting individual fan curves or slowing down some of the fans with Low-Noise Adaptors.

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.

Recommendations for the optimal placement of our G2 PPA and PPB fans, as included in the Sx2-PP sets, can be found in this FAQ.