The NF-A14x25r G2 PWM Sx2-PP is a set of two NF-A14x25r G2 PWM round-frame 140mm fans for push-pull operation on heatsinks such as the NH-D15(S) or NH-U14S. When two fans operate in a push-pull configuration running at almost the same, constant speed, their acoustic interaction can lead to undesirable harmonic phenomena such as intermittent vibrations or periodic humming due to interference, which can be heard as beat frequencies. Therefore, the two NF-A14x25r G2 PWM fans contained in the Sx2-PP set are slightly offset in speed (+/- ~25rpm) in order to avoid such phenomena. This fine-tuning is made possible by the fans’ novel etaPERF™ motor and its NE-FD6 driver IC that achieves extremely precise speed control and, thanks to its SupraTorque™ feature, can leverage extra torque headroom to keep the exact desired speed even when working against significant flow resistance such as on heatsinks. With one fan running ~25rpm slower (PPA) and one ~25rpm faster (PPB), the overall cooling performance can be kept the same while efficiently preventing intermittent vibrations or periodic humming. Since the perception of beat frequencies can vary depending on psychoacoustic preferences, users can easily further optimise acoustics by slowing the PPA fan down a little more through PWM control or the supplied Low-Noise-Adaptor, which will result in an even smoother sound signature due to the increased offset.