> regardless of their RPM. It's not regardless: 5400RPM == 90Hz 7200RPM == 120Hz The human perception curve shows that the humans hear much better the soun

> regardless of their RPM. It's not regardless: 5400RPM == 90Hz 7200RPM == 120Hz The human perception curve shows that the humans hear much better the sounds at 120Hz than at 90Hz, https://en.wikipedia.org/wiki/Equal-loudness_contour https://en.wikipedia.org/wiki/Equal-loudness_contour That means that for the same reported noise level of e.g. 30dB a 5400RPM drive is much more quiet for humans than a 7200RPM drive. That is how the people recognized that these HDs aren't 5400RPM -- they sounded "too loud" so they took the sample of the noise and drawing the histogram they proved that the peak was at 120Hz. As far as I know, nobody managed to produce a 7200RPM drive which does not have a peak at 120Hz. If you know some that doesn't have such characteristic, please write, everybody will celebrate that achievement.