Thanks Gaspath for saving my already dodgy eyesight! Your information has, I suspect, also provided the answer to my original post. As I am not an engineer I may just have interpreted your numbers wrong, but assuming that N1 represents the speed of the inner, ie fan, shaft, then at 100% the Trent's fan is rotating a whole 50% faster than the fan on the GE90! If this is true then it is no surprise that it makes a very different noise. Since the difference in diameter is small (about 12%), the Trent's tip speed must be considerably greater than the GE's. I don't have the patience to do the calculation, but this alone, when converted to noise frequencies, would surely explain the difference? Over to the physicists/engineers......