Large turbofan noise
With apologies in advance if this has been raised before........
As a frequent pax/TAP/sad git (choose your label) living under the LHR outbound flightpath for 09R (and the occasional 09L) I can't help but notice the significant difference in t/o noise between BA's "old" 777s with GE90s and the newer, Trent-powered version. To put it succinctly, the former ones "whistle" whilst the latter ones "growl". The question is, why such a difference in tone from engines of similar size and power? Is it the Trent's extra shaft, perhaps, or that it has a snubberless fan, that makes its sound so much lower in frequency (and, I hate to say, somewhat noisier)? Can any engine techies out there enlighten please?