Finally something this SLF member (who happens to be a sound engineer) can comment on!
I did some research on the Heathrow "quota count" and found the following quote:
For this scheme, aircraft are grouped into 'QC' bands dependent on their noise performance measured during certification - the noisier aircraft falling into the higher QC bands. The value of the bands doubles every 3 decibels, to reflect the nature of noise measurement - i.e. a doubling in noise energy for every 3 decibel increase.
While it's true that 3dB(SPL) change represents a doubling (or halving) of noise energy, because of the way the human ear works, 3dB also represents roughly the smallest difference in sound level the average punter can detect. (A trained sound person can usually hear 1.5-2.0dB changes).
The dramatic "quartering" of the sound level from the A380 or B787 will represent a 6dB difference, which, in sound terms is a very small change.
The spin doctors strike again!
Bob