funnily enough a lot of people who live outside that definition still see themselves as in the Greater LHR area........
The 55 Lden contour corresponds to the World Health Organisation's definition of "severe annoyance".
The contour for 50 Lden equates to "moderate annoyance". I haven't seen this modelled for Heathrow, but it will typically be 2-3 times the area of the 55 Lden contour, though of course the relative size of the population affected won't necessarily be exactly in the same ratio.