SONA, a study the Government commissioned from the Civil Aviation Authority (CAA), Survey of Noise Attitudes 2014, SONA found that people start to get annoyed by aircraft noise at lower levels than previously officially recognised. This is reflected now in Government policy which acknowledges that people can get annoyed by aircraft noise when it averages out over a 16 hour day at 54 or 51 decibels. Previously government argued it was 57 decibels. Some people get annoyed at even lower levels. The World Health Organisation report, published in October 2018, argues for the lower levels:
The chart above, taken from the study, compares the results of the Government sponsored 1982 ANIS Study with the new SONA study. SONA found 9% of people are highly annoyed when the average is 54 decibels. In geographical terms around Heathrow that goes as far as about Clapham to the east and about 16 miles to the west: about 65,000 people in total. The lower average of 51% extends about as far as Peckham.
The full study http://publicapps.caa.co.uk/docs/33/CAP%201506%20FEB17.pdf