HC Deb 24 November 1975 vol 901 c13W
Lord James Douglas-Hamilton

asked the Secretary of State for Trade what average, and maximum, noise levels in perceived noise decibles are being experienced at the boundaries of the areas within which noise insulation grants are applicable around Heathrow and Gatwick Airports, respectively.

Mr. Clinton Davis

The boundaries of the noise insulation grant schemes at Heathrow and Gatwick are related to NNI contours. The NNI figure at any particular point is a combination of the number of aircraft flying over it and their known noise characteristics; for example, a point on the 55 NNI contour could arise from 300 flights a day with an average noise of 98.5 PNdB or 30 flights a day with an average noise of 113 PNdB. Thus, there are no average or maximum noise levels common to all points along the noise insulation grant scheme boundaries, nor are measurements of peak noise along these boundaries available.