Originally Posted by DaveReidUK
As a general rule, that's true. Whether it's the net effect here depends on all other things being equal.
The nub of this thread seems to suggest that they won't necessarily be.
Well, that's treating the sound as a point source, and non-directional.
Maybe someone can say whether they think those are too simplistic or poor assumptions. Yes it's moving, and continuous, but at any moment the sound reaching the observer has come from a specific point.
I also suggest that the greatest saving, due to 6.6% extra distance, will be where the aircraft is directly overhead. Where the observer is out at the side the increase in distance will be less. At say half a mile sideways aircraft on 3.2 and 3 degree approach angles are only about 1.5% different in distance from the observer. At one mile to the side this is 0.44%
A 1 or 2 dB difference is difficult for people to distinguish between two sounds played a few seconds apart. A few hours apart, assuming the trial starts at the beginning of one morning, only specialist instruments can compare. And locations half a mile and one mile to the side of the approach line will experience 0.12 dB and 0.04 dB differences. Utterly imperceptible. Where do the 2dB claims come from?
If the chosen angle means that aircraft are nearer to a pure glide approach and thus using less or idle power until the point of deploying more drag then yes that will be quieter, but two sounds the same level at source will not be distinguished by human ear and brain.
So are we talking about being able to use less power? Is that why prospective savings are greater?
What is a typical glide angle of these things? Has it worsened from 20 to 18 to justify a steeper angle being chosen? Will they also consider a range of slightly steeper angles for stronger winds?