I live a few miles from Southampton Airport.
If a similar trial was happening here, I just calculated the aircraft would be about 90 feet higher overhead and thus 0.50dB quieter. I don't think any of the local residents would notice, not even me who is used to looking up at aircraft.
I gather the numbers are quite close to the natural glide angle for some jets, hence the mention of CDA. More drag and power is going to have to happen in the latter part of the approach though, and there will be limits to how close that transition can be brought.
So the trial will have negligible effect on the people most affected.
I don't think the human ear and brain can notice a 0.5 dB difference in a few seconds let alone a few days. Like guessing the weight of two cakes a few grams different. What is the
PR point of promising an improvement that will turn out to be imperceptible?
I thought I would check my assertion about levels. I found discussion on a hifi type forum, where they proposed tests with 1 dB plus or minus or zero change in mid piece. Some discussion of which music or just pink noise. Someone said that 0.5 to 3 dB is said to be undetectable. Nobody actually polled any results. The discussion closed with someone talking of reading about a test with MRI scanning of people told to imagine they were sampling an expensive wine, and there was raised activity in parts of the brain that sense pleasantness. Placebo effect.
So the people who suggest this trial has already been deemed a success may well be right. But maybe it just depends on telling enough people.