Some airports adopt good operational practices. Many don’t.
There are a number of operational practices which can reduce the noise from planes.
Arrivals
Continuous Descent Approach (CDA)
Aircraft can descend in one of two ways: either in a step-by-step fashion or using CDA. CDA can cut noise by between 2.5 and 5 decibels. 
At some European airports there is widespread use of CDA. At others it is hardly used at all. This update from EuroControl looks at the use of CDA at the busiest 25 airports in Europe. Oslo comes top with Paris Charles de Gaulle bottom.
The angle of descent
International rules require aircraft to land a 3 degree angle. Some airports, such as London City with smaller planes and tall buildings to avoid, are permitted to land more steeply. On their final approach to London City planes land at 5.5 degrees. Heathrow is one of the very few large airports to try a steeper angle. It trialled a 3.2 degree approach. It was operationally possible but only had a small impact on noise levels – up to 0.5 decibel reduction. What could make more of a difference is what is known as a ‘two-segment approach’. This involves planes using a steeper angle – perhaps even 5 degrees – further away from the airport before reverting to 3 or 3.2 degrees for the last few miles as the plane steadies to land on the runway. Until trials have been done it is not possible to be certain of the noise improvements but they could be significant.
Lowering the landing gear
For modern aircraft landing gear is the dominant noise source on approach. When landing gear has been lowered noise levels increase by between 3 and 5 decibels. So, the later landing gear is lowered, the less noise. In order to get a stabilized approach landing gear needs to go down at an altitude of 1000–2000ft, about 2–3 minutes before touchdown. It is difficult for an airport to enforce the point at which landing gear is lowered but it is possible to highlight airlines and aircraft which lower it sooner than necessary.’
Departures
Rate of ascent
Common sense would suggest that the faster you climb, in order to get as high as you can as soon as possible, would reduce noise levels. In essence, that’s correct but it is a little more complex. If a plane uses all its power to climb steeply on leaving the runway, that will benefit most communities directly under the flight path. But it has four downsides: it will increase noise for people very close to the airport. It will increase air pollution levels in the vicinity of the airport. It will have a significant impact on the wear and tear of engines. It will spread the noise so that communities living either side of the flight path up to about 4,000ft will get more noise
If all elements of best practice are introduced, it will make a big difference to noise levels
If some airports, can introduce best practice, why can’t they all?