A couple of reasons for this are:
- The invention of liners, which are small honeycomb-shaped mesh pieces installed on engines to absorb sound.
- The advent of high-bypass turbofan engines, which are both quieter and more fuel-efficient.
The report also found that the effective perceived noise levels of aircraft – the noise from an aircraft experienced by people on the ground – has been reduced by a factor of three since 1972 and is trending downward.
It’s definitely impressive, but noise will always be a by-product of air travel no matter how many advancements are made.
Why do aircraft make noise?
Sources of aircraft noise
There are five phases of flight, all of which generate different types of noise. For each phase, the two primary sources of noise are aerodynamic (when air passes over the frame) and engine.
Have you ever stuck your hand out the window of your car while driving? The sound of the air hitting your hand is an example of aerodynamic noise – air passing over a moving object.
When the palm of your hand is perpendicular to the ground, the sound is a lot louder than when it’s parallel. This is because more surface area is coming into contact with the air. The same thing is happening during the departure and landing phases when the flaps, wheels and wheel well doors are pushing air. Aerodynamic sound contributes greatly to noise levels in the departure and landing phases, and there’s not much we can do about it.
When taking off and landing
Engine noise is divided into two distinct kinds:
- The sound of the engine’s combustion process
- The sound of air passing through the engine
From the ground, the inner mechanics of the engine create the hum you hear and a high-pitched whistle is created by the turbine blades rotating at a very high RPM (revolutions per minute).
Going back to your hand out the window of a moving car, you’ll notice that it’s more difficult to push your hand forward when your palm is perpendicular.
During the approach phase of flight, more of the airplane surface is perpendicular to the ground, and the flaps are also deployed to slow the airplane down. The flaps generate drag, so there’s even more resistance.
This requires that the engines work harder to move the airplane forward – much like you have to work harder to push your hand down. This creates more noise on the ground.