Ask the Expert

They don’t. In fact, if you were to compare a jet engine today with one from 40 years ago, you’d find that it is about 100 times quieter for the same engine power. Still, airplanes remain quite loud and so the government and industry have set an ambitious goal to make airplanes so quiet by 2020 that their noise wouldn’t be heard outside the airport perimeter above the other prevailing noises.

There are two main sources of noise from an airplane. One is the engines. The other is the body, or airframe. The airflow exhausting out of the engine and over the body of airplane is never steady and creates pressure waves (which our ears sense as sound) in the surrounding air as the plane moves through. During takeoff, when the plane needs to climb rapidly, the dominant source of noise is the engines. When the plane is landing, the landing gear and airframe components for maintaining high lift generate most of the noise. Making airplanes quieter has required research on each one of these noise sources. In my group here at Stanford, we develop high-fidelity computer simulation tools that allow us to figure out the noise generated by unsteady flows using basic physical principles. In the future, such tools can help develop new noise reduction concepts.

The major improvement in engine noise over the last 40 years has come mostly from changing the airflow through the engines. During takeoff the noise produced by the propulsive jet exhaust is dominant and its level depends very heavily on the jet exhaust speed. The invention of the turbo-fan engine, which generates most of its thrust by pushing the air flow with a large fan and takes its power from a much smaller jet engine core, provided a simultaneous increase in engine efficiency and lower noise. Increasing the size of the fan allows more air to be drawn through the fan and bypasses the hot and high-pressure airflow through the core engine. Overall exhaust velocity is reduced while allowing for similar or increased engine thrust. This trend has been used in modern jet engines. You may have noticed that the size (diameter) of the engine on the Boeing 777 airplane is larger than the fuselage diameter of the 737.

Another way to make jets less noisy is to redesign the shapes of the jet exhaust nozzles. The engines on Boeing’s new 787, for example, have newly shaped nozzles at the back of the engines with toothy or wavy “cutouts” called chevrons. These chevrons alter the airflow coming out of the engine so that less noise is generated in the frequencies that can travel long distances through the air before they weaken. Another reason to try to shift the noise to certain frequencies is that some frequencies are more annoying to people than others.

The idea that structures on a plane can be altered and redesigned to “manage” noise is also being applied to the problem of airframe noise during landing. As a plane begins its approach, the pilot must do two things that unfortunately create extra noise. One is moving the slats and flaps on the wings to preserve lift for a smooth descent at low speed. The other is deploying the landing gear, which is designed to be strong and reliable, rather than quiet.

As with the engine chevrons, the trick for designers is to figure out what shapes perhaps can be added to the body that will change the flow of air, making it less unsteady, in order to reduce the worst causes of noise. Taking a cue from nature, some designers at NASA and elsewhere have been experimenting with making the trailing-edge of slats more brush-like—similar to the feathers on the wings of birds—to reduce noise by reducing sharp changes in airflow. With landing gear, the requirements of easy inspection and maintenance further constrain possible noise reduction concepts.

In our work we develop computer models of the airflows and other physics that underlie all of these sources of noise. If we can gain a fundamental understanding of what causes all of this noise, then we will have many new ideas on how to reduce the noise and we can test those ideas on the computer to see if they help. Hopefully within a decade, airplanes will be much quieter than they are now.