Sources of Jet Engine Noise

There are several sources of noise to consider when looking at Jet Engines. The two major sources of noise are the Jet exhaust, and Fan noise when considering a bypass engine. In addition to these sources, noise is caused by the engine shell or nacelle, the combustion core and the turbine. Figure S1 Below shows a cutaway of a bypass engine with each of these noise sources labeled.
Figure S1: Cutaway of a bypass jet engine showing sources of noise

As the fan pulls air into the nacelle, some noise is generated simply due to the interaction between the fan and the passing air. This noise is similar to the noise heard from a propeller driven aircraft. After the air passes the fan it is split down two ducts, the fan duct and the core duct. Air going into the fan duct is swirling due to the rotation of the fan blades, so vanes called stators are placed inside the ducts to straighten out the flow and avoid a loss of momentum. As the turbulent swirling air hits the stators a slapping noise is created. This is a large source of engine noise which occurs at the rate of the passing fan blades. This is known as Blade Passage Frequency or BPF.

On the other hand, the air passing through the core duct is compressed by going through a series of rotors which force the flow of the air to create compression, and stators which again straighten out the flow of the air. Rotor stator interaction is another major source of engine noise. The compressed air is then pushed into the combustion chamber and mixed with fuel and then burned. This combustion is another source of noise, although not a major contributor.

Finally the flows from both the fan duct and the core duct are exhausted out the rear of the engine and allowed to mix with the ambient air. The high velocity jet exhaust mixing with the low velocity ambient air creates turbulence which is the cause for jet exhaust noise. This jet exhaust noise is another major contributor to engine noise. Fortunately with the introduction of high bypass engines jet noise is becoming less and less of a contributor to total engine noise since it allows for slower mixing of the exhaust gases, but the increased fan size has made the fan noise increase dramatically an has become a new point of concern since the tips of the fan blade can pass sonic speed even though the engine is subsonic. In figure S2 below is a bar graph showing how each aircraft component contributes to overall jet aircraft noise.
Figure S2: bar graph of component contribution to total aircraft noise during Takeoff and Approach
As it was mentioned earlier, high bypass ratio jet engines have significantly reduced the overall magnitude of commercial jet engine noise. However, the large bypass fan has become a major contributor to overall engine noise. Figure S3 below shows the relation between a high and low bypass fan with relative component noise magnitudes represented by the loops connected to the engines centerline. It can clearly be seen here that the fan is a major contributor to the high bypass engine noise, but the magnitude of noise is significantly lower than that of a low bypass engine. It can also be observed that the jet exhaust of the high bypass engine contributes much less to the total noise than in the low bypass. This effect is due to the mixing of the high velocity exhaust gas with the lower velocity bypass air to slow overall exhaust speed which decreases the amount of turbulence with the outside air as the exhaust gas leaves the engine
Figure S3: comparison of high and low bypass ration turbofan engines where loops represent the magnitude of noise from each major component.