Methods of Jet Engine Noise Reduction

Since the introduction of the high bypass engine, the amount of noise disturbance due to jet engines has been reduced dramatically. However, as bypass ratios continued to grow, the amount of noise from bypass fans on a turbofan engine began to dominate as an engine noise source. While jet engine exhaust is still a major cause of engine noise, recently much more research has been concentrated on the reduction of fan noise. In addition to fan noise research, much effort has been put into the study of noise reduction nozzles to accelerate exhaust mixing by increasing the mixing area. Currently most of the studies which aim at reduction of fan noise are based around active control, while exhaust nozzles continue to dominate the research for exhaust noise reduction. In addition to these methods the engine shell or nacelle and ejector shroud are being insulated to help dampen noise within the engine.

An example of an acoustically insulated ejector shroud is shown below in Figure R1. Generally the insulation material is constructed in a honey comb fashion such as those shown if Figure R2. However the honey comb configuration can be quite varied. Although other types of insulation are available such as the bulk absorber in Figure R2 below, the different configurations of honey comb are most common and have been proven as very effective sound dampeners.
Figure R1: example of acoustically insulated ejector shroud Figure R2: Several examples of acoustic insulation

Use of exhaust nozzles is the main method of reducing jet engine exhaust noise. An example of a commercially available exhaust nozzle is given in Figure R3. Jet exhaust nozzles help to reduce exhaust noise by allowing the bypass air to more thoroughly with the high velocity exhaust gas reducing the overall velocity of the of the jet exhaust and therefore reducing the amount of turbulence with the surrounding outside air.

Figure R4 is a cutaway of the same exhaust nozzle. This cutaway clearly show how the bypass air can be channeled in through the outside surface of the nozzle then allowed to mix thoroughly with the high velocity exhaust gas passing through the core duct. These types of exhaust nozzles can produce a decrease in exhaust decibel levels by several decibels some times as many as 5 or 10 dB..

Another commonly used exhaust nozzle is the chevron nozzle. This nozzle has several triangular shaped fins around the exhaust exit as seen in figure R5. The effect of the triangular shaped fins at the exhaust exit is to allow the exiting exhaust to mix over a larger exit area by letting the exhaust gasses exit over a much wider angle which in turn allows for faster mixing as well as faster cooling of the exhaust gasses. For this reason, the chevron nozzle has predominately been used on military aircraft. This military use is greatly due to the fact that while the nozzle does make the aircraft quieter with very little performance losses, the faster cooling created by the chevron nozzle also greatly reduces the heat signature of the aircraft on which it is used.
Figure R3: example of commercially available exhaust nozzle
Figure R4: Cutaway of an exhaust nozzle showing how mixing of bypass and core duct gasses are mixed
Figure R5: Example of a chevron exhaust nozzle
While the chevron nozzle is predominately used for military applications, recently extensive research has been undertaken to consider using chevron nozzles on high performance commercial aircraft such as a new supersonic transport which is being developed to possibly replace the extremely loud Concord.
In addition to research concerning exhaust nozzles, extensive research has begun in the area of fan noise reduction. Since fan noise has become a major source of engine noise due to the high bypass ratios being used on aircraft today, as was said before, active noise control for bypass fans is being developed by the Glenn Research Center under contract with NASA. Below in Figure R6 is a photograph of the Active Control Fan which is being tested at the Glenn Research Center.
Figure R6: Photograph of the Active Control Fan which is being tested at the Glenn Research Center
The research being conducted at the Glenn Research Center is based on the principle that one sound can be canceled out by the opposite sound wave to produce a flat wave. The Glenn Research Center is currently trying to write programs which can better model the noise which will be produced by the fan at different speeds and with different stator configurations. The goal of this research is to achieve a 6 dB decrease in engine noise which would decrease the amount of aircraft noise substantially as shown in Figure F1 in the FAA regulations section. While still in persuit of their final goal of a 6 dB reduction in fan noise, the Glenn Research Center has already achieved a 3dB reduction in fan noise, accompanied with a 3 dB reduction in exhaust noise using similar methods. The active control of exhaust is done by predicting the noise that will be created by the gas which has already exited the engine and sending out the cancellation noise with the exhaust before the noise is actually created by turbulence in the mixing air. For this reason, since the noise has to be predicted before the noise is actually created, this form of active control is much harder to successfully control.