Noise reduction in a jet engine

[nuub]

I am in the process of writing a report on the Physics of Jet engines and I have come across a problem I cannot comprehend. In the exhaust of many jet engines they allow the atmospheric air to mix with the air from the engine to reduce exhaust noise. From what I understand this allows the gasses to mix faster, but why does this help reduce noise?

So far I think it is to do with the increased speed of mixing means that the pressure wave built up if of a higher frequency and so perhaps it is outside our hearing range? This doesn't seem quite right.

If anyone knows of how it actually works, or any other mechanisms that are employed to reduce noise please tell me.

 

[dronezero]

Well this may be a little late, but I will give it a shot anyway. From my understanding what causes the vast majority of the noise from a jet engine is due to the fact that you have the very fast exhaust air and the slower realitive wind directly in contact with each other. This creates a strong shear and produces vast amounts of noise. So what they do is route some air around the acutal combustion chamber. This creates a very fast exhaust air surrounded by the medium speed air that was routed around the chamber, which is in turn surrounded by the relative wind. This lessens the amount of shear between any two levels of the air.

I hope that makes sense; I am terrible at explaining things.

 

[russ_watters]

I hope that makes sense; I am terrible at explaining things.

That sounded pretty good - welcome to the site.