Expert: Paul Soderman - 6/11/2009

Question
QUESTION: Where noises come from when an airplane passes above us in sky? Are they most come from engines or they are for moving through in different densities of air? And different engines make different noises or not?

ANSWER: Mehrdad
The primary source of aircraft noise is the propulsion system.  For a jet engine that would include the jet exhaust noise and the inlet fan noise.  A propeller makes a different sound.  Near to landing, noise can also be heard from the airframe components such as flaps, ailerons, landing gear, etc. Aircraft sound does pass through a range of air densities, but that has virtually no effect on the noise levels.  Different engines make different noises.
Paul

---------- FOLLOW-UP ----------

QUESTION: Hello dear Paul
Altogether, how much changes we have in frequencies from rise to landing? Are these changes huge or they are epsilon that we could waive them? I mean could we say that for example several same 747 with same speeds have a known same range of noise frequencies in different airports?
Thanks


ANSWER: There is usually a large difference in acoustic spectra between takeoff and landing because takeoff requires full engine throttle which generates strong jet noise whereas landing requires much less throttle.  On landing the fan noise and airframe noise tend to become important.  

Yes, you could assume that different 747's at the same speed and distance have similar spectra.  This ignores differences due to weight or weather or flap setting for example, but it gives you an approximate spectrum for that aircraft.  You should be aware, however, that there is a family of 747 aircraft with several different configurations that have been developed over the years.

Perhaps if I knew what you are trying to accomplish I could be more helpful.
Paul

---------- FOLLOW-UP ----------

QUESTION: I want to know which factors are exists that prevent us to devise an instrument to eliminate or reduce the noises?
Yes, thank you for point to “family of 747” I will read about them.

Answer
I spent most of my career with NASA trying to answer that very question, along with researchers from industry and other research labs.  The problem is quite complicated because it is directly linked to aircraft performance.  I suggest researching the many books and technical papers on this subject.  Maybe you can make a contribution to the science of aeroacoustics.
Paul