In article <airliners.1996.2574@ohare.Chicago.COM>, jcb@interlog.com says...
>Can anyone explain to this non-pilot what actually creates the
>sound and why the sounds are different?  I seem to recall from
>being at the airport that the high whine is more pronounced when
>close to the planes and the dull roar more pronounced from a
>distance.
>
>Is the whine a mechanical sound (turbines or something) of metal
>parts rubbing on one another?  Is the roar the actual sound of
>combustion or of air rushing over the metal surfaces of the
>engine?

While there are many sources for the noise, the answer is dependent on
the type of engine and the power at which it is operating.  At takeoff
power the predominant noise source is from the exhaust jet (and is
proportional to the ehaust jet velocity). On approach at lower power
settings the higher pitched "tones" are from the fan, esspecially on
high bypass turbofan engines.