Re: Engine noise question

Date:         01 Dec 96 04:08:51 
From:         Steve Lacker <slacker@arlut.utexas.edu>
Organization: applied research laboratories
References:   1
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Jeff Bowen wrote:
>

> 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?


The actual production of sound from a jet engine is EXTREMELY complex!
Several (dozens!) of factors actually come into play to some degree. The
inlet and exhaust portions of the system both contribute. The actual
stream of moving air aft of the engine produces sound. Each blade on the
fan produces sound as air flows over it, and the fact that the blades
are *rotating* modulate the sound also. Interactions between inlet guide
vanes (if present) and turbine blades produce sound. Interactions
between the fan and first-stage compressor blading produce sound. What
you probably *dont* ever hear is true mechanical noise (parts rubbing)
because if it were ever loud enough to hear over all the other sounds,
something would be coming apart in the engine.


To VASTLY oversimplify things, I think its reasonably safe to say that
"most" of the very high-pitched sounds (whine) are due to engine inlet
phenomena (particularly the banshee-wail caused by the inlet guide
vane/fan blade interaction on engines like JT8D's). Big fan engines
(PW2000, CF-6, JT9D, etc) don't have much of this component because
there are no inlet guide vanes and also the first rotating component in
the front of the engine is a slow-turning fan. The frequency of sound
produced by the inlet of the engine tends to be proportional to the
"blade rate" (rpm*number of blades) on the first compression stage in
the engine, and a fan has a low blade rate compared to a compressor
turbine. They do have an interaction between the fan blades and
stator/compressor systems further back in the engine, but this sound is
somewhat contained by the fact that large volumes of air are being
pulled into the engine. The very low "rumble" that you feel in your
chest is more related to exhaust stream turbulence. Big fans have this,
but the slow-moving fan air shrouds the high-speed core exhaust and
reduces the phenomenon. Lower-bypass fans (like JT8Ds and JT3Ds) have
more of this sound because less bypass air is present to shroud the core
flow.

Pure jets like the J-57 have very pronounced high-pitched screams
because the very first fan in the front of the engine is a high-rpm
compressor, not a slow turning fan (remember frequency is proportional
to blade rate). Their exhaust streams are not shrouded by bypass air
either, so they *also* have a very loud low-frequency rumble... in short
they are just plain LOUD no matter how you slice it.

--
Stephen Lacker
Applied Research Laboratories, The University of Texas at Austin
PO Box 8029, Austin TX 78713-8029
512-835-3286	slacker@arlut.utexas.edu