From kls Sun Dec 1 04:08:51 1996 Newsgroups: sci.aeronautics.airliners Path: bounce-back Date: 01 Dec 96 04:08:51 From: Steve Lacker Subject: Re: Engine noise question References: Message-ID: Approved: kls@ohare.Chicago.COM Sender: kls@ohare.Chicago.COM Organization: applied research laboratories 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