From: email@example.com (Ed Hahn) Organization: The MITRE Corporation, McLean, Va. Date: 02 Jun 94 00:36:16 References: 1 Followups: 1 2
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In article <airliners.1994.1269@ohare.Chicago.COM> firstname.lastname@example.org (i am me) writes: Hi! This might sound like a dumb question, but what is the difference that causes the 2 powerplants on the md 80 to seem to be noiser than the 3 powerplants on the boeing 727??? I recently rode on both planes on a short trip and it sounded louder on the md 80 than on the 727. Thanks morris ------ I think the answer depends on where you sat and what interior configuration the aircraft had. First off, the center engine of the B727 is located behind the aft entry door, and thus sound insulation in the door and pressure vessel makes the sound contribution of this engine small compared to the side engines. Second, the nacelle location of the B727 is located slightly more aft than that of the MD80 with respect to the rear of the cabin. (The outline drawing I have shows the leading edge of the pylon at about the middle of the aft lavatories on the B727, while the leading edge of the MD80 pylon is about even with the last row of seats. The rear of the cabin on the B727 is about a quarter way back on the nacelle. The rear of the cabin on the MD80 is about halfway back.) Third, while the MD80 engines (JT8D-217 or -219) are quieter than the B727 engines (JT8D-7, -9, etc), it seems to me that most of the engine exhaust noise would not be a big factor in cabin noise, compared with the structural vibration noise, as the nozzle is located well aft of the cabin in both cases. This is not true for aircraft with wing mounted engines, whose structural noise coupling should yield a much quieter aircraft if exhaust noise was ignored. My personal observations: Sitting near the front of these aircraft, you CANNOT hear the engine noise in a MD80; all you can really hear is the wind noise. This is not true of the B727. Near the rear of the aircraft, I haven't noticed that one aircraft is significantly noisier than the other - they are both somewhat unpleasant. Note that the noise falloff as you move forward is more pronounced on an MD80 than a B727. (This makes sense since the MD80 engines are quieter on the whole. The narrower cabin (3-3 vs 2-3 seating) on the MD 80 might also contribute). For certain interior configurations, you may get an even slightly more skewed effect. Delta, for instance, has a "mini-cabin" in the back of their MD80s, located aft of galleys located on both sides of the aisle. American's MD80s instead have all of their galleys on the left side of the aircraft in the rear, and only have seats on the right side. Seems to me that acoustically, the "mini-cabin" of the Delta configuration would have greater potential for noise, but would make the rest of the cabin quieter. (As a side note, Delta employees seem to get stuck in the "mini-cabin"). Picture: DELTA MD-80: /----------------------------/ /Lav|IO IO |Galley| IO IO IO |---|IO IO |------| IO IO IO | Direction of Flight -> |---|IO IO |------| IO IO IO |Lav|IO IO |Galley| IO IO IO \ |IO IO | | IO IO IO \----------------------------/ AMERICAN MD-80: /----------------------------/ /Lav| Galley | IO IO IO |---|--------------| IO IO IO | Direction of Flight -> |---| IO IO IO IO IO IO IO IO |Lav| IO IO IO IO IO IO IO IO \ | IO IO IO IO IO IO IO IO \----------------------------/ Not to scale, of course. Hmm. This is a lot longer than I thought it would be. Enjoy! ed //////// Ed Hahn | email@example.com | (703) 883-5988 \\\\\\\\ The above comment reflects the opinions of the author, and does not constitute endorsement or implied warranty by the MITRE Corporation. Really, I wouldn't kid you about a thing like this.