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From: barr@ash.mmm.ucar.EDU (Keith Barr)
Subject: Re: aircraft noise
Date: 05 Feb 93 13:55:08 PST

kls@ohare.Chicago.COM (Karl Swartz) says:
> This has a dramatic impact on noise because the majority of the noise
> comes from the hot exhaust gasses of the combustion process.  With
> more of the thrust coming from the cooler bypass airflow the engine
> is quieter.  In addition, the cool bypass air surrounds the hotter
> combustion gasses and tends to contain the noise.  Further improvement
> have come as the manufacturers have learned how to better control this
> phenomenon.

When I was in school, I had a class on jet propulsion.  In that class we
were taught that sound was tightly related to the speed of the
flow out the back of the engine.  In fact, we were shown that sound is
roughly related to a constant times the velocity raised to the seventh!
Therefore, even a small slowdown in the velocity can make a great deal of
improvement in the overall sound.  If you work through the math, a 10%
reduction in velocity results in approximately a 50% decrease in noise.

In modern high-bypass turbofans the core flow is still very fast, but the
bypass air is flowing at a considerbly slower rate.  I am not sure if the
bypass air "shields" the core flow, or if the sound decrease is based
more upon mass flow (much lower mass flowing through the core of a newer
engine).  I might be able to come up with some rough speed differences
(given in mass flow rates of course) for different bypass ratios and
engines if people are interested.
 _____________________________           _____
| Keith Barr                  \           \   \__      _____
|           \___________\   \/_______\___\_____________
| Commercial/AS&MEL/Inst/IGI   /           < /_/   .....................  `-.
|_____________________________/             `-----------,----,--------------'
When you think how well basic appliances work, it's   _/____/
hard to believe anyone ever gets on an airplane.--Calvin

Newsgroups: sci.aeronautics.airliners
From: (Ed Hahn)
Subject: Re: md80 vs boeing 727 (noise)
Date: 02 Jun 94 00:36:16

In article <airliners.1994.1269@ohare.Chicago.COM> (i am me) writes:


   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.


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

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:

/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

/Lav|    Galley    | IO IO IO
|---|--------------| IO IO IO
|                              Direction of Flight ->

Not to scale, of course.

Hmm.  This is a lot longer than I thought it would be.  Enjoy!

////////   Ed Hahn | | (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.

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