Re: Engine noise question

Date:         01 Dec 96 04:08:51 
From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
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In article <airliners.1996.2574@ohare.Chicago.COM>, jcb@interlog.com (Jeff
Bowen) wrote:

> I was walking my dog in the park this afternoon in Toronto and
> listening to the sound of passenger jets going over - a common
> experience as I am about 20 miles from Pearson airport.
>
> Some of the planes had a high-pitched whine and some a dull roar.
> All were above the cloud cover.
>
> Can anyone explain to this non-pilot what actually creates the
> sound and why the sounds are different?


The wise-ass answer is that they're different for the same reason car
engines sound different from each other- the engines ARE different.  But
an attempt at a real answer is this: there are two parts to a modern
turbofan engine, the jet core itself and the big fan up front that is
driven by the tubojet core.  I don't know the percentages- I'm sure
someone in this group will- but a large percentage of the thrust of a
high-bypass fanjet comes from the fan, not the jet engine core.  Fan air
flows back through "C" ducts on either side of the jet core and "shrouds"
the jet exhaust, which is one reason modern turbofans are much quieter
than their predecessors.

The fans are different sizes and diameters and have differing numbers of
blades, so they all sound different.  The "snarl" or "buzz" you hear from
a large plane like a 747, 777, MD-11, etc. as they take off is really the
fan noise, not the jet core noise, which is the roar part.

So the noise will vary with the size and type of fan, the distance the
plane is from you (the jet roar carries farther than the fan buzz or
whine), and the direction the plane is flying in relation to you.  Older
planes like DC-9s and 727s have lower-bypass engines than the
new-generation planes, so they get more of their thrust from the core, and
the volume of shielding fan air is not as great.  Therefore, you get a lot
more roar than buzz from these planes, and the roar will hang in the air
longer.  Not a scientific answer, but I hope it helps...

C. Marin Faure
   author, Flying a Floatplane