Re: Engine noise question

Date:         01 Dec 96 04:08:51 
From:         Carl Peters <cpeters@i1.net>
Organization: Internet 1st, Inc
References:   1
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Jeff Bowen wrote:

> 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 differences in sound are related to the engine size, speed of moving
parts, type of engine (different degrees of bypass), speed and size of
efflux, and your position in relation. High pitched noises typically are
more prominent from the front, and are higher and louder in the older,
low bypass engines. They result from the low pressure front fan(s) mainly.
The larger engines with a large front fan usually turn at lower RPM's,
and thus produce less shrill (as a doc in the USAF, I have seen hearing
loss commonly associated with personnel that worked the old B-52 flight
lines - 8 screaming old engines, small diameter, no bypass on earlier
versions).

The deeper, rumble sounds typically are from the high speed jet efflux,
created by shear with the air around this core. In the larger engines with
high bypass (eg a typical engine on the 747 or 767 may pass 5 times the
air volume around the engine core from the 'low' speed front fan, than
what passes through the engine itself - in essence an engine driving an
enclosed prop with many blades), the front fan bypass air surrounds and
blankets the high speed core efflux, thus attenuating the noise. In
addition, I believe the exhaust velocity is quite a bit lower. This is
readily seen at the airport, behind the threshold - a 747 with 4 large
bypass engines producing 60,000 lbs thrust each can sound much quieter on
takeoff than a DC-9 with 2 low bypass JT8-D's at 14,000 lbs thrust.

Hope this helps,

Carl Peters, M.D.