Re: identifying engines by sound

From: (Tareq R. Matalqa)
Organization: The Ohio State University
Date:         14 Aug 95 03:43:28 
References:   1
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In article <airliners.1995.1214@ohare.Chicago.COM> (doug wakefield) writes:
>Recently on a trip to Florida, I travel down and back to DC on 757s.  The
>sound at takeoff was very different. I know this subject has been covered
>before but would like just a quick refresher.
>The engines going done had a very distingctive medium to low frequency,
>rather loud hum during takoff and climb out.
>I've heard these on Airbuses.  The return was lacking that hum.


Well the reason that you heared a different sound is the takeoff was done
with the envirnimental control system packs off  (ECS) in other words the
sound of the engines was not obstructed by any air conditioner sounds and
any other air flow sounds.

Preforming a no pack takeoff provides the engines with more power.  and
after airborn and an obstruction has been cleared or any other reason the
packs had been off the pilot will turn them back on.