Engine noise

Date:         27 Dec 96 13:32:25 
From:         "Robert M. Sherry" <sherryr@a.cs.okstate.edu>
Organization: Oklahoma State University, Computer Science
Followups:    1 2 3
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While waiting for a pre-dawn flight last week, I noticed that
both Stage 2 and 3 jets appear (to me, at least) to produce
almost equal amounts of low-frequency noise...When a DC-9
made a takeoff run, the windows rattled, and there was a
noticeable tremor in the floor. A few minutes later, an AA
Fokker 100 lifted off, accompanied by the same ground-shaking
sounds. A 737-300 was a bit quieter, but not by much...It's
worth noting that the un-hushed DC-9s (and one old 737) were
easy to pick out by their shrill metallic whine, while the
others (especially the F100) were all thunder, and no scream.

So just out of curiousity, are there different standards for
sound *frequency* in the stage 2/3 specs, or is it purely a
measure of decibels? Given what I know from tinkering with
speakers, a low frequency sound may not be percieved as as
loud as one of higher frequency, even if both are of the
same power. So are engine manufacturers more likely to try
and block the high frequency fan whine than the overall
exhaust roar?