Re: Engine noise

Date:         27 Dec 96 19:09:39 
From:         s_odle@earthlink.net
Organization: Earthlink Network, Inc.
References:   1
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On 27 Dec 96 13:32:25 , "Robert M. Sherry" <sherryr@a.cs.okstate.edu>
wrote:

>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?

There are many more variables than what you might think.

1. A particular aircraft's Stage 2 or Stage 3 limit is a function of
its maximum takeoff gross weight.  There a particular DC-9 may have a
lower limit than another DC-9 (same model even) simply because of its
lower max takeoff weight.

2. Actually the "whine" or tone is eliminated because in the
calculations done (to the raw noise data) to produce the certification
noise levels. Tone have a large effect on the fianl number.

3. You can'y compare two aircraft at any given airport due to the
difference in each aircraft performance, takeoff procedures, etc.