Re: aircraft noise

From:         stadler@Apple.COM (Andy Stadler)
Organization: Apple Computer Inc, Cupertino, CA
Date:         10 Feb 93 03:11:25 PST
References:   1
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In article <airliners.1993.137@ohare.Chicago.COM> kls@ohare.Chicago.COM
 (Karl Swartz) writes:
>>why is it that small-ish 727s and 737s seem to make so much noise on
>>take off and even fly-by, compared to the apparently MUCH quieter
>>747s? This has been my distinct impression on numerous occasions. Is
>>is just more efficiently designed engines, or better noise insulation,
>>or what ?
>
>By 737 you probably mean the 737-100/-200 variants, as the newer
>-300/-400/-500 versions are much quieter.

In addition to Karl's well-written description of the different noise
characteristics of low- and high-bypass engines, there's something else
to consider:  Relative size.

Smaller jets (older 737s, 727s, DC-9, etc) tend to use lower-bypass engines.
Larger jets (DC-10, 747, etc) tend to use higher-bypass engines.  The thing
to remember is not only are these engines quieter, but there's also an
optical illusion - the larger airframes appear to be closer than they 
really are.  This is the same reason a 747 appears to "hang" in the air -
its huge size makes it appear much closer, which screws up your perception
of speed.

In summary, the larger jets seem quieter because they are farther away than
you think they are.  AND because they have quieter engines.

--Andy    stadler@apple.com