Re: aircraft noise

From:         barr@ash.mmm.ucar.EDU (Keith Barr)
Date:         05 Feb 93 13:55:08 PST
References:   1 2
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kls@ohare.Chicago.COM (Karl Swartz) says:
> This has a dramatic impact on noise because the majority of the noise
> comes from the hot exhaust gasses of the combustion process.  With
> more of the thrust coming from the cooler bypass airflow the engine
> is quieter.  In addition, the cool bypass air surrounds the hotter
> combustion gasses and tends to contain the noise.  Further improvement
> have come as the manufacturers have learned how to better control this
> phenomenon.

When I was in school, I had a class on jet propulsion.  In that class we
were taught that sound was tightly related to the speed of the
flow out the back of the engine.  In fact, we were shown that sound is
roughly related to a constant times the velocity raised to the seventh!
Therefore, even a small slowdown in the velocity can make a great deal of
improvement in the overall sound.  If you work through the math, a 10%
reduction in velocity results in approximately a 50% decrease in noise.

In modern high-bypass turbofans the core flow is still very fast, but the
bypass air is flowing at a considerbly slower rate.  I am not sure if the
bypass air "shields" the core flow, or if the sound decrease is based
more upon mass flow (much lower mass flowing through the core of a newer
engine).  I might be able to come up with some rough speed differences
(given in mass flow rates of course) for different bypass ratios and
engines if people are interested.
 _____________________________           _____
| Keith Barr                  \           \   \__      _____                  
| barr@ncar.ucar.edu           \___________\   \/_______\___\_____________    
| Commercial/AS&MEL/Inst/IGI   /           < /_/   .....................  `-.
|_____________________________/             `-----------,----,--------------'
When you think how well basic appliances work, it's   _/____/               
hard to believe anyone ever gets on an airplane.--Calvin