"crackle"

Date:         04 Sep 99 11:38:28 
From:         ebarber <ebarber@wombat.eng.fsu.edu>
Organization: Florida State University
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I am interested in a so called phenomenom called "crackle".  It was
proposed some 25 years ago by a gentleman named Ffowcs-Williams, that at
jet take off and periodically during flight that the engines would make a
"crackling" sound.  He stated that this sound was analogous to water
hitting hot oil.  It is said that through laboratory test, this sound was
proven not to be induced by the combustion process, but was rather a
characteristic of the jet (fluid) flow.

Has anyone in their years of experience, actually heard this "crackle"
sound?  Have any postulations as to the origin of the sound if it
actually exist?  Heard any mention of this "crackle" in text or literature
and have a possible reference other than Ffowcs-Williams 1975 paper?

Any insight into this matter would be greatly appreciated.