Re: "crackle"

Date:         16 Sep 99 16:42:29 
From:         Don Stauffer <>
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It is my impression that by far the majority of the sound from a jet
engine, except for obvious fan harmonics in high bypass engines, is due
to turbulent interaction of exhaust column with surrounding air.  And,
yes, I believe there is a fair body of literature on this subject.  It
is part of the technology for developing quieter jet aircraft.  Now, one
theory, specifically dealing with the crackle, is that the transmission
of sound through the air is subject to spectral filtering.  It is
frequently noted that the crackling sound is prevelant on very cold
days, (we have a lot of them in Minneapolis), and that the air
temperature affects both the frequency and, by temperature gradients,
the actual propogation path of the sound.  There has been much less
published, to my knowledge, on this later subject, but I have heard some
people talking of papers on the subject.

Check Journal of Acoustics as a starting place, or any conferences on
jet noise.

Don Stauffer in Minneapolis