>>>>> "SD" == Scott Dorsey <klu...@panix.com
SD> Part of it depends on how you weight it.
SD> If the airplane was a uniform isotropic sound source and it went by, the
SD> level of sound would be inversely proportional to the square of the
SD> distance, and you could figure out the distance with a little bit of
SD> trig by drawing a right triangle whose base is the distance of the plane
SD> from the space directly above you, whose height is the altitude, and whose
SD> hypotenuse is the distance.
Yes the hypotenuse is 15 km.
SD> BUT... and where this gets interesting.... is that although some sounds
SD> from an aircraft (like the noise caused by turbulence over the body) can
SD> be considered an isotropic source, not all of them can be.
SD> Jet engine noise is much louder behind the plane than in front of it,
SD> louder in front of it than directly below it, and the frequency spectrum
SD> varies a lot with angle too.
Ah ha, so the sudden onset, and then gradual decline, that I hear on the
ground is not due to some speed of sound / Doppler style thing, but the
SD> Props are different.... and recprocating engines and turboprops have
SD> different engine noise characteristics even though the prop noise is
SD> more or less the same.
Too few props here for me to comment. I'm talking about the jets going
by me on
SD> If you go to ntrs.nasa.gov
and put in ANOPP, you can see some reports on
SD> typical examples for modern jets.
Wow there's a lot there. Thanks!