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Originally posted by Thermo Klein
I followed the roads to get to the object and could barely recognize the bottom of a helicopter (pretty sure) but couldn't hear any sound. I felt I could see the rotors spinning. Enough to say definitely not a UFO, and maybe 80-90% it was a helicopter.
The sighting lasted several minutes, during which I didn't hear any sound, not even when I pricked up my ears.
We checked the weather conditions for the evening of December 11, 1989 to see if the wind or the presence of an inversion layer may help explain why no sound was heard. The official weather data  tell us that their was a light to moderate breeze in the evening, and near gale toward midnight. The direction of the wind was south to south-southeast. When Lt. Col. AMOND first spotted the panel-shaped lights in the distance, the object was in the west. At that moment, the wind, coming from a generally southern direction, may have carried the sound away from the witnesses. When the object executed the 360 degree turn, it was closer by and in the south-southwest. Under these circumstances one would expect that at least some of the noise had reached the couple. However, from his letter of December 19, we know that Lt. Col. AMOND restarted the engine of the car immediately after the big light appeared. It is therefore conceivable that, during this phase, the sound of the car engine drowned the sound of the unidentified object.
Research in Austria by a Belgian university team (I didn't keep URLs for this source but we can google it), namely "Comparison of measurements and predictions of sound propagation in a valley-slope configuration in an inhomogeneous atmosphere", T. Van Renterghem and D. Botteldooren, Ghent University, P. Lercher, Medical University of Innsbruck, Austria found that:
"In the case of an upward refracting atmosphere, turbulent scattering into the acoustic shadow zone that is formed becomes important. Neglecting this effect often results in unrealistically large attenuations. (...) Based on experiments, it was found that the sound pressure level relative to free field propagation stays more or less constant in the acoustical shadow zone formed by an upward refracting atmosphere. This constant value depends on the geometry of the problem and on the strength of the turbulence. A value of -20 dB is common in acoustical literature. (...) [but] (...) The temperature profiles observed (...) contain upward and downward refracting parts [whose] influence ranges from -3 dBA to +10 dBA [+10 for the downward refracting parts]".
According to one report, a classified stealth helicopter was being tested at the Groom Lake Air Force base as early as 1990. The code name for the helicopter as "T.E.-K," standing for "Test and Evaluation Project K." The F-117 stealth fighter was reportedly known as "T.E.-A," and the B-2 stealth bomber, known as "T.E.-B." The 2/6/95 issue of Aviation Week & Space Technology reported that the Air force had a silent NOTAR helicopter and a stealth helicopter inside Area 51.