[jambutty]

Originally Posted by Boeing Guy

Okay,
JB the SR71 flys at 90,000 feet well above commercial traffic, it would be too dangerous below 50,000 we are all up there.

Of course the SR71 magically appears at 90,000 feet after take off. Rather it climbs like any other aircraft and in doing so is under the control of air traffic control, either civilian or military, which will make sure that each aeroplane has enough airspace so as not to be a danger to other aircraft.

At 90,000 feet that is all the more reason why its engine noise wouldn’t be heard at ground level.

Sound, as we know, will not travel through a vacuum and struggles through a partial vacuum or a rarefied atmosphere. Unless my math has deserted me, 90,000 feet is about 17 miles. At between about 20,000 feet and 90,000 feet the atmosphere is thinish and gets even thinner the higher that you go.

So the next question is will sound travel some 17 miles in our atmosphere let alone through a good portion of rarefied air? If it was incredibly loud and well above the noise of a jet engine at full throttle it might. The level of sound is inversely proportional to the distance travelled. But the frequencies of the noise is also against long distance travel.

It is well known that the foghorns that once upon a time blared out their mournful sound to alert ships of proximity to land were of a very low frequency. It was proved that low sounds travel further than higher sounds but even these are limited to a handful of miles, if that. I wouldn’t recommend standing next to a shore based foghorn when it blasts out its sound. The ringing in you ears would take on a new meaning.