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  • Plane takes off, reaches mach 1.3, flies 1K miles, accelerates to mach 2.1, flies 100 mi. How many sonic booms?

    Posted on November 19th, 2010 admin No comments

    First correct and complete answer was from @jchaager: A plane flying at mach 1 or faster produces one continuous sonic boom which “follows” the plane.  At least two people said a supersonic plane makes 1 sonic boom without specifying that it was continuous.  Not sure they got it or not.

    An airplane, or any other object traveling at the speed of sound or faster produces a sonic boom continually until it decelerates below mach 1.  There is a common misconception that a plane produces a singular sonic boom when it “breaks the sound barrier.”  Not true.  There is an almost equally common misconception that a plane produces another sonic boom at each whole multiple of speed of sound, as it “breaks the second sound barrier” at mach 2, and so on.  Not true.

    An object moving faster than the speed of sound produces a shock wave which is perceived by an observer as a “sonic boom” as it passes.  An observer hears a sonic boom after the airplane passes, even though the plane may have “broken the sound barrier” hundreds of miles away.

    One would have hoped that the famous “double sonic boom” of a Space Shuttle on final approach would cure the “sonic boom/sound barrier” myth.  The double sonic boom of the Space Shuttle is caused by its curving approach.  The shuttle passes the observers twice while still supersonic.

    Here in Middle Georgia, sonic booms from U.S. Air Force fighters are a weekly occurrence, but I have a couple of noteable personal experiences with sonic booms.

    One day (many years ago) while hunting, I heard sharp “crack” in the trees overhead.  About two seconds later, I heard gunshot on next hill.   A moment later, this was repeated:  A “crack” overhead followed by a distant gunshot two seconds later.  What was happening here was that another hunter a mile or so away was shooting at something (and missing), and the bullets were passing my position.  Each bullet arrived at my location before the sound of the shot that fired it.  As the supersonic bullet passed me, I heard its tiny sonic boom.  I didn’t hear the gunshot until that sound had time to travel the mile or so from the shooter’s position to mine.

    Now, these bullets “broke the sound barrier” within the barrel of the gun about a mile away from me.  But I heard the sonic boom as the bullet passed me.  It took a total of about five seconds for the sound of the gunshot – and any sound the bullet may have made by “breaking the sound barrier” – to reach me, but I heard the sonic boom from the bullet 20 feet over my head long before the sound of the gunshot reached me.

    There is a very odd phenomenon that may be observed after a sonic boom if the airplane making it passed very close by.  I observed this once when a U.S. Navy F-14 “buzzed” my ship while supersonic at mast-top level.  The plane had approached from the port side while I was on the starboard side, so I never saw it coming.  After the hellaceous blast of the sonic boom, I saw the plane departing very rapidly off the starboard side,  and I heard the sound of its departing engines following it.  But I also  noticed what sounded like another plane’s engines departing rapidly off the port side of the ship, in the direction from which the F-14 had come.

    What was happening here was that the sounds made by the plane were “catching up” to my position in reverse order.  Sounds made 10 seconds before the plane passed us were arriving at the ship after the sounds made 9 seconds before the plane passed, and so on.  This is a perfectly straightforward consequence of the supersonic flight of the plane, but it produces a very disconcerting effect.  The observer hears the sounds of the departing plane in “normal” fashion, but the sounds the plane made as it approached are “played in reverse” as earlier sounds catch up to later sounds.  It sounds like an invisible airplane flying away.

    Ever since that day, whenever I see an airplane approaching but I can’t hear it, I plug my ears and cringe.  I have never again heard a sonic boom from mast-top level, but I really, really don’t want to.

    More about sonic booms:  http://www.sciencetriviatweets.com/?p=1008

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