What's a Sonic Boom?
kristiandupont.com
kristiandupont.com
This is not anywhere near a correct description of a sonic boom. A sonic boom is not "a large burst when [an object] “breaks the sound barrier”." It is a phenomenon that is associated with any object traveling at or above the speed of sound. A related phenomenon governs the crack of a whip, but is associated with unsteady flow around a subsonic object. (Yes, you read that right. The business end of a bullwhip does not need to travel faster than the speed of sound to produce an audible crack.)
What really happens in a sonic boom is, for a slender body traveling above the speed of sound, the local flow is turned "suddenly," and the freestream is not able to accommodate it in a continuous fashion. There then arises an oblique shock wave, a very small region where fluid properties change nearly instantaneously; Direction, temperature, pressure, and entropy all change. Because the change is not isentropic, there is a hard lower limit to parasitic drag associated with supersonic motion through a fluid.
Interestingly, there is an expansion fan at the widest part of, for example, an aircraft body that is isentropic. It's a broad region where temperature, pressure, and direction change gradually. Thus, there are three 'sonic booms' when an aircraft passes overhead at a speed greater than the speed of sound. There is one sharp one as the nose passes you, due to a compression shock, and another softer one as the widest part passes you, due to an expansion fan, and a third one as the tail passes you, due to another compression shock. This is analogous to Mach diamonds in a rocket exhaust stream.
This is all for sufficiently lax definitions of "passing you," as the shock and expansion fans propagate at the local Mach angle. You'll be looking downstream at the local Mach angle when you hear the boom, if you're following the craft with your eyes.
"When an object passes through the air, it creates a series of pressure waves in front of it and behind it, similar to the bow and stern waves created by a boat. These waves travel at the speed of sound, and as the speed of the object increases, the waves are forced together, or compressed, because they cannot "get out of the way" of each other, eventually merging into a single shock wave at the speed of sound. "
I had this misunderstanding for years (I blame nearly every description of sonic booms), and it made it pretty confusing for me to understand the hubbub about the Concorde traveling over land. I had thought it'd only generate exactly two sonic booms, upon accelerating past Mach 1.0, and decelerating again, and figured they could arrange it so those were located in sparsely populated areas. It wasn't until I realized sonic booms continue to happen during the entire course of the Concorde's supersonic flight that it made any sense.
There is, I believe, a sudden change in aerodynamics when an aircraft passes the sound barrier - it IS a barrier of sorts. Aircraft had the power to pass this barrier, the problem was they would no longer fly correctly, or they would rip them selves apart, or some such thing once they hit mach 1.0.
People naturally got the idea that this phenomenon was related to the sonic boom itself.
I guess I should stress that my point was simply the joke that nature has an unhandled exception - not an actual introduction to Doppler effects or anything.. (Of course, it does still bother me to get it wrong like that :-)