How Everything Works
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Question 979

What is the frequency, amplitude, wavelength, etc. of a sound wave at the sound barrier? — KT, Ocean Springs, MS
The sound barrier is something of a myth that dates to the early days of transonic flight. As early airplanes approached the speed of sound, they suffered various flight instabilities—a significant rise in air drag and a tendency for supersonic shock waves to interfere with the operations of control surfaces. Exceeding the speed of sound appeared problematic at the time and the expression "the sound barrier" came into common use. However, there is no real sound barrier. Once Yeager had exceed the speed of sound in an experimental plane, it became clear that the speed of sound was not a firm barrier.

However, there is one peculiar thing that does happen once a plane has exceeded the speed of sound. You can no longer hear the plane coming because it is outrunning its own sound waves. Instead of having its sound spread out in front of it, the plane has its sound swept back in a cone behind it. The edges of this cone are a shock wave and you experience a sudden pressure rise as this cone passes across you—you hear a sonic boom. A supersonic plane carries this conical shock wave with it at all times and everyone hears a sonic boom as this shock wave sweeps across them. What you should remember is that the sonic boom doesn't occur when the plane "breaks the sound barrier"; the sonic boom is a continuous feature of a supersonic plane that you hear as its shockwave passes you by.


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