How Everything Works
How Everything Works How Everything Works

Question 544

What is a shockwave and a sonic boom? - EL
A plane that is flying faster than the speed of sound is outrunning its own sound. As a result, its sound spreads out behind it as a conical structure, with the plane located at the apex of that cone. This cone moves along with the plane. Since the planes sound is all contained inside the cone, you can't hear the plane until the cone passes by you. When the edge of the cone does pass you, you hear a great deal of sound all at once. In fact, there is a pressure jump right at the surface of the cone (sound and pressure are closely related) and this cone itself is a shockwave. As the shockwave (or cone surface) passes you, you hear a loud booming sound, a "sonic boom". Note that the sonic boom occurs when the shockwave passes your ears, not when the plane "breaks the sound barrier". When you hear the sonic boom depends on where you are relative to the moving plane, so different people hear it at different times.

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