I know it comes from breaking the sound barrier, but why a boom instead of complete silence?
I have the understanding it creates a physical wave too.
Bonus questions: Does the pilot hear it?
Can the same thing be achieved underwater?
Related: I know the speed of sound is faster under water, does any reaction happen theoretically at the air speed of sound underwater?
In: 310
Aerodynamicist here.
Imagine standing on the shore of a lake and watching a boat go by. As the front of the boat pushes through the water, it creates a bow wave. That wave then “propagates” away from the boat to both sides. If you were to look straight down on the boat from above, the bow wave would look like a V-shape. As long as the boat is moving, the bow wave is continuously forming at the bow and then moving away to the sides.
https://en.m.wikipedia.org/wiki/Wake_(physics)#/media/File%3AFjordn_surface_wave_boat.jpg
If you’re standing on the shore, sometime after the boat goes by, it’s bow wave will eventually propagate far enough to reach your position. You’ll know it because the gentle lapping waves on the shore will be replaced by big splashy waves for just a moment, then calm back down.
Now think of the airplane pushing through the air. Just like the boat, the nose tip of the plane creates a bow wave. On a slow airplane, the bow wave is a gentle ripple of many small waves as the air gets nudged away, just like the boat. But when a plane goes supersonic, the air can’t move out of the way fast enough, so all the waves coalesce into a single big wave called a “shock”. But the shock wave is still essentially like the bow wave from the boat, and it propagates out in a V shape just the same.
https://en.m.wikipedia.org/wiki/Oblique_shock#/media/File%3AShockwave_pattern_around_a_T-38C_observed_with_Background-Oriented_Schlieren_photography_(1).jpg
And just like at the lake, the shock wave from the nose of the airplane will propagate all the way to where you are on the ground. As it passes over you, your ear will perceive the wave in the air as a loud and sharp sound, aka a sonic boom.
From this concept, incomplete as it is, you’ll be able to avoid a couple of common misconceptions. For one, a sonic boom isn’t a single event that occurs at the moment a plane transitions from subsonic to supersonic. Any time an airplane is traveling at supersonic speeds, it is dragging a sonic boom behind it continuously just like the boat and the bow wave. The other is that the pilot never hears it. To hear it, the wave has to pass over your ear when the plane passes, but the pilot is traveling at the same speed as the rest of the aircraft (at least he’s supposed to be), so it never sweeps over him the way it does people on the ground.
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