What is really happening when an aircraft surpasses the sound speed limit for us to hear and see a small ‘boom’?

Question

What is really happening when an aircraft surpasses the sound speed limit for us to hear and see a small ‘boom’?

In: Physics

Answer 1

So an airplane is constantly putting out sound. Sound moves at a specific speed, which for subsonic flight is faster than the plane itself is moving.

Let’s say a plane is in subsonic flight, starting at point 1. At point 1, one “piece” of sound leaves the engines and moves forward (at the speed of sound). The airplane moves to point 2, one yard further than point 1. In the time it took the plane to move 1 yard, the sound from point 1 has moved, say, 10 yards. That’s for regular flight.

Now let’s say a plane is flying at exactly the speed of sound. At point 1, the “piece” of sound leaves the engines and moves forward. The airplane moves to point 2 and emits another “piece” of sound, as it would in any situation. However, in this case the sound from point 1 reaches point 2 at the exact same time as the plane. So when the plane reaches point 2 and emits the second “piece” of sound, now you have 2 pieces of sound in the same spot (since the 1st one is moving right alongside the plane). Now you’ve doubled the intensity of the sound since you have twice the amount (the sound from point 1 and from point 2 are in the same place at the same time).

Now in real life sound isn’t being created once every yard, but rather continuously. So instead of 2 “pieces” of sound, you have a bunch all in the same place at the same time. So now you have a fast buildup of sound, and that added intensity is the sonic boom.

Answer 2

When an object surpasses the speed of sound in a given medium, it leaves a “bubble” of empty space that collapses almost immediately. This is because the molecules of the medium are pushed out of the way too quickly to fill in the space left in the wake of the moving object.

With aircraft, it’s pushing the air out of the way so quickly that it’s leaving a vacuum pocket behind it. We hear the air collapsing the pocket and see the shockwave from the air carrying the sound of closing that pocket.