What is the threshold effect?

Mach effect that can be found is the following effect derived from visual effect:

When the brightness jumps, there will be a feeling of edge enhancement, and the bright side will be brighter and the dark side will be darker. Mach effect will lead to local threshold effect, that is, on the bright side of the edge, the false perception threshold of pixels near the edge is 3~4 times higher than that far away from the edge, so it can be considered that the edge masks its neighboring pixels.

When you say shock wave, it should be Mach cone! That is, when flying at supersonic speed, the front end of the plane stirs up a shock wave in the air. This shock wave is conical, which is a cone wrapped with sound. Because the cone top is supersonic, the sound wave propagating forward can't pass through the cone top, but the front end of the sound wave propagating around just constitutes the cone surface of this cone, so you won't hear the sound of the plane before the Mach cone sweeps you, but when the cone surface sweeps you, you will hear a deafening sound like an explosion.