(I'm going to use the term "sound cone" for the area you can hear an aircraft when it's at Mach ≥ 1, illustrated by this picture. Not considering the shockwave.)
So as explained in this answer, the angle that an oblique shockwave will form at is dependent on 2 things.
- The Mach number before the shock
- How much the flow has to turn in the shock
So this means that the shock angle will change with speed, but you can also change the shock angle by changing the aircraft shape (which would change the flow turning angle, therefore changing the shock angle).
The sound cone angle depends only on the speed, if I'm correct. When going Mach ≥ 1, the sound waves can't travel forwards relative to the plane because the plane is going faster than the sound, but they can travel sideways and back, evidenced by the picture above.
My confusion is here : Why does the sound cone angle always equal the shock angle?
(You can't hear an aircraft until after the shockwave)
Is it just a coincidence that the shock angle (determined by the flow turning angle and Mach number) equals the sound cone angle which only depends on the speed?
For an example (at Mach 1 in example) -- If you were to only change the nose shape of an aircraft, that would change the flow turning angle, and consequently the shock angle. You didn't change your speed, so the sound cone angle (again pictured above) wouldn't change. Wouldn't this mean at some point the sound cone and shockwave would meet, because they are not parallel? If they did intersect, the shock would travel through the sound cone and in that case you might be able to hear the aircraft before the shockwave.
My theory up to this point is that sound (say sound from a jet engine) can't travel through a shockwave, so it wouldn't matter if the shock and sound cone were parallel, because the sound can't travel through it anyway.