When an object is flying in the air at a mach number ($M$) greater than 1, a shock wave is continuously produced and the mach cone makes a particular angle, $\theta_M$, with the ground (or normal). An observer on the ground will hear a shock wave when their relative angle with the flying object reaches $\theta_M$. But why does the observer not hear continuous shock waves, which are continuously produced by the flying object? If the observer is now inside the mach cone, are not the shock waves propagating spherically symmetric and would not the observer still hear a shock? Would not an observer even directly behind the flying object hear the shock wave using this same logic?
Physically, are the gas molecules just all bunched/compressed exactly at the mach angle for some reason? Based on the derivation of $\theta_M = \arcsin (1/M)$, it appears this is the angle of the flying object relative to when a sound is first heard, but should not an observer continue hearing a sound? In the picture, the black solid lines appear to indicate distinct waves which should be able to be heard by any observers for either subsonic or supersonic motion. I would've thought anyone intersecting the black lines would hear the sound associated to regular waves ($M<1$) or a shock discontinuity ($M>1$). But is that true?
Perhaps I am misunderstanding where the shock front itself exists and this is a crucial premise to the above. Is not a new shock front continuously generated at each time and location? And does not each of these new shock fronts continuously expand spherically? Unless I am mistaken, this is what the pictures depicts and one should hear multiple shocks (which is contrary to experience).