Focused on audible sound, as opposed to sight.
A sound, or a sound signal
Just below, or just above the speed of sound (0.8 < Ma < 1.2 approximately).
Travelling through a fluid (commonly air) at a speed where regions of supersonic and subsonic flow coexist simultaneously over different portions of the surface of the moving object in question.
Passing from subsonic to supersonic, or vice versa.