Consider a source moving at the speed of sound (Mach 1). The sounds it produces will travel at the same speed as the source, so that in front of the source, each new wavefront is compressed to occur at the same point. A listener placed in front of the source will not detect anything until the source arrives. All the wavefronts add together, creating a wall of pressure. This shock wave will not be perceived as a pitch but as a ‘thump’ as the pressure front passes the listener.
Pilots who have flown at Mach 1 have described a noticeable “barrier” that must be penetrated before achieving supersonic speeds. Traveling within the pressure front results in a bouncy, turbulent flight.
Now consider a sound source moving at supersonic speed, i.e., faster than the speed of sound. In this case, the source will be in advance of the wavefront. So a stationary listener will hear the sound after the source has passed by. The shock wave forms a Mach cone, which is a conical pressure front with the plane at the tip. This cone creates the sonic boom shock wave as a supersonic aircraft passes by. This shock wave travels at the speed of sound, and since it is the combination of all the wavefronts, the listener will hear a quite intense sound. However, supersonic aircraft actually produce two sonic booms in quick succession. One boom comes from the aircraft’s nose and the other one from its tail, resulting in a double thump.
The speed of sound varies with temperature and humidity, but not directly with pressure. In air at sea level, it is about 343 m/s. But in water, the speed of sound is far quicker (about 1,484 m/s), since molecules in water are more compressed than in air and sound is produced by the vibrations of the substance. So the sound barrier can be broken at different speeds depending on air conditions, but is far more difficult to break underwater.