An airplane flying well below the speed of sound has ample warning of the pressure pulses created in the air. But as the airplane approaches the speed of sound, the pressure pulses merge closer together in front of the airplane and little time elapses between the time the disturbance is sent out and the airplane's arrival time. At the speed of sound, the pressure pulses merge together ahead of the airplane into a "shock wave" that is an almost instantaneous line of change in pressure, temperature, and density.

Credits - NASA