Sonic Boom arises because a supersonic airplane moves faster than the pressure disturbances, or sound waves, it propagates. A stationary source (left) emits spherical sound waves that move outward like concentric ripples. If the source moves less that the speed of sound (middle), waves emitted at successive positions are crowded in the direction of movement. They overtake the moving source and "warn" the air of its approach. But disturbances from the earlier emissions of a supersonic source (right) cannot overtake the source, which arrives without warning and creates a shock wave. The spheres become tangent to the sides of the shock wave cone.