The speed of sound plays a fundamental role in compressible gas dynamics: it is the speed at which a small amplitude disturbance propagates through a gas. Sound is actually a pressure oscillation, areas of compression and rarefaction passing through a medium (gas, liquid, or solid). When a body is moving faster than the speed of sound, it compresses the sound waves radiating out from it. These build up into a large pressure front which is the shock wave. As the body passes the observer, the effect of this shock wave is a sudden spike in pressure. In the atmosphere, this is the so-called sonic boom.

Imagine the following thought experiment. Take a platform, and move it at a constant speed through a uniform gaseous medium. Arrange a sound speaker on platform, so that it is continually emitting sound. If the platform is at rest, the sound...