Not the Roar of the Lion
March 01, 2001
March may sometimes come in like a lion, but the roar one hears may not come from a ferocious gust of wind but rather from a supesonic aircraft. The photo above shows sound waves emanating from a jet moving faster than 760 miles per hour (the approximate speed of sound in air at sea level). The circular rings are camera artifacts. As a jet accelerates faster than the speed of sound, it pushes on the sound waves in front of it. However, since sound waves can't move faster than the speed of sound, they pile up against each other. These piled up waves are known as shock waves, the largest of which form at the tip and tail of the plane. A listener on the surface hears the sound, all at once, as the accumulated cone of shock waves passes - the waves emitted over a longer period of time reach the listener's ear about at the same time.