Consider a sound source, for instance an airplane, moving towards a stationary observer with a speed vs. If the speed of sound in air is v, the Mach number is defined as the ratio of the two speeds. If vs is greater than v, the Mach number becomes greater than unity, and the speed is called supersonic. Then, according to the Doppler formula, the sound's observed frequency is negative, which is practically impossible. Instead, the observer perceives no sound until the source has passed them. The sound waves interfere constructively along the lines of a three-dimensional cone. Within the cone, they interfere destructively. Thus, the cone's edge is a sharp divider of regions of no sound and destructive interference. The resultant pattern is called a shock wave. Applying geometric concepts, the sine of the cone angle equals the inverse of the Mach number. The sudden arrival of the shock wave is known as a sonic boom. Because of the high pressure, sonic booms created by moving airplanes can shatter windows and rattle our nerves.