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Shock Waves and Sonic Booms

Shock Waves

A shock wave is a pressure wave that propagates faster than the sound speed of the medium in which it propagates. A shock wave is a typical example of nonlinear phenomena which occurs when energy is deposited and instantaneously released in very confined regions of gases, liquids and even solids.

These strong pressure waves occur in any elastic medium such as air, water, or a solid substance, produced by supersonic (or near supersonic) aircraft, explosions, lightning, or other phenomena that create violent changes in pressure. Shock waves differ from sound waves in that the wave front, in which compression takes place, is a region of sudden and violent change in stress, density, and temperature. Because of this, shock waves propagate in a manner different from that of ordinary acoustic waves. In particular, shock waves travel faster than sound, and their speed increases as the amplitude is raised; but the intensity of a shock wave also decreases faster than does that of a sound wave, because some of the energy of the shock wave is expended to heat the medium in which it travels.

When an aircraft travels at subsonic speed, the pressure disturbances, or sounds, that it generates extend in all directions. Because this disturbance is transmitted earthward continuously to every point along the path, there are no sharp disturbances or changes of pressure. At supersonic speeds, however, the pressure field is confined to a region extending mostly to the rear and extending from the craft in a restricted widening cone (called a Mach cone). As the aircraft proceeds, the trailing parabolic edge of that cone of disturbance intercepts the earth, producing on earth a sound of a sharp bang or boom--with silence before and after. When such an aircraft flies at a low altitude, the shock wave may be of sufficient intensity to cause glass breakage and other damage. The intensity of the sonic boom is determined not only by the distance between the craft and the ground but also by the size and shape of the aircraft, the types of maneuvers that it makes, and the atmospheric pressure, temperature, and winds. If the aircraft is especially long, double sonic booms might be detected, one emanating from the leading edge of the plane and one from the trailing edge.

Sonic Booms

If the speed of the source is greater than the speed of sound, another type of wave phenomenon will occur: the sonic boom. A sonic boom is a type of shock wave that occurs when waves generated by a source over a period of time add together coherently, creating an unusually strong sum wave. An analogue to a sonic boom is the V-shaped bow wave created in water by a motorboat when its speed is greater than the speed of the waves. In the case of an aircraft flying faster than the speed of sound (about 764 miles per hour), the shock wave takes the form of a diamond-shaped cone in three-dimensional space called the Mach cone. The Mach number is defined as the ratio of the speed of the aircraft to the speed of sound. The higher the Mach number--that is, the faster the aircraft--the smaller the angle of the Mach cone.

Mach Number

In fluid mechanics, the ratio of the velocity of a fluid to the velocity of sound in that fluid is defined as Mach, named after Ernst Mach (1838-1916), an Austrian physicist and philosopher. In the case of an object moving through a fluid, such as an aircraft in flight, the Mach number is equal to the velocity of the object relative to the fluid divided by the velocity of sound in that fluid. Mach numbers less than one indicate subsonic flow; those greater than one, supersonic flow. Fluid flow, in addition, is classified as compressible or incompressible on the basis of the Mach number. For example, gas flowing with a Mach number of less than three-tenths may be considered incompressible, or of constant density, an approximation that greatly simplifies the analysis of its behavior. For Mach numbers greater than one, shock wave patterns develop on the moving body because of compression of the surrounding fluid. Streamlining alleviates shock wave effects.

Sources: Britannica.com and NASA