A wall of superimposed sound waves along the leading edge of an aircraft traveling at the speed of sound in air.
The Doppler effect, discovered by Austrian physicist Christian Johann Doppler in 1842, is the change in the observed frequency of a wave, of sound or light, for example, due to relative motion between the observer and the wave source. When observer and source approach each other, the emitted frequency of the waves is measured to be higher, due to the velocity of approach; the greater the relative speed, the greater the frequency shift. When the source and observer are receding from each other, the emitted frequency is measured to be lower, in direct proportion to the velocity of recession.
Although the Doppler effect applies to all types of waves, it is particularly noticeable for sound waves. When an ambulance speeds by, for example, the pitch, or frequency, drops noticeably. The effect is most easily explained by considering water waves on a placid pond created by a small insect jiggling its legs. The insect’s movement creates a pattern of equally spaced concentric rings; each ring represents the crest of a wave traveling outward from the insect at constant speed. If the insect is traveling toward the left while jiggling its legs, the wave pattern is distorted; the rings are no longer concentric, but the centers of consecutive waves are displaced in the direction of motion. Although the insect has not changed the frequency with which it jiggles its legs, an observer at a position toward the insect’s left encounters a higher frequency of waves because the waves are compressed in the direction of motion. An observer at a position to the insect’s right perceives a lower frequency of waves. The waves are spread farther apart because the insect is moving away from the observer.
Although sound waves are invisible and spread into three dimensions, the same principle applies. When a source of sound approaches, the perceived frequency, or pitch, is higher than the emitted frequency, and the opposite is true for a receding source.
If the insect discussed above were to swim across the water at the same speed as the velocity of water waves while continuing to jiggle its legs at a constant frequency, the wave crests would be superimposed on one another directly in front of the insect rather than moving ahead of it. This wall of water may be considered a wave barrier, because the insect would have to exert considerable effort to swim over this barrier in order to travel at a speed greater than the wave velocity. However, after the insect had surmounted the barrier by exceeding the wave velocity, the water ahead would be smooth and undisturbed.
When an aircraft travels at the speed of sound in air, it also encounters a barrier of superimposed sound waves. The compression waves are stacked up along the leading edge of the aircraft, requiring some additional thrust for the aircraft to punch through. After the plane exceeds the velocity of sound, however, there are no further barriers to inhibit additional acceleration; the airplane may travel at supersonic speed.
As the jiggling insect travels through water with a speed greater than the wave velocity, it produces the pattern in which each consecutive wave crest, represented by a circle, is located outside the previous crest. The wave crests overlap to form larger crests. This small wall of water, called a bow wave, has a solid “V” shape.
When an aircraft flies at a supersonic speed, the overlapping spherical sound waves form a cone of air pressure that grows in size until intercepted by the ground. This thin conical shell of compressed air is termed a shock wave. Just as a person floating in a tranquil lake will be hit by the bow wave of a speed boat traveling faster than the speed of water waves, people on the ground will be struck by the shock wave of a supersonic aircraft. This wave, called a sonic boom, is heard as a sharp cracking thunderclap.
The sound of a subsonic aircraft is perceived by a listener on the ground as a continuous tone. The shock wave produced by a supersonic airplane, consisting of many superimposed waves, occurs like an explosion in a single burst. Both processes consist of a burst of high-pressure air that creates a loud, unpleasant noise. In actuality, the shock waves produced by supersonic aircraft create a double sonic boom; the shock wave from the bow of the plane is a pulse of increased pressure that is followed a fraction of a second later by a negative-pressure pulse from the trailing edge of the aircraft. Overall, the pressure wave has the general appearance of the letter “N.” This pressure shock wave is produced during the entire course of a supersonic flight and not only during the time when it passes the sound barrier, as is mistakenly believed. Because the width of the sonic boom trail is about 20 miles, and its length is the flight path, sonic booms can create considerable problems. First, there is the annoyance factor of people being startled or awakened by the loud, explosive noise. Because of sonic booms’ intense and rapid pressure changes, sonic booms can destroy property in inhabited areas. Broken windows and structural damage are not uncommon. Finally, sonic booms can be problematic even in uninhabited regions; they have been known to topple rock structures in national parks.
The speed of sound at sea level is 760 miles per hour. The speed of sound decreases with increased altitude, so that at 50,000 feet, sound travels at 660 miles per hour. Because the wall of pressure termed the sound barrier differentiates subsonic from supersonic flight, the speed of sound is defined as a velocity of Mach 1. Mach 2, then, would be twice the speed of sound, and so on.
Although several attempts were made in the early 1940’s to exceed the sound barrier, early jet planes of the period were not powerful or sturdy enough to succeed. When an aircraft reaches Mach 1, strong local shock waves form on the wings, and the flow of air around the plane becomes unsteady. As a result, the airplane is subjected to severe buffeting that interferes with the plane’s stability and renders it difficult to control. In 1943, U.S. aeronautical engineers began working on the first airplane specifically designed to surmount these problems and withstand the tremendous air pressure of Mach 1 in order to obtain supersonic flight. This goal was realized on October 14, 1947, when Captain Charles E. “Chuck” Yeager of the U.S. Air Force smashed through the sound barrier in a Bell X-1 rocket plane. Although many supersonic flights at ever-increasing speeds were made over the next decade, the speed never exceeded Mach 2.5, because friction caused by the rapidly moving air overheated the outer shell of the airplanes.
Using jet engines specifically designed for supersonic flight, the North American F-100 Super Sabre jet fighter became the first jet capable of flying at supersonic speeds in level flight. The first supersonic bomber, the Convair B-58 Hustler, became operational in 1956. By 1963, the X-15 rocket plane was able to fly 67 miles above the earth’s surface at a speed exceeding Mach 6. The world’s first supersonic transport (SST) plane, the Tupolev Tu-144, was tested by Soviet pilots in 1968. Britain and France jointly constructed the Concorde SST, which was designed to fly at Mach 2 and began commercial service in 1969. Since that time, however, the number of supersonic flights has been limited due to the high cost of fuel and the problems of sonic booms. In the United States, commercial supersonic flights are now restricted to transoceanic flights.
Anderson, John D. Introduction to Flight: Its Engineering and History. New York: McGraw-Hill, 1978. An introductory text that considers the theoretical questions of aerodynamics, including the design and construction of airplanes planned for different purposes. Dwiggins, Don. Flying the Frontiers of Space. New York: Dodd, Mead, 1982. A readily accessible history of American experimental aircraft from 1947 to the early 1980’s. Kerrebrock, Jack. Aircraft Engines and Gas Turbines. 2d ed. Cambridge, Mass.: MIT Press, 1992. A technical description requiring some familiarity with physics or engineering of the power plants necessary for supersonic flight. Kryter, K. D. Noise and Man. New York: Academic Press, 1970. This work includes a complete description of sonic booms, their effects on people and structures, and the potential sonic hazards of SST overland flights. Strong, W., and G. R. Plitnik. Music, Speech, Audio. Provo, Utah: Soundprint, 1992. An easy-to-read introduction to the science of acoustics that contains a complete explanation of the physics of the Doppler effect and sonic booms in easy-to-understand descriptive terms.
Aerospace industry, U.S.
Andrei Nikolayevich Tupolev