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Sonic Boom
A sonic boom is a thunder-like sound produced when an aircraft travels faster than the speed of sound. Air is a fluid and is pushed apart with great force as an aircraft traveling at supersonic speeds cuts through the air, forming a shock wave of compressed air, similar to the bow waves created by a boat as it cuts through the water.
The shock wave produced by an aircraft is in the shape of a cone with its vertex located at the nose of the aircraft and pointing in the direction of travel of the aircraft. The cone spreads out behind the aircraft and increases in diameter as distance behind the aircraft increases. The cone shaped shock wave moves along with the aircraft at the same speed as the aircraft. The shock wave creates a sonic boom at each point in space that it passes. The air pressure within the shock wave is usually only a few pounds per square foot greater than normal atmospheric pressure. This is about the same pressure difference experienced by a change in elevation of about 20 to 30 feet. This additional pressure above normal atmospheric pressure is called overpressure. If the overpressure were released slowly it would produce little sound. When this overpressure is quickly released during a very short time interval it generates a sonic boom. This can be compared to a tank containing pressurized air. If the air is slowly released only a slight hissing noise will be heard. If on the other hand, the tank suddenly bursts open, instantly releasing all the pressurized air inside it, it will create a loud explosion. Thus it is the rate of change of air pressure, the instantaneous change in air pressure from normal pressure to overpressure and back again to normal air pressure that creates the sonic boom.
All aircraft traveling faster than the speed of sound generate at least two shock wave cones—one at the nose and one at the tail of the aircraft. Additional shock waves are produced at other locations on the aircraft that cut through the air such as the canopy, inlets, wings, and vertical tails. These shock waves tend to merge into a strong nose shock wave and a strong tail shock wave. Both the nose and tail shock waves have similar pressure and the time interval between the two waves as they reach the ground primarily depends on the size of the aircraft and its altitude above the ground. Each shock wave generates a separate sonic boom. However, since both shock waves pass a location very quickly, a stationary observer may only hear one sonic boom, since the human ear has difficulty distinguishing the sudden sound changes, the sonic booms "appear" to merge together and only one sonic boom is heard. The sound generated by each sonic boom becomes more distinguishable as the size and mass of the aircraft increases. Vehicles such as the space shuttle with considerably larger size and heavier mass than most aircraft generate two distinct sonic booms.
Factors Affecting Sonic Boom Intensity
The intensity of sonic booms are affected by :
· Weight, size and shape of the aircraft.
· Altitude
· Attitude—orientation of the aircraft’s axes relative to the its direction of motion.
· Flight path.
· Atmospheric and weather conditions.
As the size and weight of the aircraft increases, the intensity of the sonic increases. This is because a larger aircraft displaces more air, and a heavier aircraft needs a greater force of lift to sustain flight. Thus creating a louder and stronger sonic boom.
As the altitude of the aircraft increases the intensity of the sonic boom at ground level decreases. The shock cone gets wider as distance behind the aircraft increases. The sonic boom cone spreads out beneath the aircraft about one mile for each 1000 feet decrease in altitude. Thus an aircraft flying faster than the speed of sound at an altitude of 60,000 feet will produce a sonic boom cone 60 miles wide at ground level. As the cone gets wider the force contained in it is spread before reaching the ground, and the less intense the sonic boom is at ground level. The lateral spreading of the sonic boom is dependent only upon altitude, speed and the atmospheric conditions. Increasing altitude is the most effective method of reducing sonic boom intensity at ground level.
The ratio of aircraft length to maximum cross sectional area of the aircraft also affects sonic boom intensity. The longer and more slender the aircraft, the weaker the shock waves. On the other hand the fatter and more blunt the aircraft is the stronger the shock wave produced.
As speed increases above Mach 1.3 only small changes in the shock wave strength result. The direction of propagation and strength of shock waves are affected by wind, speed, direction, air temperature and pressure. At speeds just over Mach 1, their affect can be significant, but their affect is small at speeds greater than Mach 1.3. Distortions in the shape of the sonic boom signatures can also be affected by air turbulence near the ground. This will cause variations in overpressure levels.
The motion of the aircraft can cause distortions in shock wave patterns. Maneuvers such as pushovers, S-turns and accelerating can amplify the intensity of the shock wave. Hills, valleys and other topographic features can create multiple reflections of shock waves thus affecting intensity.
Overpressure Data
Overpressure in pounds per square foot is used to measure sonic booms. Overpressure is the amount of pressure above normal atmospheric pressure.
· Normal air pressure is 2,116 psf or 14.7 psi.
· At 1 psf of overpressure, no damage to structures occurs.
· At 1 to 2 psf of overpressure occur from aircraft flying at supersonic speeds and at normal operating altitudes. Overpressure above 1.5 psf is irritating to people.
· At 2 to 5 psf some minor damage can occur to structures.
· As overpressure increases, the chance of structural damage increases. Structures in good condition can withstand overpressures of up to 11 psf.
· 20 to 144 psf are created by aircraft flying at supersonic speeds at altitudes of less than 100 feet. Such levels of overpressure have been experienced by humans without injury.
· At 720 psf damage to eardrums results. At 2160 psf lung damage occurs.
The following over pressures have been measured by several aircraft:
· 0.8 psf for the F-104 at Mach 1.93 and 48,000 feet.
· 0.9 psf for the SR-71 at Mach 3 and 80,000 feet.
· 1.25 psf for the Space Shuttle at Mach 1.5 and 60,000 feet during landing approach.
· 1.94 psf for the Concorde SST at Mach 2 and 52,000 feet.
Click here to see Schlieren Photography of showning shock wave structure.
Bibliography