Concorde Sonic Boom
A sonic boom is the thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound or supersonic. Air reacts like a fluid to supersonic objects. As objects travel through the air, the air molecules are pushed aside with great force and this forms a shock wave much like a boat creates a bow wave. The bigger and heavier the aircraft, the more air it displaces. There are several factors that can influence sonic booms -- weight, size, and shape of the aircraft or vehicle, plus its altitude, attitude and flight path, and weather or atmospheric conditions. A larger and heavier aircraft must displace more air and create more lift to sustain flight, compared with small, light aircraft. Therefore, they will create sonic booms stronger and louder than those of smaller, lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves will be. Altitude determines the distance shock waves travel before reaching the ground, and this has the most significant effect on intensity. As the shock cone gets wider, and it moves outward and downward, its strength is reduced.
The Concorde SST produced an overpressure or 1.94 pounds, flying at a speed of Mach 2 at an altitude of 52,000 feet. Sonic booms are measured in pounds per square foot of overpressure. This is the amount of the increase over the normal atmospheric pressure which surrounds us (2,116 psf/14.7 psi). At one pound overpressure, no damage to structures would be expected. Overpressures of 1 to 2 pounds are produced by supersonic aircraft flying at normal operating altitudes. Some public reaction could be expected between 1.5 and 2 pounds.
The Congress first gave the Federal Aviation Administration authority to regulate aircraft noise and sonice booms in 1968. The FAA received its first authority to control the design of aircraft for noise purposes in Public Law 90-411, 82 Stat. 389, July 21, 1968). Since March 1973, supersonic flight over land by civil aircraft has been prohibited in the United States. The regulations applicable to supersonic aircraft are found in 14 CFR part 36, Subpart D, ``Noise Limits for Supersonic Transport Category Airplanes,'' and 14 CFR part 91, Subpart I, ``Operating Noise Limits.'' The noise certification levels for the Concorde airplane are in part 36. This regulation requires that the noise levels of the airplane must be reduced to the lowest levels that are economically reasonable, technologically practicable, and appropriate for a Concorde type design. Part 91 prohibits civil aircraft operation at greater than Mach 1 over the United States. Part 91 also imposes flight limitations to ensure that civil supersonic flight entering or leaving the United States will not cause a sonic boom to reach the surface within the United States.
After various reports of audible and infrasonic disturbances on the East Coast of the U.S. beginning in December 1977, some speculation arose that the reflection of upward-traveling booms from the thermosphere might be the cause of some of these events, and that significant changes in the upper atmosphere might be caused by the upward-propagating sonic booms from supersonic aircraft. From 1976 through 1978 there were reports of audible and infrasonic disturbances in the eastern U.S. and Canada. Analysis showed that at least some of these could be correlated with the scheduled flights of the Concorde. These disturbances could not have been the direct sonic boom, since the Concorde flights were adequately slowed at sufficient distance from the coastline, but they could have been shock waves which had reflected from the water, and then traveled to a height of 50 to 100 kilometers before refracting back toward the ground. Alternately, the boom might have been initially headed upward.
Infrasound generated by the sonic boom from the inbound Concorde supersonic transport was recorded at Palisades, New York (Lamont-Doherty Geological Observatory) in 1977. It registered as a series of impulses from distances varying from 165 to about 1000 kilometers. Refraction effects determined by temperature and wind conditions return the signal to the surface from both stratospheric (40 to 50 kilometers) and thermospheric (100 to 130 kilometers) levels. The frequency of the recorded signal is a function of the level of reflection; the frequency decreases from impulse stretching as the atmosphere becomes more rarified relative to the sound pressure. The horizontal trace velocity of the signal across the array of instruments is equal to the acoustic velocity at the reflection level.
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