A
point to be noted at this stage is that a sonic boom does not only happen
the instant an aircraft surpasses the speed of
sound. Sonic booms are observed when any
aircraft, which is traveling faster than the speed of sound,
passes overhead. It is not a sign that the aircraft just overcame the sound
barrier, but rather a sign that the aircraft is traveling faster than sound.
It is also to be noted that a sonic boom is heard only after the aircraft
passes. This is because if the plane is flying at less than the speed of
sound, the sound of the plane travels ahead of the plane. Thus, people on
the ground can hear the plane coming toward them. However, the sound
of a plane flying faster than the speed of sound cannot be heard on the
ground until the aircraft has passed.
All
aircraft generate two cones, one at the nose and the other at the tail.
They are usually of similar strength and the time
interval between the two as they reach the
ground is primarily dependent on the size of the aircraft and
its altitude. Most people on the ground cannot distinguish between the two
and they are usually heard as a single sonic boom. Sonic booms created
by vehicles the size and mass as the space shuttles are very distinguishable
and two distinct booms are easily heard.
Mach
numbers:
Mach numbers
are used to describe the speed of planes flying near or above
the speed of sound. A Mach number is found by dividing the speed of an
airplane by the speed of sound at the plane's altitude. For example, the
Mach number of a plane flying at 1,520 mph at sea
level would be 2. Modern airliners
cruise at an altitude of about 35,000 feet (9,000 meters) and
a speed of about Mach 0.80 to Mach 0.85. Flight that is slightly faster
or slower than Mach 1 is known as transonic
flight. A plane that is significantly
slower than Mach 1 is subsonic whereas a plane that is significantly
faster than Mach 1 is supersonic, and that which is at or faster than
about Mach 5 is hypersonic.
Impact
on human beings:
The
noise levels of sonic booms are varied. Sonic booms are measured in pounds
per square foot of overpressure. Overpressure is the amount of increase
over normal atmospheric pressure that surrounds us. Here are a few
parameters to judge the level of sonic booms according to the level of damage
they can inflict.
At one
pound of overpressure, which is produced by supersonic aircraft flying
at normal operating altitudes, there will be no damage to structures. At
an overpressure between 1.5 - 2 pounds it would be annoying to the people
living in the vicinity of where it occurs. When
there is an overpressure
of 2 - 5 pounds rare minor damage could occur. From this point
onwards, the likelihood of structural damage increases. Tests however
reveal that sound structures have been undamaged even by pressures
up to 11 pounds. Sonic booms produced by aircraft flying supersonic
at altitudes of less than 100 ft, creating between 20 and 144 pounds
of overpressure, have been experienced by humans without injury. There
could be damage to the ears when overpressures reach 720 pounds. Lung
damage will occur when there are overpressures of 2160 pounds.
Factors
influencing sonic boom:
The
main reason that we hear a sonic boom is because of the shock waves generated
by the aircraft when it crosses the speed of sound. However there
are several other factors that can influence sonic booms - weight, size
and shape of an aircraft, the altitude and flight path at which the aircraft
is flying and the outside temperature and pressure.
A
larger and correspondingly heavier aircraft displaces more air and creates
more lift to sustain flight thereby generating
stronger shock waves. It will therefore
create sonic booms stronger and louder than those of smaller and lighter
aircraft.
The
altitude of an aircraft determines the distance shock waves travel before
reaching the ground. This is a significant factor on the intensity of the
shock wave. As the shock cone gets wider, and it moves outward and downward,
its strength gets diminished. Accordingly, the higher the aircraft,
the greater the distance the shock wave has to travel, reducing the intensity
and audibility of the sonic boom. Among
the factors influencing sonic booms, an
increase in altitude is the most effective method for reducing
the intensity of sonic boom.
The
width of the cone beneath the aircraft is approximately one mile for each
1000 feet of altitude. For example, an aircraft flying supersonic at
50,000 feet can produce a sonic boom cone about 50
miles wide. The sonic boom, however, will
not be uniform. Maximum intensity is directly beneath the
aircraft, and decreases as the lateral distance from the flight path increases
until it ceases to exist because the shock waves refract away from
the ground. The lateral spreading of the sonic boom depends only upon
altitude, speed, and the atmosphere - and is independent of the aircraft's
shape, size and weight.
The ratio
of aircraft length to maximum cross sectional area also influences the
intensity of the sonic boom. The longer and more slender the aircraft, the
weaker would be the shock waves. The
shorter and stockier the
vehicle,
the stronger would be the shock waves. Increasing speeds above Mach
1.3 results in only small changes in shock wave strength. The direction
of travel and strength of shock waves are influenced by wind, speed,
and direction, and by air temperature and pressure. At speeds slightly
greater than Mach 1, their effect can be significant, but their influence
is small at speeds greater than Mach 1.3. Distortions in the shape of
the sonic booms can also be influenced by local air turbulence near the
ground. This, too, will cause variations in the
overpressure levels.
Sonic
boom lines:
Aircraft
maneuvering can cause distortions in shock wave patterns. Some maneuvers
pushovers, acceleration, and 'S' turns - can amplify the intensity
of the shock wave. Hills, valleys, and other terrain features can create
multiple reflections of the shock waves and affect intensity.
On
maps and globes of the earth, the locations
of the equator, the Arctic and Antarctic
circles, and the tropics of Cancer and
Capricorn are clearly marked. These
circles indicate certain characteristics of
the apparent journey that the sun makes
over the earth's surface each year. There are two other conjured
circles on the earth's surface that few people know about. They are
called "sonic boom lines," and they indicate the latitudes where the
earth's surface velocity is equal to the speed of
sound.
The
rotation of the earth causes many parts of the surface to
spin at velocities that exceed the speed of sound. Since the
circumference of the earth is roughly 24,000 miles, the surface
speed at the equator due to the rotation of the earth
is about 1,000 miles per hour (well above the speed of
sound). Using simple trigonometry, it is easy to compute the
latitude where the earth's surface is moving at precisely the
speed of sound. At sea level and zero degrees Celsius that
latitude is 44 degrees 21 minutes, and a circle drawn around
the earth at that latitude is a sonic boom line. There are two such lines;
one in the Northern Hemisphere and the other in the Southern Hemisphere.
Unlike the equator and tropics, the exact locations of the sonic boom
lines vary according to local altitude and air temperature. Although these
effects are very small, they are enough to make the exact locations of
the boom lines laborious to determine and in continuous flux. That is why
cartographers refuse to put these lines on their maps.
Technically,
the sonic boom lines are the intersections of an imaginary cylinder
with the earth's surface. The center line of the cylinder is aligned with
the earth's rotational axis and the surface of the cylinder (which represents
all points that are rotating at the speed of sound) pierces the earth's
surface at the two sonic boom latitudes. The two supersonic atmospheric
regions that are directly above the sonic boom lines at the earth's
surface are called, the 'nether regions'. These small areas of the atmosphere
dissipate the sonic boom shock waves into outer space, thus preventing
the shock waves from causing a continuous, annoying boom to the
people on the surface. The earth's motion around the sun and the sun's motion
through the cosmos do not affect the sonic boom phenomenon nor produce
one of their own. This is because sound waves cannot travel through
the vacuum of outer space.
Though
sonic booms are not often heard these days in most inhabited parts of
the world it is a field that required wide study in order to enable engineers
to design aircrafts that would cause the minimum sonic boom and
thereby minimum discomfort and damage to people.
