Sonic boom is
an impulsive noise similar to thunder. It is caused by an
object moving faster than sound -- about 750 miles per hour at
sea level. An aircraft traveling through the atmosphere
continuously produces air-pressure waves similar to the water
waves caused by a ship's bow. When the aircraft exceeds the
speed of sound, these pressure waves combine and form shock
waves which travel forward from the generation or "release"
point.
As an aircraft flies at supersonic speeds it is
continually generating shock waves, dropping sonic boom along
its flight path, similar to someone dropping objects from a
moving vehicle. From the perspective of the aircraft, the boom
appears to be swept backwards as it travels away from the
aircraft. If the plane makes a sharp turn or pulls up, the
boom will hit the ground in front of the aircraft.
The
sound heard on the ground as a "sonic boom" is the sudden
onset and release of pressure after the buildup by the shock
wave or "peak overpressure." The change in pressure caused by
sonic boom is only a few pounds per square foot -- about the
same pressure change we experience on an elevator as it
descends two or three floors -- in a much shorter time period.
It is the magnitude of this peak overpressure that describes a
sonic boom.
There are two types of booms: N-waves and
U-waves. The N-wave is generated from steady flight
conditions, and its pressure wave is shaped like the letter
"N." N-waves have a front shock to a positive peak
overpressure which is followed by a linear decrease in the
pressure until the rear shock returns to ambient pressure. The
U-wave, or focused boom, is generated from maneuvering
flights, and its pressure wave is shaped like the letter "U."
U-waves have positive shocks at the front and rear of the boom
in which the peak overpressures are increased compared to the
N-wave.
For today's supersonic aircraft in normal
operating conditions, the peak overpressure varies from less
than one pound to about 10 pounds per square foot for a N-wave
boom. Peak overpressures for U-waves are amplified two to five
times the N-wave, but this amplified overpressure impacts only
a very small area when compared to the area exposed to the
rest of the sonic boom.
The strongest sonic boom ever
recorded was 144 pounds per square foot and it did not cause
injury to the researchers who were exposed to it. The boom was
produced by a F-4 flying just above the speed of sound at an
altitude of 100 feet.
In recent tests, the maximum
boom measured during more realistic flight conditions was 21
pounds per square foot. There is a probability that some
damage -- shattered glass, for example, will result from a
sonic boom. Buildings in good repair should suffer no damage
by pressures of less than 16 pounds per square foot. And,
typically, community exposure to sonic boom is below two
pounds per square foot. Ground motion resulting from sonic
boom is rare and is well below structural damage thresholds
accepted by the U.S. Bureau of Mines and other agencies.
Characteristics
The energy range of sonic boom
is concentrated in the 0.1 - 100 hertz frequency range that is
considerably below that of subsonic aircraft, gunfire and most
industrial noise. Duration of sonic boom is brief; less than a
second -- 100 milliseconds (.100 seconds) for most
fighter-sized aircraft and 500 milliseconds for the space
shuttle or Concorde jetliner .
The intensity and width
of a sonic boom path depends on the physical characteristics
of the aircraft and how it is operated. In general, the
greater an aircraft's altitude, the lower the overpressure on
the ground. Greater altitude also increases the boom's lateral
spread, exposing a wider area to the boom. Overpressures in
the sonic boom impact area, however, will not be uniform. Boom
intensity is greatest directly under the flight path,
progressively weakening with greater horizontal distance away
from the aircraft flight track.
Ground width of the
boom exposure area is approximately one mile for each 1,000
feet of altitude; that is, an aircraft flying supersonic at
30,000 feet will create a lateral boom spread of about 30
miles. For steady supersonic flight, the boom is described as
a carpet boom since it moves with the aircraft as it maintains
supersonic speed and altitude.
Some maneuvers, diving,
acceleration or turning, can cause focusing of the boom. Other
maneuvers, such as deceleration and climbing, can reduce the
strength of the shock. In some instances weather conditions
can distort sonic booms.
Sonic Boom
Refraction
Depending on the aircraft's altitude, sonic
booms reach the ground two to 60 seconds after flyover.
However, not all booms are heard at ground level. The speed of
sound at any altitude is a function of air temperature. A
decrease or increase in temperature results in a corresponding
decrease or increase in sound speed.
Under standard
atmospheric conditions, air temperature decreases with
increased altitude. For example, when sea-level temperature is
58 degrees Fahrenheit, the temperature at 30,000 feet drops to
minus 49 degrees Fahrenheit. This temperature gradient helps
bend the sound waves upward. Therefore, for a boom to reach
the ground, the aircraft speed relative to the ground must be
greater than the speed of sound at the ground. For example,
the speed of sound at 30,000 feet is about 670 miles per hour,
but an aircraft must travel at least 750 miles per hour (Mach
1.12, where Mach 1 equals the speed of sound) for a boom to be
heard on the ground.
Background
The Air Force
has conducted faster-than-sound test flights since 1947, and
today most Air Force fighter aircraft are capable of
supersonic speed. Consequently, supersonic training flights
that simulate actual combat conditions are necessary to ensure
the success and survival of aircrews during wartime. However,
Air Force procedures require that, whenever possible, flights
be over open water, above 10,000 feet and no closer than 15
miles from shore. Supersonic operations over land must be
conducted above 30,000 feet or, when below 30,000 feet, in
specially designated areas approved by Headquarters United
States Air Force, Washington, D.C., and the Federal Aviation
Administration.
Public Interest
Responsibilities
The Air Force continues to expand its
knowledge of sonic boom. Continuing research specifically
addresses modeling the generation of a sonic boom and its
impact on the environment -- people, domestic animals,
wildlife, and historical, unconventional and conventional
structures. This research provides the Air Force with tools to
mitigate sonic boom disturbances through flight operations
planning and land use compatibility planning.
Point of Contact Armstrong Laboratory, 2610 Seventh St.,
Wright-Patterson AFB OH 45433-7201, DSN 785-3664 or (513)
257-3664.
