# Acoustics/Sonic Boom

![](Acoustics_sonic_boom.JPG "Acoustics_sonic_boom.JPG")

!Warplane passing the sound
barrier.

A **sonic boom** is the audible component of a shock wave in air. The
term is commonly used to refer to the air shocks caused by the
supersonic flight of military aircraft or passenger transports such as
Concorde (Mach 2.2, no longer flying) and the Space Shuttle (Mach 27).
Sonic booms generate enormous amounts of sound energy, sounding much
like an explosion; typically the shock front may approach 100 megawatts
per square meter, and may exceed 200 decibels.

!When an aircraft is near the sound barrier, an unusual cloud sometimes
forms in its wake. A Prandtl-Glauert Singularity results from a drop in
pressure, due to shock wave formation. This pressure change causes a
sharp drop in temperature, which in humid conditions leads the water
vapor in the air to condense into droplets and form the
cloud._-_filtered.jpg "When an aircraft is near the sound barrier, an unusual cloud sometimes forms in its wake. A Prandtl-Glauert Singularity results from a drop in pressure, due to shock wave formation. This pressure change causes a sharp drop in temperature, which in humid conditions leads the water vapor in the air to condense into droplets and form the cloud."){width="200"}

## Cause of sonic booms

As an object moves through the air it creates a series of pressure waves
in front and behind it, similar to the bow and stern waves created by a
boat. These waves travel at the speed of sound, and as the speed of the
aircraft increases the waves are forced together or \'compressed\'
because they cannot \"get out of the way\" of each other, eventually
merging into a single shock wave at the speed of sound. This critical
speed is known as Mach 1 and is 1,225 km/h (761 mph) at sea level.

In smooth flight, the shock wave starts at the nose of the aircraft and
ends at the tail. There is a sudden increase in pressure at the nose,
decreasing steadily to a negative pressure at the tail, where it
suddenly returns to normal. This \"overpressure profile\" is known as
the N-wave due to its shape. We experience the \"boom\" when there is a
sudden increase in pressure, so the N-wave causes two booms, one when
the initial pressure rise from the nose hits, and another when the tail
passes and the pressure suddenly returns to normal. This leads to a
distinctive \"double boom\" from supersonic aircraft. When maneuvering
the pressure; distribution changes into different forms, with a
characteristic U-wave shape. Since the boom is being generated
continually as long as the aircraft is supersonic, it traces out a path
on the ground following the aircraft\'s flight path, known as the **boom
carpet**. frame\|A cage around the engine reflects any shock waves. A
spike behind the engine converts them into
thrust. frame\|To
generate lift a supersonic airplane has to produce at least two shock
waves: One over-pressure downwards wave, and one under-pressure upwards
wave. Whitcomb area rule states, we can reuse air displacement without
generating additional shock waves. In this case the fuselage reuses some
displacement of the
wings.

A sonic boom or \"tunnel boom\" can also be caused by high-speed trains
in tunnels (e.g. the Japanese Shinkansen). In order to reduce the sonic
boom effect, a special shape of the train car and a widened opening of
the tunnel entrance is necessary. When a high speed train enters a
tunnel, the sonic boom effect occurs at the tunnel exit. In contrast to
the (super)sonic boom of an aircraft, this \"tunnel boom\" is caused by
a rapid change of subsonic flow (due to the sudden narrowing of the
surrounding space) rather than by a shock wave. In close range to the
tunnel exit this phenomenon can causes disturbances to residents.

## Characteristics

The power, or volume, of the shock wave is dependent on the quantity of
air that is being accelerated, and thus the size and weight of the
aircraft. As the aircraft increases speed the shocks grow \"tighter\"
around the craft, and do not become much \"louder\". At very high speeds
and altitudes the cone does not intersect the ground, and no boom will
be heard. The \"length\" of the boom from front to back is dependent on
the length of the aircraft, although to a factor of 3:2 not 1:1. Longer
aircraft therefore \"spread out\" their booms more than smaller ones,
which leads to a less powerful boom.

The nose shockwave compresses and pulls the air along with the aircraft
so that the aircraft behind its shockwave sees subsonic airflow.

However, this means that several smaller shock waves can, and usually
do, form at other points on the aircraft, primarily any convex points or
curves, the leading wing edge and especially the inlet to engines. These
secondary shockwaves are caused by the subsonic air behind the main
shockwave being forced to go supersonic again by the shape of the
aircraft (for example due to the air\'s acceleration over the top of a
curved wing).

The later shock waves are somehow faster than the first one, travel
faster and add to the main shockwave at some distance away from the
aircraft to create a much more defined N-wave shape. This maximizes both
the magnitude and the \"rise time\" of the shock, which makes it seem
louder. On most designs the characteristic distance is about 40,000 ft,
meaning that below this altitude the sonic boom will be \"softer\".
However the drag at this altitude or below makes supersonic travel
particularly inefficient, which poses a serious problem.

## Abatement

In the late 1950s when SST designs were being actively pursued, it was
thought that although the boom would be very large, they could avoid
problems by flying higher. This premise was proven false when the North
American B-70 *Valkyrie* started flying and it was found that the boom
was a very real problem even at 70,000 ft (21,000m). It was during these
tests that the N-wave was first characterized.

Richard Seebass and his colleague Albert George at Cornell University
studied the problem extensively, and eventually defined a \"figure of
merit\", **FM**, to characterize the sonic boom levels of different
aircraft. FM is proportional to the aircraft weight divided by the
three-halves of the aircraft length, FM = W/(3/2·L) = 2W/3L. The lower
this value, the less boom the aircraft generates, with figures of about
1 or lower being considered acceptable. Using this calculation they
found FM\'s of about 1.4 for Concorde, and 1.9 for the Boeing 2707. This
eventually doomed most SST projects as public resentment, somewhat blown
out of proportion, mixed with politics eventually resulted in laws that
made any such aircraft impractical (flying only over water for
instance).

Seebass-George also worked on the problem from another angle, examining
ways to reduce the \"peaks\" of the N-wave and therefore smooth out the
shock into something less annoying. Their theory suggested that body
shaping might be able to use the secondary shocks to either \"spread
out\" the N-wave, or interfere with each other to the same end. Ideally
this would raise the characteristic altitude from 40,000 ft to 60,000,
which is where most SST designs fly. The design required some fairly
sophisticated shaping in order to achieve the dual needs of reducing the
shock and still leaving an aerodynamically efficient shape, and
therefore had to wait for the advent of computer-aided design before
being able to be built.

This remained untested for decades, until DARPA started the **Quiet
Supersonic Platform** project and funded the **Shaped Sonic Boom
Demonstration** aircraft to test it. SSBD used a F-5 Freedom Fighter
modified with a new body shape, and was tested over a two year period in
what has become the most extensive study on the sonic boom to date.
After measuring the 1,300 recordings, some taken inside the shock wave
by a chase plane, the SSBD demonstrated a reduction in boom by about
one-third. Although one-third is not a huge reduction, it could reduce
Concorde below the FM = 1 limit for instance.

There are theoretical designs that do not appear to create sonic booms
at all, such as the Busemann\'s Biplane. Nobody has been able to suggest
a practical implementation of this concept, as yet.

## Perception and noise

The sound of a sonic boom depends largely on the distance between the
observer and the aircraft producing the sonic boom. A sonic boom is
usually heard as a deep double \"boom\" as the aircraft is usually some
distance away. However, as those who have witnessed landings of space
shuttles have heard, when the aircraft is nearby the sonic boom is a
sharper \"bang\" or \"crack\". The sound is much like the \"aerial
bombs\" used at firework displays.

In 1964, NASA and the FAA began the Oklahoma City sonic boom tests,
which caused eight sonic booms per day over a period of six months.
Valuable data was gathered from the experiment, but 15,000 complaints
were generated and ultimately entangled the government in a class action
lawsuit, which it lost on appeal in 1969.

In late October 2005, Israel began using nighttime sonic boom raids
against civilian populations in the Gaza Strip
1 as a
method of psychological warfare. The practice was condemned by the
United Nations. A senior Israeli army intelligence source said the
tactic was intended to break civilian support for armed Palestinian
groups.

## Media

These videos include jets achieving supersonic speeds.

First supersonic
flight
(info)

:   Chuck Yeager broke the sound barrier on October 14, 1947 in the Bell
    X-1.

F-14 Tomcat sonic boom flyby (with
audio)
(info)

:   F-14 Tomcat flies at Mach 1 over the water, creating a sonic boom as
    it passes.

F-14A Tomcat supersonic
flyby
(info)

:   Supersonic F-14A Tomcat flying by the USS Theodore Roosevelt CVN-71
    in 1986 for the tiger cruise.

Shuttle passes sound
barrier
(info)

:   Space shuttle Columbia crosses the sound barrier at 45 seconds after
    liftoff.

## External links

-   NASA opens new chapter in supersonic
    flight
-   \"Sonic
    Boom,\" a
    tutorial from the \"Sonic Boom, Sound Barrier, and Condensation
    Clouds\"
    (or \"Sonic Boom, Sound Barrier, and Prandtl-Glauert Condensation
    Clouds\") collection of tutorials by Mark S. Cramer, Ph.D. at
    <http://FluidMech.net> (Tutorials, Sound Barrier).
-   decibel chart including sonic
    booms

