Sonic boom
Overview
 
A sonic boom is the sound associated with the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 energy, sounding much like an explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

. The crack of a supersonic bullet
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...

 passing overhead is an example of a sonic boom in miniature.
When an object passes through the air it creates a series of pressure waves
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...

 in front of it and behind it, similar to the bow and stern waves
Bow wave
A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, poses a risk to smaller boats, and in a harbor can cause damage to shore facilities and moored ships...

 created by a boat.
Encyclopedia
A sonic boom is the sound associated with the shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 energy, sounding much like an explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...

. The crack of a supersonic bullet
Bullet
A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration...

 passing overhead is an example of a sonic boom in miniature.

Causes

When an object passes through the air it creates a series of pressure waves
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...

 in front of it and behind it, similar to the bow and stern waves
Bow wave
A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, poses a risk to smaller boats, and in a harbor can cause damage to shore facilities and moored ships...

 created by a boat. These waves travel at the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....

, and as the speed of the object 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 approximately 1225 km/h (761.2 mph) at sea level and 20 °C (68 °F).
In smooth flight, the shock wave starts at the nose of the aircraft and ends at the tail. Because radial directions around the aircraft's direction of travel are equivalent, the shock forms a Mach cone with the aircraft at its tip. The half-angle (between direction of flight and the shock wave) is given by
,

where is the plane's Mach number
Mach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...

. So the faster it goes, the finer, (more pointed) the cone.

There is a rise in pressure at the nose, decreasing steadily to a negative pressure at the tail, followed by a sudden return to normal pressure after the object passes. This "overpressure
Overpressure
The term Overpressure is applied to a pressure difference, relative to a "normal" or "ambient" pressure, in various circumstances:* In engineering: the pressure difference over the wall thickness of a pressure vessel...

 profile" is known as an N-wave because of its shape. The "boom" is experienced when there is a sudden change 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 fills out a narrow path on the ground following the aircraft's flight path, a bit like an unrolling red carpet
Red carpet
A red carpet is traditionally used to mark the route taken by heads of state on ceremonial and formal occasions, and has in recent decades been extended to use by VIPs and celebrities at formal events.- History :...

 and hence known as the "boom carpet". Its width depends on the altitude of the aircraft. The distance from the point on the ground where the boom is heard to the aircraft depends on its altitude and the angle .

For today's supersonic aircraft in normal operating conditions, the peak overpressure varies from less than 50 to 500 Pa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 (one pound per square foot 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 7,000 Pa (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
F-4 Phantom II
The McDonnell Douglas F-4 Phantom II is a tandem two-seat, twin-engined, all-weather, long-range supersonic jet interceptor fighter/fighter-bomber originally developed for the United States Navy by McDonnell Aircraft. It first entered service in 1960 with the U.S. Navy. Proving highly adaptable,...

 flying just above the speed of sound at an altitude of 100 feet (30.5 m). In recent tests, the maximum boom measured during more realistic flight conditions was 1,010 Pa (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 11 pounds per square foot or less. 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.

The power, or volume, of the shock wave is dependent on the quantity of air that is being accelerated, and thus the size and shape of the aircraft. As the aircraft increases speed the shock cone gets tighter around the craft and becomes weaker to the point that at very high speeds and altitudes no boom is heard. The "length" of the boom from front to back is dependent on the length of the aircraft to a factor of 3:2. Longer aircraft therefore "spread out" their booms more than smaller ones, which leads to a less powerful boom.

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 air being forced to turn around these convex points, which generates a shock wave in supersonic flow.

The later shock waves are somewhat 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 the boom seem louder. On most designs the characteristic distance is about 40000 feet (12,192 m), 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.

Measurement and examples

The pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

 from sonic booms caused by aircraft often are a few pounds per square foot. A vehicle flying at greater altitude will generate lower pressures on the ground, because the shock wave reduces in intensity as it spreads out away from the vehicle, but the sonic booms are less affected by vehicle speed.
Aircraft speed altitude pressure (lbf/ft2) pressure (Pa)
SR-71 Mach 3 80000 feet (24,384 m) 0.9 43
Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

 SST
Mach 2 52000 feet (15,849.6 m) 1.94 93
F-104 Mach 1.93 48000 feet (14,630.4 m) 0.8 38
Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

Mach 1.5 60000 feet (18,288 m) 1.25 60

Abatement

In the late 1950s when supersonic transport
Supersonic transport
A supersonic transport is a civilian supersonic aircraft designed to transport passengers at speeds greater than the speed of sound. The only SSTs to see regular service to date have been Concorde and the Tupolev Tu-144. The last passenger flight of the Tu-144 was in June 1978 with its last ever...

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

Richard Seebass and his colleague Albert George at Cornell University
Cornell University
Cornell University is an Ivy League university located in Ithaca, New York, United States. It is a private land-grant university, receiving annual funding from the State of New York for certain educational missions...

 studied the problem extensively and eventually defined a "figure of merit
Figure of merit
A figure of merit is a quantity used to characterize the performance of a device, system or method, relative to its alternatives. In engineering, figures of merit are often defined for particular materials or devices in order to determine their relative utility for an application...

" (FM) to characterize the sonic boom levels of different aircraft. FM is a function of the aircraft weight and the aircraft length. 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 FMs of about 1.4 for Concorde
Concorde
Aérospatiale-BAC Concorde was a turbojet-powered supersonic passenger airliner, a supersonic transport . It was a product of an Anglo-French government treaty, combining the manufacturing efforts of Aérospatiale and the British Aircraft Corporation...

 and 1.9 for the Boeing 2707
Boeing 2707
The Boeing 2707 was developed as the first American supersonic transport . After winning a competition for a government-funded contract to build an American SST, Boeing began development at its facilities in Seattle, Washington...

. This eventually doomed most SST projects as public resentment mixed with politics eventually resulted in laws that made any such aircraft impractical (flying only over water for instance). Another way to express this is wing span. The fuselage
Fuselage
The fuselage is an aircraft's main body section that holds crew and passengers or cargo. In single-engine aircraft it will usually contain an engine, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage which in turn is used as a floating hull...

 of even a large supersonic aircraft is very sleek and with enough angle of attack and wing span the plane can fly so high that the boom by the fuselage is not important. The larger the wing span, the greater the downwards impulse which can be applied to the air, the greater the boom felt. A smaller wing span favors small aeroplane designs like business jets.

Seebass and George also worked on the problem from another angle, trying to spread out the N-wave laterally and temporally (longitudinally), by producing a strong and downwards-focused (SR-71 Blackbird
SR-71 Blackbird
The Lockheed SR-71 "Blackbird" was an advanced, long-range, Mach 3+ strategic reconnaissance aircraft. It was developed as a black project from the Lockheed A-12 reconnaissance aircraft in the 1960s by the Lockheed Skunk Works. Clarence "Kelly" Johnson was responsible for many of the...

, Boeing X-43
Boeing X-43
The X-43 is an unmanned experimental hypersonic aircraft with multiple planned scale variations meant to test various aspects of hypersonic flight. It was part of NASA's Hyper-X program. It has set several airspeed records for jet-propelled aircraft....

) shock at a sharp, but wide angle nosecone, which will travel at slightly supersonic speed (bow shock
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

), and using a swept back flying wing
Flying wing
A flying wing is a tailless fixed-wing aircraft which has no definite fuselage, with most of the crew, payload and equipment being housed inside the main wing structure....

 or an oblique flying wing
Oblique wing
An oblique wing is a variable geometry wing concept. On an aircraft so equipped, the wing is designed to rotate on center pivot, so that one tip is swept forward while the opposite tip is swept aft...

 to smooth out this shock along the direction of flight (the tail of the shock travels at sonic speed). To adapt this principle to existing planes, which generate a shock at their nose cone
Nose cone
The term nose cone is used to refer to the forwardmost section of a rocket, guided missile or aircraft. The cone is shaped to offer minimum aerodynamic resistance...

 and an even stronger one at their wing leading edge, the fuselage below the wing is shaped according to the area rule
Area rule
The Whitcomb area rule, also called the transonic area rule, is a design technique used to reduce an aircraft's drag at transonic and supersonic speeds, particularly between Mach 0.75 and 1.2....

. Ideally this would raise the characteristic altitude from 40000 feet (12,192 m) to 60,000 feet (from 12,000 m to 18,000 m), which is where most SST aircraft fly.

This remained untested for decades, until DARPA
Defense Advanced Research Projects Agency
The Defense Advanced Research Projects Agency is an agency of the United States Department of Defense responsible for the development of new technology for use by the military...

 started the Quiet Supersonic Platform project and funded the Shaped Sonic Boom Demonstration
Shaped Sonic Boom Demonstration
The NASA Shaped Sonic Boom Demonstration, also known as the Shaped Sonic Boom Experiment, was a two-year program that used a Northrop F-5E with a modified fuselage to demonstrate that the aircraft's shock wave, and accompanying sonic boom, can be shaped, and thereby reduced...

 (SSBD) aircraft to test it. SSBD used an F-5 Freedom Fighter
F-5 Freedom Fighter
The Northrop F-5A/B Freedom Fighter and the F-5E/F Tiger II are part of a family of widely-used light supersonic fighter aircraft, designed and built by Northrop...

. The F-5E was modified with a highly refined shape which lengthened the nose to that of the F-5F model. The fairing extended from the nose all the way back to the inlets on the underside of the aircraft. The SSBD was tested over a two year period culminating in 21 flights and was an extensive study on sonic boom characteristics. After measuring the 1,300 recordings, some taken inside the shock wave by a chase plane
Chase plane
A chase plane is an aircraft that "chases" another aircraft, a spacecraft or a rocket during flight. Safety can be one function of a chase plane; others are to photo or video the target vehicle, or to collect engineering data from it...

, the SSBD demonstrated a reduction in boom by about one-third. Although one-third is not a huge reduction, it could have reduced Concorde below the FM = 1 limit for instance.

As a follow-on to SSBD, in 2006 a NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

-Gulfstream Aerospace
Gulfstream Aerospace
Gulfstream Aerospace Corporation is a producer of several models of jet aircraft. Gulfstream has been a unit of General Dynamics since 1999.The company has produced more than 1,500 aircraft for corporate, government, private, and military customers around the world...

 team tested the Quiet Spike
Quiet Spike
Quiet Spike is a collaborative program between Gulfstream Aerospace and NASA's Dryden Flight Research Center to investigate the suppression of sonic booms....

 on NASA-Dryden's F-15B aircraft 836. The Quiet Spike
Quiet Spike
Quiet Spike is a collaborative program between Gulfstream Aerospace and NASA's Dryden Flight Research Center to investigate the suppression of sonic booms....

 is a telescoping boom fitted to the nose of an aircraft specifically designed to weaken the strength of the shock waves forming on the nose of the aircraft at supersonic speeds. Over 50 test flights were performed. Several flights included probing of the shockwaves by a second F-15B, NASA's Intelligent Flight Control System
Intelligent Flight Control System
The Intelligent Flight Control System is a next-generation flight control system designed to provide increased safety for the crew and passengers of aircraft as well as to optimize the aircraft performance under normal conditions. The main benefit of this system is that it will allow a pilot to...

 testbed, aircraft 837.

There are theoretical designs that do not appear to create sonic booms at all, such as the Busemann's Biplane
Busemann's Biplane
Busemann's Biplane is a conceptual airframe design invented by Adolf Busemann which avoids the formation of N-type shock waves and thus does not create a sonic boom....

.

Perception and noise

The sound of a sonic boom depends largely on the distance between the observer and the aircraft shape 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 shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...

s have heard, when the aircraft is nearby the sonic boom is a sharper "bang" or "crack". The sound is much like the "aerial bomb
Aerial bomb
An aerial bomb is a type of explosive weapon intended to travel through the air with predictable trajectories, usually designed to be dropped from an aircraft...

s" used at firework displays. It is a common misconception that only "one" boom is generated during the subsonic to supersonic transition, rather, the boom is continuous along the boom carpet for the entire supersonic flight. As a former Concorde pilot puts it, "You don't actually hear anything on board. All we see is the pressure wave moving down the aeroplane - it gives an indication on the instruments. And that's what we see of Mach 1. But we don't hear the sonic boom or anything like that. That's rather like the wake of ship - it's behind us.".

In 1964, NASA and the Federal Aviation Administration
Federal Aviation Administration
The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

 began the Oklahoma City sonic boom tests
Oklahoma City sonic boom tests
The Oklahoma City sonic boom tests, also known as Operation Bongo II, refer to a controversial experiment in which 1,253 sonic booms were carried out over Oklahoma City, Oklahoma over a period of six months in 1964...

, 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
Class action
In law, a class action, a class suit, or a representative action is a form of lawsuit in which a large group of people collectively bring a claim to court and/or in which a class of defendants is being sued...

 lawsuit, which it lost on appeal in 1969.

There has been recent work in this area, notably under DARPA's Quiet Supersonic Platform studies. Research by acoustics experts under this program began looking more closely at the composition of sonic booms, including the frequency content. Several characteristics of the traditional sonic boom "N" wave can influence how loud and irritating it can be perceived by listeners on the ground. Even strong N-waves such as those generated by Concorde or military aircraft can be far less objectionable if the rise time of the overpressure is sufficiently long. A new metric has emerged, known as perceived loudness, measured in PLdB. This takes into account the frequency content, rise time, etc. A well known example is the snapping of your fingers in which the "perceived" sound is nothing more than an annoyance.

The energy range of sonic boom is concentrated in the 0.1–100 hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 frequency range that is considerably below that of subsonic aircraft, gunfire
Gunfire
Gunfire can refer to:* Gunshot, the discharge of a firearm* Gunfire , a DC comic book superhero* Gunfire , a 1934 Western starring Rex Bell* Gunfire , a 1950 Western starring Don "Red" Barry...

 and most industrial noise
Industrial noise
Industrial noise is usually considered mainly from the point of view of environmental health and safety, rather than nuisance, as sustained exposure can cause permanent hearing damage. Traditionally, occupational noise has been a hazard linked to heavy industries such as ship-building and...

. Duration of sonic boom is brief; less than a second, 100 milliseconds (0.1 second) 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 1 smi for each 1000 feet (304.8 m) of altitude (5 m/m); that is, an aircraft flying supersonic at 30000 feet (9,144 m) will create a lateral boom spread of about 30 miles (48.3 km), or at 10,000 meters a spread of 50 kilometers. 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.

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 59 degrees Fahrenheit (15 °C), the temperature at 30000 feet (9,144 m) drops to minus 49 degrees Fahrenheit (−45 °C). 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 30000 feet (9,144 m) is about 670 miles (1,078.3 km) per hour, but an aircraft must travel at least 750 miles (1,207 km) per hour (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.

The composition of the atmosphere is also a factor. Temperature variations, humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

, atmospheric pollution, and wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s can all have an effect on how a sonic boom is perceived on the ground. Even the ground itself can influence the sound of a sonic boom. Hard surfaces such as concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

, pavement, and large buildings can cause reflections which may amplify the sound of a sonic boom. Similarly grass
Grass
Grasses, or more technically graminoids, are monocotyledonous, usually herbaceous plants with narrow leaves growing from the base. They include the "true grasses", of the Poaceae family, as well as the sedges and the rushes . The true grasses include cereals, bamboo and the grasses of lawns ...

y fields and lots of foliage can help attenuate the strength of the overpressure of a sonic boom.

Currently there are no industry accepted standards for the acceptability of a sonic boom. Until such metrics can be established, either through further study or supersonic overflight testing, it is doubtful that legislation will be enacted to remove the current prohibition on supersonic overflight in place in several countries, including the United States.

A sonic boom can sometimes be referred to as a sonic wave.

Bullwhip

The cracking sound a bullwhip
Bullwhip
A bullwhip is a single-tailed whip, usually made of braided leather, which was originally used as a tool for working with livestock.Bullwhips are pastoral tools, traditionally used to control livestock in open country...

 makes when properly wielded is, in fact, a small sonic boom. The end of the whip, known as the "cracker", moves faster than the speed of sound, thus creating a sonic boom. The whip was probably the first human invention to break the sound barrier
Sound barrier
The sound barrier, in aerodynamics, is the point at which an aircraft moves from transonic to supersonic speed. The term, which occasionally has other meanings, came into use during World War II, when a number of aircraft started to encounter the effects of compressibility, a collection of several...

.

A bullwhip tapers down from the handle section to the cracker. The cracker has much less mass than the handle section. When the whip is sharply swung, the energy is transferred down the length of the tapering whip. In accordance with the formula (if the work for whipping remains constant) for kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

, the velocity of the whip increases with the decrease in mass, which is how the whip reaches the speed of sound and causes a sonic boom.

While this tapering assists in reaching speed, actually, even a completely "flat" whip can "crack": as the wave travels down the length of the whip, the distance to the tip decreases, in turn decreasing the mass in the above formula.
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