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| | The author is named Lida Kimber and she feels comfortable when folks utilize the name. Years back she moved to Tennessee but she will have to move one-day or another. The thing she loves most is attracting and today she's time for you to accept new items. Application building is her day-job now.<br><br>my page ... [https://twitter.com/zeitpop Jordan Kurland] |
| {{refimprove|date=December 2013}}
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| [[File:Dopplereffectsourcemovingrightatmach1.4.gif|thumb|The sound source is traveling at 1.4 times the speed of sound (Mach 1.4). Since the source is moving faster than the sound waves it creates, it leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound it creates.]]
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| [[File:Sonic boom.svg|thumb|right|250px|A sonic boom produced by an aircraft moving at M=2.92, calculated from the cone angle of 20 degrees. An observer hears nothing but a boom when the shock wave, on the edges of the cone, crosses his or her location.]]
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| [[File:Mach cone.svg|thumb|250px|Mach cone angle]]
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| <!-- [[File:sonicbm2.ogg||thumb|right|250px|Aircraft sonic boom. {{deletable image-caption|Monday, 21 December 2009}}]] -->
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| A '''sonic boom''' is the sound associated with the [[shock wave]]s created by an object traveling through the air faster than the speed of sound. Sonic booms generate enormous amounts of [[sound]] energy, sounding much like an [[explosion]]. The crack of a supersonic [[bullet]] passing overhead is an example of a sonic boom in miniature.
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| ==Causes==
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| When an aircraft passes through the air it creates a series of [[P-wave|pressure waves]] in front of it and behind it, similar to the [[Bow wave|bow and stern waves]] created by a boat. These waves travel at the [[speed of sound]], 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 they merge into a single shock wave, which travels at the speed of sound, a critical speed known as ''Mach 1'', and is approximately {{convert|1225|km/h|mph|abbr=on}} at sea level and {{convert|20|C|F}}.
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| In smooth flight, the shock wave starts at the nose of the aircraft and ends at the tail. Because the different radial directions around the aircraft's direction of travel are equivalent (given the "smooth flight" condition), the shock wave forms a ''Mach cone'', similar to a [[vapor cone]], with the aircraft at its tip. The half-angle between direction of flight and the shock wave <math> \alpha </math> is given by:
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| :<math> \sin(\alpha) = \frac{v_\text{sound}}{v_\text{object}} </math>,
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| where <math> \frac{v_\text{object}}{v_\text{sound}} </math> is the plane's [[Mach number]]. Thus the faster the plane travels, the finer and more pointed the cone is.
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| 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]] profile" is known as an N-wave because of its shape. The "boom" is experienced when there is a sudden change in pressure, therefore an 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 a supersonic aircraft. When maneuvering, the pressure distribution changes into different forms, with a characteristic U-wave shape.
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| 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 a [[red carpet]], 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 <math> \alpha </math>.
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| For today's supersonic aircraft in normal operating conditions, the peak overpressure varies from less than 50 to 500 [[Pascal (unit)|Pa]] (1 to 10 psf (pound 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 over pressure 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 psf) and it did not cause injury to the researchers who were exposed to it. The boom was produced by an [[F-4 Phantom II|F-4]] flying just above the speed of sound at an altitude of {{convert|100|ft|m}}.<ref>[http://proceedings.esri.com/library/userconf/proc01/professional/papers/pap284/p284.htm Analyzing Sonic Boom Footprints of Military Jets, Andy S. Rogers, A.O.T, Inc.]</ref> In recent tests, the maximum boom measured during more realistic flight conditions was 1,010 Pa (21 psf). 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 530 Pa (11 psf) or less. And, typically, community exposure to sonic boom is below 100 Pa (2 psf). 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.<ref name="Fact Sheet">USAF Fact Sheet 96-03, Armstrong Laboratory, 1996</ref>
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| 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 power of 3/2. Longer aircraft therefore "spread out" their booms more than smaller ones, which leads to a less powerful boom.<ref name=seebass>[ftp://ftp.rta.nato.int/PubFulltext/RTO/EN/RTO-EN-004/$EN-004-06.pdf Sonic Boom Minimization Richard Seebass]</ref>
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| Several smaller shock waves can and usually do form at other points on the aircraft, primarily at 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 speed|supersonic flow]].
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| 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 aircraft designs the characteristic distance is about {{convert|40000|ft|m|sigfig=2}}, 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.
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| ==Measurement and examples==
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| The [[pressure]] 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.
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| {| class="wikitable"
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| |-
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| ! Aircraft
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| ! speed
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| ! altitude
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| ! pressure (lbf/ft<sup>2</sup>)
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| ! pressure (Pa)
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| |-
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| | [[SR-71]]
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| | Mach 3
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| | {{convert|80000|ft|m}}
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| | 0.9
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| | 43
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| |-
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| | [[Concorde]] SST
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| | Mach 2
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| | {{convert|52000|ft|m}}
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| | 1.94
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| | 93
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| |-
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| | [[F-104]]
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| | Mach 1.93
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| | {{convert|48000|ft|m}}
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| | 0.8
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| | 38
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| |-
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| | [[Space Shuttle]]
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| | Mach 1.5
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| | {{convert|60000|ft|m}}
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| | 1.25
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| | 60
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| |-
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| |}<ref>[http://www.nasa.gov/centers/dryden/news/FactSheets/FS-016-DFRC.html Dryden Flight Research Center Fact Sheet: Sonic Booms]</ref>
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| ==Abatement==
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| [[File:Large-Scale Low-Boom Supersonic Inlet Model.jpg|thumb|New research is being performed at NASA's [[Glenn Research Center]] that could help alleviate the sonic boom produced by supersonic aircraft. Testing was recently completed of a Large-Scale Low-Boom supersonic inlet model with micro-array flow control. A NASA aerospace engineer is pictured here in a wind tunnel with the Large-Scale Low-Boom supersonic inlet model.]]
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| In the late 1950s when [[supersonic transport]] (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.
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| 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 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]] and 1.9 for the [[Boeing 2707]]. 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]] 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]].<ref name=seebass/>
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| Seebass and George also worked on the problem from a different angle, trying to spread out the N-wave laterally and temporally (longitudinally), by producing a strong and downwards-focused ([[SR-71 Blackbird]], [[Boeing X-43]]) shock at a sharp, but wide angle nosecone, which will travel at slightly supersonic speed ([[Shock wave|bow shock]]), and using a swept back [[flying wing]] or an [[Oblique wing|oblique flying wing]] 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]] and an even stronger one at their wing leading edge, the fuselage below the wing is shaped according to the [[area rule]]. Ideally this would raise the characteristic altitude from {{convert|40000|ft|m}} to 60,000 feet (from 12,000 m to 18,000 m), which is where most SST aircraft fly.<ref name=seebass/>
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| [[File:Northrop F-5E (modified) DARPA sonic tests 04.07R.jpg|thumb|NASA F-5E modified for DARPA sonic boom tests]]
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| This remained untested for decades, until [[Defense Advanced Research Projects Agency|DARPA]] started the [[Shaped Sonic Boom Demonstration|Quiet Supersonic Platform]] project and funded the [[Shaped Sonic Boom Demonstration]] (SSBD) aircraft to test it. SSBD used an [[F-5 Freedom Fighter]]. The F-5E was modified with a highly refined shape which lengthened the nose to that of the F-5F model. The [[aircraft fairing|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]], 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.
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| As a follow-on to SSBD, in 2006 a [[NASA]]-[[Gulfstream Aerospace]] team tested the [[Quiet Spike]] on NASA-Dryden's F-15B aircraft 836. The [[Quiet Spike]] 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]] testbed, aircraft 837.
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| There are theoretical designs that do not appear to create sonic booms at all, such as the [[Busemann's Biplane]]. However, creating a shockwave is inescapable if they generate aerodynamic lift.<ref name=seebass/>
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| ==Perception and noise==
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| 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]]s have heard, when the aircraft is nearby the sonic boom is a sharper "bang" or "crack". The sound is much like that of [[fireworks]] used for [[firework display|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 around Mach 1. But we don't hear the sonic boom or anything like that. That's rather like the wake of a ship - it's behind us.".<ref>[http://news.bbc.co.uk/2/hi/talking_point/3207470.stm BBC News interview with former Concorde Pilot (2003)]</ref>
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| In 1964, NASA and the [[Federal Aviation Administration]] 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.
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| Sonic booms were also a nuisance in North Cornwall and North Devon as these areas were underneath the flight path of Concorde. Windows would rattle and in some cases the "torching" (pointing underneath roof slates) would be dislodged with the vibration.
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| 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 one's fingers in which the "perceived" sound is nothing more than an annoyance.
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| The energy range of sonic boom is concentrated in the 0.1–100 [[hertz]] [[frequency range]] that is considerably below that of subsonic aircraft, [[Gunshot|gunfire]] and most [[industrial noise]]. 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 {{convert|1|smi|km}} for each {{convert|1000|ft|m}} of altitude (the width is about five times the altitude); that is, an aircraft flying supersonic at {{convert|30000|ft|m}} will create a lateral boom spread of about {{convert|30|mi|km}}. 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.<ref name="Fact Sheet" /> | |
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| 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 {{convert|30000|ft|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 {{convert|30000|ft|m}} is about {{convert|670|mph|km/h}}, but an aircraft must travel at least {{convert|750|mph|km/h}} (Mach 1.12, where Mach 1 equals the speed of sound) for a boom to be heard on the ground.<ref name="Fact Sheet" />
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| The composition of the atmosphere is also a factor. Temperature variations, [[humidity]], [[atmospheric pollution]], and [[wind]]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]], [[Road surface|pavement]], and large buildings can cause reflections which may amplify the sound of a sonic boom. Similarly [[grass]]y fields and lots of [[foliage]] can help attenuate the strength of the overpressure of a sonic boom.
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| 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.
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| ==Health impact==
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| {{Further|United States Navy in Vieques, Puerto Rico#Sonic booms}}
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| Some studies claim to show that sonic booms from U.S. Navy testing in Vieques, [[Puerto Rico]], increased the incidence of vibroacoustic disease, a thickening of heart tissue. However, other scientists dispute the claims.
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| ==Bullwhip==
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| [[File:Bullwhip.jpg|right|thumb|150px|An Australian bullwhip.]]
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| The cracking sound a [[bullwhip]] 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.<ref>[http://www.americanscientist.org/issues/pub/2002/9/crackin-good-mathematics Mike May, ''Crackin' Good Mathematics'', American Scientist, Volume 90, Number 5, 2002]</ref> The whip is probably the first human invention to break the [[sound barrier]].
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| 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]] <math>E_k = {mv^2}/{2}</math>, 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.
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| ==Marine biology==
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| The pistol shrimp can create [[sonoluminescence|sonoluminescent]] cavitation bubbles that reach up to {{convert|5000|K|C|-2}},<ref>{{cite journal |author=D. Lohse, B. Schmitz & M. Versluis |title=Snapping shrimp make flashing bubbles |journal=[[Nature (journal)|Nature]] |volume=413 |issue=6855 |year=2001 |pages=477–478 |doi=10.1038/35097152 |url=http://www.nature.com/nature/journal/v413/n6855/abs/413477a0.html |pmid=11586346}}</ref> which are as loud as 218 decibels; breaking the sound barrier.<ref>{{cite web|url=http://www.dailymail.co.uk/sciencetech/article-1085398/Deadly-pistol-shrimp-stuns-prey-sound-loud-Concorde-UK-waters.html|title=Deadly pistol shrimp that stuns prey with sound as loud as Concorde found in UK waters|author=David Derbyshire}}</ref>
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| ==See also==
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| *[[Cherenkov radiation]]
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| *[[Hypersonic]]
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| ==References==
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| {{Commons category|Sonic boom}}
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| {{Reflist}}
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| {{Use dmy dates|date=September 2010}}
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| {{DEFAULTSORT:Sonic Boom}}
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| [[Category:Sound]]
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| [[Category:Aerospace engineering]]
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| [[Category:Aerodynamics]]
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| [[Category:Shock waves]]
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