Just because this seems to be a topic that a lot of people have misconceptions about
What a sonic boom is:
Whenever an object moves through air, it will displace some of that air. This results in a change of pressure. You can observe this by seeing its effects, such as leaves being dragged behind a bus or being pushed out of the way in front of it. You can sometimes hear it, too. When something passes by, it may be accompanied with the “woosh” sound of air being moved.
When moving at subsonic speeds, the air in front of an object starts getting pushed out of the way before the object reaches it. The pressure that builds in front of it is transmitted forward before the object reaches the air. Thus, instead of a single pressure wave, there is a gradual increase in pressure. However, there is a limit to how fast the air can transmit pressure changes and that limit is the speed of sound. Sound, after all, is really just a change in air pressure. When one molecule in the air is pushed on, it pushes the one next to it, transmitting this change. The speed at which this occurs depends on altitude, humidity and temperature. At sea level, it comes out to being about 340 meters per second or about 770 miles per hour. It is lower at higher altitudes.
When an object, such as an aircraft, approaches the speed of sound, the waves of pressure become compressed into a smaller area. This is because the air can not transmit the pressure change very far in front of the aircraft before it reaches that location. As an aircraft passes about 75% of the speed of sound, this effect becomes noticeable and begins to affect the control of the aircraft. As it comes even closer to the speed of sound, it enters what is called the “transonic region.“ Early aircraft, such as fighters during the Second World War were not designed to maintain control and stability when faced by this pile up of air pressure. On occasion they entered the transonic region in dives and when this happened, they often lost control and crashed.
When the moving body, such as an aircraft, reaches the speed of sound, the pressure change can no longer be transmitted ahead of the aircraft at all. Now it is moving as fast or faster than the gas molecules can be pushed out of the way. There is no longer a pile-up of pressure waves, but instead, they all merge into one pressure wave, which travels with the aircraft. The air goes from uncompressed to fully compressed almost instantaneously, as the aircraft passes.
In fact, there are two pressure waves, one from the front of the aircraft and one from the rear. Depending on the design, there may be other, smaller waves generated by points in the aircraft. These pressure waves are the sonic boom. They are close enough together that the double wave may only be perceived as a single boom.
The boom is shaped like a cone. It’s continuous and travels with the aircraft as long as it is moving at supersonic speeds. All observers along the aircraft’s path experience the boom. For example, if an aircraft flew at supersonic speed from California to New York, observers in California would hear a boom, followed by observers in Nevada then observers in Utah and so on. Each would hear it after the aircraft passed over them.
A good illustration of how a sonic boom works comes from the design of the “Quiet spike” an extension to aircraft developed by Gulfstream aviation. The idea behind the Quiet spike is to decrease the sonic boom by pushing some of the air out of the way before the main body of the aircraft arrives. This way, it behaves more like a subsonic aircraft. Rather than having a single pressure wave, a more gradual increase in pressure occurs.
The Quiet Spike does not entirely eliminate the sonic boom, but it does reduce the intensity. It is hoped that a design that incorporates the quiet spike and also uses a gradually tapered aircraft body and other “boom shaping” design features can significantly reduce the sonic boom effect from supersonic aircraft.
The effects of a sonic boom:
In TV shoes and movies, sonic booms are often shown as being extremely violent, smashing windows and even knocking people to the ground. This is not necessarily the case. The intensity of the boom depends on the size of the body that creates it, its speed and its distance from observers. It also is reduced by the thinner atmosphere at high altitudes, which does not transmit the pressure wave as effectively as air can at sea level.
Rifle bullets produce a sonic boom, but this sonic boom is relatively small and therefore is only very loud to those nearby. Bull whips can also produce a sonic boom, which is what the “crack” that can be heard actually is. Again, the effect is pretty modest.
Aircraft moving at supersonic speed naturally produce a much greater effect. If an aircraft is large enough and moving at supersonic speeds at a relatively low altitude, it can indeed produce a pressure wave that is capable of causing ground-level damage. Broken windows and other such damage generally do not occur when aircraft are flying at altitude. However, there have been incidents where extensive damage has occurred from low level supersonic flights. In general, most buildings will not suffer any structural damage, even from a very low level sonic boom, although broken glass is possible.
Last year, a pair of Brazilian fighters buzzed the capital city at low altitude and high speed as part of an annual ceremony. It was an impressive display for onlookers, but the aircraft generated a sonic boom that smashed every pane of glass in the facade of the Supreme Federal Court building. It remains unclear whether this was due to the aircraft unintentionally exceeding their planned speed or simply not realizing the potential for damage from high speed low altitude passes. In either case… oops.
The damage from an aircraft traveling at supersonic speed and at low levels is unlikely to be severe enough to make it a viable weapon of war, but it has been used in hostile areas as a show of force or to startle the enemy. Israel has been accused of using sonic booms as a way of antagonizing Palestinian communities. The strongest sonic boom to which humans have been exposed was produced during a test flight, in which an F-4 Phantom exceeded the speed of sound just one hundred feet above the ground. This resulted in very strong overpressure, but researchers, who were directly exposed to the boom, suffered no injury.
The effects of a sonic boom from an aircraft traveling at tens of thousands of feet is much less dramatic. It can still be heard at ground level and may even be felt or produce some shaking, but the boom would not produce any substantial damage. It is primarily just an issue of noise.
In the 1950’s and 1960’s, it was hoped that issues related to sonic booms could be avoided by simply flying at a sufficiently high altitude. Unfortunately, even at very high altitudes, the boom is still quite perceptible at ground level. This was discovered during initial tests of the XB-70, which produced noticeable sonic booms even when flying above 70,000 feet. It is part of the reason that American projects to produce a supersonic airliner were canceled.
The boom from a large supersonic aircraft can be described as being similar to a loud thunder clap, although it is one single boom and not the rolling, booming sound one associated with thunder. It is not unlike the sound of fireworks or a cannon.
The objection to sonic booms and the reason that supersonic overflight is not permitted over most populated areas is simply due to the fact that the population finds them annoying and obtrusive. It really has nothing to do with damage on the ground or potential for injuries. In the early days of planning for super sonic transports, it had been hoped that the public would simply grow accustomed to the sound of sonic booms and that they would not be a major nuisance. Unfortunately, this did not happen.
In 1964 the United States Air Force and the Federal Aviation Administration undertook a test program to determine how a metropolitan area would react to regular sonic booms. Oklahoma City was chosen and for a period of six months a variety of US Air Force aircraft overflew the city, multiple times per day, at supersonic speeds. Initially, the tests were taken in stride and most residents of the city were not bothered by being part of the experiment. However, as time went on, many became highly intolerant of the booms. In the end, 73% of the population said they could live with sonic booms indefinitely with no problems. However, a small percentage filled complaints or sued.
Despite the fact that nobody was injured and claims of damage were limited to a handful of broken windows (which may or may not actually have been caused by the sonic booms) the Oklahoma tests were heavily criticized and used as an example of the government unethically subjecting the population to torturous testing. A huge amount of negative publicity was generated. Ultimately, the Oklahoma tests lead to the United States and many other countries completely restricting supersonic overflight of land, except for a few isolated military test areas.
A similar problem occurred in the early days the Concorde. Early flights included supersonic speeds over a portion of the UK, which included North Cornwall and North Devon. Although the actual damage from the booms was extremely light, being limited to a few roof slates, dislodged by the vibrations, the public did not take well to the sonic booms. The area was dubbed “boom alley” and residents quickly began filing lawsuits and complaints.
Ultimately, complaints over the sonic booms lead to the Concorde being restricted to supersonic flight over open ocean only. This is one of the major reasons that the Concorde program was a financial failure, selling only fourteen aircraft. Since the Concorde is designed especially for supersonic flight, it is not a very efficient aircraft at subsonic speeds. Its great advantage in travel time is diminished when it must fly a route that takes it over large areas of land. It is therefore totally unsuitable for any service except that between coastal cities. Airlines do not like aircraft with such poor flexibility, and since the Concorde was relatively expensive and of nominal capacity to begin with, forcing it to fly at full speed over water only completely killed any hope of wide adoption.
The public seems to be far more bothered by sonic booms than of other noises of an equal level. Loud airplanes are more likely to be tolerated when the sound comes as a constant drone that fades in and out. Car doors slamming or other everyday sounds may be much louder at the distances they occur from observers, but the sudden, low frequency boom of supersonic travel and the associated reverberations are something that many in the population just won’t stand for.
Until a truly quiet supersonic aircraft is produced, which generates no perceptible sonic boom at ground level, or very little perceptible sonic boom, it is unlikely that supersonic transport will be viable. There are designs that have been proposed and are being tested, yet no full size quiet supersonic aircraft has yet been built.
Some common myths about sonic booms and supersonic travel:
- There is a “sound barrier” that was broken or is broken by aircraft - There really is no “barrier.” It is simply an issue of whether an aircraft is designed in a manner that makes it capable of maintaining stability and control as it approaches and exceeds the speed of sound. Early aircraft did not have swept wings, had large props and were generally not aerodynamically compatible with such speeds. This belief may be the result of an early misunderstanding of the Prandtl–Glauert singularity, which would predict pressures to become infinite at the speed of sound. However, the Prandtl–Glauert transformation does not apply at such speeds, so the prediction of infinite pressure is false.
- A sonic boom is experienced as a violent phenomenon on board the aircraft – If flying at supersonic speeds, a passenger on the aircraft would have no indication of the sonic boom other than the airspeed indicator. The plane travels with the boom, so it is perfectly smooth.
- An aircraft flying at supersonic speeds creates a violent and damaging sonic boom – As stated above, this is not usually the case. At low altitude, it may break large plates of glass, but at higher altitudes, even this will not happen. It will not produce any physical injury to humans, although it is loud enough to cause some hearing damage, although usually not from one incident. At high altitudes, it will produce no danger of damage to hearing. At worst, it is an annoyance.
- A visible cloud of condensation indicates that an aircraft has gone supersonic – The pressure generated by a fast-moving aircraft can compress the air enough to force moisture to condense, producing a visible cloud. However, this does not really have anything to do with supersonic travel. Such clouds do sometimes occur at or near Mach-1, but they can occur at lower speeds too. It depends on atmospheric conditions.
- There is a sudden change in flight characteristics at Mach 1 – There are differences in the flight characteristics of supersonic versus subsonic flight, but it does not happen all at once. This is largely due to the fact that as an aircraft approaches the speed of sound, portions of the aircraft will experience supersonic air flow before the entire aircraft actually exceeds the speed of sound.
This entry was posted on Saturday, February 23rd, 2013 at 5:33 pm and is filed under Bad Science, Good Science, History, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.
View blog reactions