SONIC BOOM

The term 'sonic boom' is commonly used to refer to the shocks caused by the supersonic flight of a military aircraft or passenger transports such as Concorde (Mach 2.03, no longer in service) and the Space Shuttle (up to Mach 27). Sonic booms generate enormous amounts of sound energy, sounding much like an explosion; typically the shock front may approach 167 megawatts per square meter, and may exceed 200 decibels. Thunder is a type of natural sonic boom, created by the rapid heating and expansion of air in a thunder storm.

Contents

Causes

Characteristics

Abatement

Perception and noise

Bullwhip

References

Causes

When an object passes through the air, it creates a series of pressure waves in front of it 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 approximately 1,225 kilometers per hour (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 rise 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 because of its shape. The "boom" is experienced when there is a sudden rise 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'.

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 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 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 the boom seem louder. On most designs the characteristic distance is about 40,000 feet (12,000 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.

Abatement

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 premise 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 (21,000m). 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 aircraft length to the power of 3/2 (W/L^3/2). 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 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 large supersonic aeroplanes 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-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, Boeing X-43) shock at a sharp, but wide angle nosecone, which will travel at slightly supersonic speed (bow shock), and using a swept back flying wing or an 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 40,000 feet 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 started the Quiet Supersonic Platform project and funded the Shaped Sonic Boom Demonstration (SSBD) aircraft to test it. SSBD used an 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 have reduced 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. It has large similarity with the scramjet and may be implemented by increasing the size of engine inlets and using a large bypass. Or the flow can be even shocked down to subsonic speeds (like in the MiG-21 to reduce the shock of the fuselage) and compressed by a big fan to achieve efficient thrust both at low subsonic speeds and supersonic speeds. It will produce a shock at nonzero lift and it does not seem a good idea for the wing tips though.

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 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.

Bullwhip

The cracking sound a bullwhip makes when properly wielded is, in fact, a sonic boom. The end of the whip, known as the ''"cracker"'', moves faster than the speed of sound, thus resulting in the sonic boom ^[1]. The whip was the first human invention to break the sound barrier.
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 for kinetic energy (), 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.

References

1. http://www.americanscientist.org/template/AssetDetail/assetid/17894