Sonic boom

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 2.03, no longer flying) 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.

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, because of 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.

Cause

As an object moves 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. Supersonic shockless engine.PNG|thumb|right|A nacelle around the engine reflects any shock waves. A spike behind the engine converts them into thrust.]] Supersonic shockless fuselage.PNG|thumb|right|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 air displacement can be reused 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 (such as the Japanese Shinkansen). In order to reduce the sonic boom effect, a special shape of the traincar 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 (from the sudden narrowing of the surrounding space) rather than by a shock wave. Close to the tunnel exit this phenomenon can cause 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 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.

The nose shockwave compresses and pulls the air along with the aircraft so that the aircraft behind its shockwave is in 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, 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 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 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 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 feet to 60,000 feet (from 12,000 m to 18,000 m), which is where most SST aircraft 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 (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. 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 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.

In late October 2005, Israel used F-16 jet planes to create sonic booms over the Gaza Strip 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, especially those firing Qassam rockets into Israeli population centers. [http://www.frontpagemag.com/Articles/ReadArticle.asp?ID=20236

Media

These videos include jets achieving supersonic speeds.

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

Sound | Aerodynamics | Noise pollution