This maximizes both the magnitude and the "rise time" of the shock, which makes it seem louder. 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. 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 due to the air's acceleration over the top of a curved wing). 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. The nose shockwave compresses and pulls the air along with the aircraft so that the aircraft behind its shockwave sees subsonic airflow. Longer aircraft therefore "spread out" their booms more than smaller ones, which leads to a less powerful boom. The "length" of the boom from front to back is dependent on the length of the aircraft, although to a factor of 3:2 not 1:1. At very high speeds and altitudes the cone does not intersect the ground, and no boom will be heard. As the aircraft increases speed the shocks grow "tighter" around the craft, and do not become much "louder". 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. In close range to the tunnel exit this phenomenon can causes disturbances to residents. In contrast to the (super)sonic boom of an aircraft, this "tunnel boom" is caused by a rapid change of subsonic flow (due to the sudden narrowing of the surrounding space) rather than by a shock wave. When a high speed train enters a tunnel, the sonic boom effect occurs at the tunnel exit. In order to reduce the sonic boom effect, a special shape of the train car and a widened opening of the tunnel entrance is necessary. In this case the fuselage reuses some displacement of the wings.Ī sonic boom or "tunnel boom" can also be caused by high-speed trains in tunnels (e.g. Whitcomb area rule states, we can reuse air displacement without generating additional shock waves. 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. 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. When maneuvering the pressure distribution changes into different forms, with a characteristic U-wave shape. This leads to a distinctive "double boom" from supersonic aircraft. We experience the "boom" when there is a sudden increase 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 "overpressure profile" is known as the N-wave due to its shape. There is a sudden increase in pressure at the nose, decreasing steadily to a negative pressure at the tail, where it suddenly returns to normal. In smooth flight, the shock wave starts at the nose of the aircraft and ends at the tail. This critical speed is known as Mach 1 and is 1,225 km/h (761 mph) at sea level. 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. Cause of sonic booms Īs an object moves through the air it creates a series of pressure waves in front and behind it, similar to the bow and stern waves created by a boat.
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