The Science Behind Sonic Boom
When any aircraft travels through the air it continuously produces air-pressure waves similar to the waves seen behind a ship when its bow ploughs through the water. When an aircraft exceeds the speed of sound, known as supersonic speed or Mach I, at about 760 miles per hour measured at sea level, it continually generates shockwaves which drop as a trail of sonic boom along its flight path. In relation to the aircraft, sonic boom sweeps backwards, away from the aircraft, in a cone shape. A person on the ground will hear the sonic boom as an explosion, similar to a clap of thunder, when the shock waves on the edges of the cone cross his location.
When an aircraft is traveling in smooth flight at supersonic speed, the shock wave starts at the aircraft’s nose, and ends at the tail. The sudden rise in pressure at the nose of the aircraft decreases steadily to become a negative pressure at the tail, returning to normal after the aircraft passes. The supersonic boom produced in steady flight conditions is known as an N-wave because of its shape. The supersonic boom is created by a sudden change in air pressure, so an N-wave causes a double boom, one resulting from the initial pressure rise of the aircraft nose cutting through the air, and the second as a result of the pressure returning to normal when the tail passes. When an aircraft is carrying out maneuvers at supersonic speed, the air pressure distribution takes on a characteristic U-wave shape.
The power of the shock wave and resultant sonic boom is largely dependent on the quantity of air that is being displaced, which is in turn dependent on the size, weight and shape of the aircraft, as well as the speed at which it is traveling. Other factors influencing the volume at which a sonic boom is heard on the ground include temperature variations, pollution, humidity, wind and even the surface of the ground itself.
Currently, due mainly to the noise factor, supersonic flight is prohibited over several countries, including the United States. Ongoing research and development may lead to quieter supersonic flight, but until it can be guaranteed that that supersonic flight and its accompanying sonic boom will not unduly distress the communities they plan to fly over, it is unlikely that we will see any airplanes flying at supersonic speed over our urban neighborhoods.