Sonic boom
An aircraft travelling through the atmosphere produces air-pressure waves similar to a ship's bow-wave. When the aircraft exceeds the speed of sound, these waves combine to form shock-waves. The sonic boom is the sudden onset and release of pressure after shock-wave build-up, or "peak overpressure". There are two types of pressure-waves known as N-waves and U-waves. The N-shaped pressure-wave is produced by the coalescing of shock-waves from several parts of the aircraft (nose, canopy, inlet, wing, tail etc), and a second shock at the rear of the aircraft. The two vertical portions of the "N" are shock-waves, so generally a double boom is heard. The U-wave, or focused boom, is generated from manoeuvring during flight, and has a pressure wave shaped like a 'U'.
Peak overpressure
This varies from less than 4.9kg/m² (1lb/ft2) to around 49kg/m2 for an N-wave boom, and for U-wave booms can be up to two to five times as large. The value for Concorde is 6.9kg/m2 at Mach 2, while the estimated target for a Mach 1.6 business jet is 2kg/m2. The energy of the sonic boom is focused in the 0.1-100Hz range, and the duration varies from less than 0.1s for the average fighter to around 0.5s for Concorde. The speed of sound is affected by air temperature, a decrease or increase in which results in a corresponding change in sound velocity. At 30,000ft (9,150m) the speed of sound is around 590kt.
Source: Flight International
