The air reacts as a fluid to supersonic objects. As these objects travel through the air, the molecules are pushed aside with great force and this forms a shock wave, the sonic boom. The same phenomenon is seen with a boat creating a wake in the water. The bigger and heavier the plane, the more air it displaces.
The cause of the sonic boom
The shock wave forms a ‘cone’ of pressurised or accumulated air molecules, which move outwards and backwards in all directions and extend down to the ground. When this cone spreads across the landscape along the flight path, it creates a continuous sonic boom along the entire width of the base of the cone. The rate at which pressure is released, after it has been accumulated by the shockwave, is known as the ‘sonic boom’.
The change in air pressure associated with a sonic boom is only a few grams per square meter, about the same change in pressure experienced when driving a two- or three-story lift.
It is the rate of change, the sudden change in pressure, that makes the sonic boom audible.
All aircraft generate two cones, at the nose and at the tail. They are usually of similar strength. The time interval between the two when they reach the ground depends mainly on the size of the aircraft and its altitude. While some people on the ground may perceive the sound as a single sonic ‘boom’, many sonic booms produced by NASA research flights are easily heard as distinct. This type is typical of the space shuttle
There are several factors that can affect sonic booms: weight, size and shape of the aircraft or vehicle, as well as its altitude, attitude and flight path, and weather or atmospheric conditions. A larger and heavier aircraft has to move more air and create more lift to sustain flight, compared to a small and light aircraft. Therefore, they will create louder and noisier sonic booms than smaller and lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves.
Altitude effect and sonic boom “carpet”
Altitude determines the distance the shockwaves travel before reaching the ground, and this has a significant effect on the intensity. As the shock cone widens and moves outwards and downwards, its force is reduced. Generally, the higher the aircraft, the greater the distance the shockwave has to travel, reducing the intensity of the sonic boom.
The width of the “carpet” behind the aircraft is approximately 1.5 kilometres for every 300 metres of altitude. The maximum intensity for conventional supersonic aircraft is below the aircraft. It decreases as the lateral distance from the flight path increases, until it ceases to exist. The lateral spread of the sonic boom depends on altitude, speed and atmosphere and is independent of the shape, size and weight of the vehicle.
Size, speed and atmosphere
As described above, the size and weight of the aircraft influence sonic booms. The ratio of aircraft length to maximum cross-sectional area also influences the intensity of the sonic boom. The longer and thinner the aircraft, the weaker the shock waves. The larger and blunter the vehicle, the stronger the shock wave can be.
Meanwhile, an increase in speed above Mach 1.3 leads to only small changes in the strength of the shock waves. The direction of travel and strength of the shock waves are influenced by wind, speed and direction, as well as air temperature and pressure. At speeds slightly above Mach 1, their effect can be significant, but their influence is reduced at speeds above Mach 1.3.
Distortions in the shape of sonic boom signatures can also be influenced by local air turbulence near the ground. This will also cause variations in the overpressure levels. Aircraft manoeuvres can cause distortions in the shock wave patterns. Some manoeuvres, such as pushovers, acceleration and S-curves, can amplify the intensity of the shock wave. Hills, valleys and other terrain features can create multiple reflections of shock waves and can also affect intensity.