Sonic Booms

On the 16th of June 2013, Cambridgeshire, Essex and Hertfordshire Police and Fire services received numerous calls of a possible explosion. Residents of certain areas of East Anglia heard a loud detonation, reported windows shaking (in some cases smashing) and one tree was reported to have fallen. In my own house (near Cambridge), the windows shook and my family heard two loud explosions. These were later confirmed by the MOD to be a sonic boom caused by jets scrambling from RAF Coningsby in Lincolnshire when communication was lost with a Heathrow-bound plane.

What is a Sonic Boom?

A sonic boom is the sound associated with the shock waves created by an object traveling through the air faster than the speed of sound (768mph). Sonic booms generate enormous amounts of sound energy, sounding much like an explosion.

When an object, in this case a jet, passes through the air it creates a series of pressure waves in front of it and behind it, caused by the object pushing the air out of the way. This is similar to the bow and stern waves created by a boat. These waves travel at the speed of sound. As the speed of the object increases and approaches the sound barrier, the waves are forced together, or compressed, because they cannot get out of the way of each other fast enough. Eventually they merge into a single shock wave, which travels at the speed of sound. The shock wave is released in a cone shape, a Mach cone, with the aircraft at its tip. The pressure inside the cone is not very different to normal air pressure. However the reason that the shock wave is audible is because the pressure is suddenly released resulting in a sound similar to an explosion.

Effects of a Sonic Boom

This shock wave is heard as a loud bang, and can be powerful enough to break windows. Two sonic booms are sometimes heard in the case of jets, as one is generated by the nose of the plane and one by the tail of the plane as it travels through the air. The two booms are often so close together that they are just perceived as one sound.

Besides from the loud band, sonic booms have a visible effect as well. This can be seen as a vapour cone that surrounds the plane, which occurs due pressure drops around the plane, causing the surrounding air to cool. Cooler air is less able to hold moisture, and so the moisture in the air condenses out and forms a cone shaped cloud behind the plane. Jets going at speeds close to the sound barrier can also cause light waves to bend due to pressure differentials. This is because as the air speed increases, air pressure and temperature drops causing a change in density which can be enough to bend light waves.

Factors Affecting a Sonic Booms

There are several factors that can influence sonic booms, including the weight, size, and shape of the object. For aircraft it also includes altitude, attitude and flight path, and weather or atmospheric conditions.

A larger and heavier aircraft displaces more air and create more lift to sustain flight, compared with small, light aircraft. Therefore, they will create sonic booms stronger and louder than those of smaller, lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves will be.

Altitude determines the distance shock waves travel before reaching the ground, which then affects the intensity of the sonic boom. As the Mach cone gets wider, its strength is reduced. The higher the aircraft, the greater the distance the shock wave must travel, reducing the intensity of the sonic boom.

The ratio of aircraft length to maximum cross sectional areaalso influences the intensity of the sonic boom. The longer and more slender the aircraft, the weaker the shock waves. The fatter and more blunt the vehicle, the stronger the shock wave can be (i.e. the more aerodynamic it is, the weaker the shock wave).

The direction of travel and strength of shock waves are influenced by wind, speed, and direction, and by air temperature and pressure.

Hard surfaces such as concrete, pavement, and large buildings can cause reflections, which may amplify the sound of a sonic boom. Similarly grassy fields and lots of foliage can help attenuate the strength of a sonic boom.

Other Objects that Cause Sonic Booms

Concorde

Concorde was able to break the sound barrier during its flights and was therefore able to cause a sonic boom. Sonic booms were a problem in North Cornwall and North Devon as these areas were underneath the flight path of Concorde. Windows would rattle and in some cases the “torching” (pointing underneath roof slates) would be dislodged with the vibration. It was proposed that Concorde, before it was decommissioned, could open up new flight paths to Australia. However the plans were scrapped due to the fact that few other countries would allow Concorde to fly at supersonic speeds due to sonic booms being disruptive and potentially damaging.

Space Shuttles

Space shuttles are able to break the sound barrier but can only create sonic booms whilst in Earth’s atmosphere. Due to its larger size and mass, two sonic booms (one for the nose and one for the tail) can be easily heard as opposed to jet planes.

Bull Whip

The end of a whip is able to break the sound barrier and so produces a sonic boom. We hear this as the cracking sound the whip makes. This was probably the firs human invention to break the sound barrier.

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