When an object moves through the air, the noise it creates spreads out in all directions, moving away in waves that travel at the speed of sound. But what happens when the object is moving faster than the speed of sound? In this case, the sound waves that are trying to move ahead get overtaken and create a shock wave that is dragged along behind the object. This causes what we call a sonic boom.
Jacopo Bertolotti has created a really nice animation that shows this. It’s available on wikipedia and on his own twitter animation collection (https://twitter.com/search?q=%23PhysicsFactlet). The animation is also shown below for convenience.
To understand this effect it is useful to think about what we mean when we talk about the speed of sound.
Sound is simply a form of kinetic energy that radiates from a source and travels as fast as the medium will allow. In air, the energy is transmitted by the movement of air particles that are a certain distance apart. The further apart the particles are, the longer it will take for the energy to be transferred from one particle to the next one. This means that at high altitudes (where the air is thinner and particles are further apart) sound energy will be transferred slower and the speed of sound will be less than at sea level. The same will be true if the air is very cold as there will be less random thermal movement to collide the particles together.
Moving faster than the speed of sound means pushing the particles aside, to make room for the advancing object, faster than the particles are normally able to move. This causes a region of compression that stores energy where the particles are much closer together than in surrounding areas.
The region of compression starts at the front of the object and stops at the back of the object. This means that there are actually two booms; one where the air starts being over-compressed and the other where the air relaxes back from its over-compressed state. The two boundaries form cones that are close together (which is why we usually hear only one sonic boom, not two). And of course it’s actually a continuous release of energy that just happens to sound like a single “boom” as the cone(s) pass the listener.
The angle of the cones depends on the object’s speed, with faster objects making more pointed cones that touch the ground much further behind the moving object. This means the object is long gone by the time its boom is heard on the ground.
You can’t see sound as such and the idea of “cones” probably sounds a bit strange so I’ve chosen a video clip that shows what we’re talking about: you can access the video on YouTube by clicking here.
There is also a great fact sheet about sonic booms from NASA. You can read it here.
As far as the GCSE Physics course is concerned, this is much more than you need to know about the behaviour of sound waves but I hope it helps to explain why the speed of sound has a fixed value (and what happens when an object moves faster).
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