Distinction between shockwaves and sound waves

McLaren Rulez

Hi,

I would like to know about what exactly constitutes a shockwave. Let me write out what I understand of it. There are two explanations I have come across.

1) The shockwave is caused when gas particles move faster than the speed of sound due to some external factor (a plane, an explosion or so). Let's try the plane. When it starts to go faster than the speed of sound, there is a buildup of pressure around the front of the plane (and a corresponding drop around the rear). This outward shockwave travels faster than the speed of sound because the plane pushes the particles faster than the speed of sound. Wikipedia, among others, calls this the shockwave here.


2) The shockwave is just a superposition of regular sound waves. When the plane travels faster than sound, the wavefronts can interfere constructively. This IS the shockwave according to some explanations. Wikipedia's explanation for the sonic boom is that it is a shockwave and then it describes this thing above. You can see that here

I have two questions:

a) Which one is the "shockwave", 1) or 2)?

b) Also, going back to 1), after that region of high pressure gets away from the plane, I believe it propagates as particles moving faster than sound until it degenerates into a normal sound wave. For an observer, will he then hear the wave described in 1), then silence, then the superposed waves described in 2) since the wave in 1) gets ahead due to its superior speed?

Andrew Mason

1 describes the shock wave. Despite what the second article suggests, the sonic boom and the shock wave are different phenomena. The shock wave is not a sound wave. The sonic boom is behind the shockwave and is a compression wave (ie sound). The airplane makes sound but because the source of the sound is moving faster than the speed of the sound wave, if you are on the ground you get a whole lot of crests of the sound wave piling up as the plane passes over (and after the sharp crack of the shock wave).

AM

sophiecentaur

This forum has a lot of posts about 'classification' and I think people worry too much about it.
I think the term 'shock wave' mainly refers to how it was generated, rather than the wave structure itself. If you can hear it, then it must be a 'sound wave'. There isn't a lot of point in losing sleep whilst trying to classify such things. In any serious discussion which involves either or both terms, the context should make clear what is being discussed.
You get the same thing with X rays and Gamma radiation. They can both have exactly the same frequency but Gamma radiation is sourced from nuclear decay whilst X rays tend to be caused by other mechanisms (e.g. electrical discharge). A detector can't tell the difference.

Andrew Mason

I think there are a lot of people who would disagree with this statement. There are fundamental differences. If a shock wave was a sound wave it would travel at the speed of sound. It doesn't. It travels at the speed of the source. The reason we can hear a shock wave has more to do with the way our ears work. If I record the crack of a rifle bullet going past my ear and play it back, I will get a sound wave. But that sound wave is not a shock wave. It is not possible to reproduce that original bullet wave structure from a recording.

AM

sophiecentaur

OK, but that's when (and where) it's being generated, surely? Dunno. By the time you get to hear it, it's sound but, as you say, when it's right in front of an aircraft it's going supersonic.
Once it's been shed off the front, doesn't it travel outwards at sonic speed, though, the angle of the V being due to the ratio of the aircraft to the speed of sound?

Description 2, would seem to be wrong if the definition involving faster than sound is right. By the time 2. applies, the wave is a short (sound) wave train, travelling at sonic speed which can be analysed in terms of a series of continuous waves.

In the case of an explosion, though, I completely take your point and I imagine the shock wave will gradually slow down till the pressure inside the sphere approaches Atmospheric, at such a point one could say it was a normal sound wave as it's not being 'driven forward'.

Andrew Mason

If a shock wave hits our eardrum, it is travelling at supersonic speed. The reason we hear a sound is because it causes our ear drum to move.

The shock wave moves forward from the nose of the object (eg. airplane, bullet) as a supersonic straight line. The perpendicular distance from the shock wave back to the path of the source will increase at the speed of sound. But so long as the shock wave forms a straight line from the nose of the source, it will travel with the same speed as source. The wave front eventually loses energy and speed and starts curving until it reaches the speed of sound and becomes a sound wave.

AM

sophiecentaur

This is interesting. You learn sunnink every day guv. I understand that the front of the V goes supersonic, but where is there a 'static' pressure difference (higher temperature plus explosive gases), as with an actual explosion, which could propel the bulk of air so that the wave would go faster than sonic towards the side of an aircraft? (Perhaps that is not a relevant remark?) Wiki talk of non-linear behaviour of the medium, which makes sense under extreme pressure. I think the supersonic portion of the wave produced by an aircraft would have to be really quite small. Wiki says that the shock wave dissipates realtively quickly but I can't actually find numbers for it. With an expanding cone, I imagine the energy flux drops off with an 1/x law. There are pictures on one site showing disc-like clouds of condensation, about twice the dimension of an aircraft, revealing the actual shock wave. From that, I deduce that you'd need to be very close to get the shock wave rather than just the sound.
The animations, showing those increasing circles don't make it very clear to me. I guess animations have their limits.

Andrew Mason

This is a very good point. The shock wave in air will not extend that far from the source. You have to be very close to the bullet path to hear the supersonic crack of the bullet (which arrives when the bullet does, ahead of the sound of the muzzle blast). What you hear from the supersonic airplane is not the supersonic shockwave but an accumulation of sound wave fronts hitting your ears at the same time. I too would be interested to know how far the shockwave extends from the nose of the source. It would depend on the speed of the source and the density of the air etc.

AM

McLaren Rulez

Thank you for your replies. I think it was important to figure out which was which because I was studying Cerenkov radiation. And nearly every description of that uses the sonic boom as an analogy.

As it turns out, there is no shockwave in Cerenkov radiation. Just the case of 2). I guess this is one of those places where Wikipedia isn't 100% accurate.

Thanks for clearing that up for me!

AlephZero

That depends on the size of the source. When out of doors in some quiet parts of the UK countryside, you sometimes hear a quiet "whump whump" from the shock wave of a military fast jet flying at high altitude - too high to see it or hear any conventional engine noise. The sound is quite distinctive from anything like a distant shotgun. Those shockwaves must have travelled several miles to ground level.

On a different note (bad pun intended) the reason for the characteristic "bright" sound of brass instruments like trumpets and trombones has been shnown to be shock waves generated within the instrument (and FWIW brass players have claimed this was the case for years before anybody proved it was true). Again, these can travel a long way compared to the size of the instrument that generates them.

The paper: http://alexandria.tue.nl/repository/...les/617406.pdf
A slow motion video of the shockwave: http://www.bbc.co.uk/news/science-environment-13574197

sophiecentaur

Just because you can hear it doesn't mean it is still a shock wave. It may have started as a shock wave but change in speed and dispersion may still produce a characteristic 'sound' of the original shock wave. If I were to record that sound and then replay it to you, would you say the what came out of the loudspeaker was also a shock wave?

I think the statement in wiki that the shock wave only survives over a relatively short distance is very reasonable and doesn't conflict with experience.

Also, the reference to non-linear behaviour makes sense. Once the pressure levels become excessive, the modulus of a gas will become higher (non-ideal and, in the limit, the gas would be incompressible), which would, of course, increase the speed of the wave. Below this level, you would expect the speed to become sonic.