Meteor striker over Russia

[Eric Mc]

I am pretty sure that the multiple bangs were caused by the fact that the object was breaking up and that individual smaller pieces were created their own, smaller, sonic booms. The secondary bangs are clear in some of the footage on youtube.

[Simpo Two]

PW said:

You know sound is a shock wave...

But more extreme - do waves from an explosion travel faster than waves from someone talking? It's not a constant like the speed of light.

[FurtiveFreddy]

Simpo Two said:

But more extreme - do waves from an explosion travel faster than waves from someone talking? It's not a constant like the speed of light.

The speed of sound waves can vary depending on the density of the medium the waves are travelling through.

Shock waves from ground-based explosions or earthquakes, for instance, behave differently because they travel through the earth, which is denser than air, but the speed will still vary depending on what type of solid the wave is propagating through.

The shock wave from the meteor exploding would travel near enough at the speed of sound because it's propagating through the atmosphere, which is gaseous.

Apart from anything, the video clips show the shock wave arriving at the same time as the sound waves, which is different to clips you may have seen of a large ground-based explosion, where the shock wave can often be seen travelling across the ground and arriving at the viewing point before the air-borne sound waves do.

So, the sound waves from an explosion and someone talking should travel at the same speed, but the shockwave of the explosion through the ground probably will travel faster.

Edited by FurtiveFreddy on Monday 4th March 10:42

[hairykrishna]

PW said:

Simpo Two said:

do waves from an explosion travel faster than waves from someone talking? It's not a constant like the speed of light.

The speed of sound varies depending on specific medium it is travelling through, but is a fundamental property of that medium - knowing the medium you know the speed of sound.

If the speed of sound was not constant, but dependant on the source of the sound, there would be no such concept as "the speed of sound".

So yes, the sound from an explosion travels at the exact same speed as the sound of someone talking.

It is a constant, just like the speed of light. (Which also depends on the specific medium it is travelling through).

That's true for sound waves but not for shock waves. A shock wave is traveling faster than the speed of sound in the medium.

[hairykrishna]

FurtiveFreddy said:

The shock wave from the meteor exploding would travel near enough at the speed of sound because it's propagating through the atmosphere, which is gaseous.

Shock waves can propagate through the atmosphere at many times the speed of sound.

The asteroid, along with it's bow shockwave, was traveling at >10km/s ~15km up. That's something like mach 60.

[hairykrishna]

PW said:

Think about it - if the shockwave travelled faster than the speed of sound you wouldn't get sonic booms, you would get a "silent boom" as the hypothetical hypersonic pressure wave hit you, followed by the sound later.

Nope. Firstly, you can hear a sonic boom even though it's not a sound wave - it's still a area of increased pressure, it just doesn't travel in the same way. Secondly, what you normally hear as a 'sonic boom' isn't often a shock wave. It's the sound waves that the shockwave has ended up as.

If shock waves only travel at the speed of sound why do you hear a sonic boom at roughly the same time a supersonic plane passes by? If a plane's doing mach 2 and its shockwave was only doing mach 1 it'd quickly leave it a long way behind.

[hairykrishna]

What you perceive as sound is a change in pressure. A shockwave is a sudden change in pressure but it doesn't travel in the same way as a sound wave. It's a different thing.

Judging by the tone of your post you're convinced I'm wrong so there's little point in arguing with you. Try actually reading the wiki page you grabbed your earlier quote from.

[Carfolio]

But what if you were standing on a conveyor belt going at the speed of sound?

[hairykrishna]

PW said:

hairykrishna said:

Judging by the tone of your post you're convinced I'm wrong so there's little point in arguing with you.

I expect nothing less on PH.

Why not try and educate me, rather than arrogantly flouncing off because it's beneath you to discuss science in a science forum?

I'm not an expert, I'm willing to be corrected, but what you've posted makes no sense to me.

hairykrishna said:

sound is a change in pressure. A shockwave is a sudden change in pressure but it doesn't travel in the same way as a sound wave.

That's contradictory.

A is a pressure wave and can only travel at speed S
B is also a pressure wave, but can travel any speed it likes because... "it's different".

How does it travel in a different way?

It's "sudden" is your explanation - that doesn't mean anything.

2 objects at different mach speeds passing at the same distance - the shockwave/sound reaches you at different times or not?

I wasn't 'arrogantly flouncing off'. I was busy. You tried to make your point by quoting from the wikipedia article about shockwaves. That same article contained all of the information required for you to educate yourself.

It's not contradictory. They are different. A sound wave is a compression wave that propagates at the local speed of sound and leaves the state of the medium unchanged i.e. it's adiabatic (assuming ideal gas to make things simpler). A shockwave front on the other hand is like a thin (~1 mfp thick) traveling layer of highly compressed gas. Wiki diagram;



It's very close to a step discontinuity in density and pressure - it's non-linear. It changes the state of the medium in a thermodynamically irreversible way, generally leaving the gas it passes through hotter and at a higher pressure.

For a more straightforward argument, here's a picture of some bow shockwaves;



These shockwaves travel with the object. The objects are traveling in excess of the speed of sound. Hence the shockwave is traveling faster than the speed of sound. No? Hence my question earlier about why 'sonic booms' arrive at roughly the same time as the aircraft producing them - why don't they get left far behind if shockwaves travel at mach 1?


If two objects were traveling at different velocities >mach 1 and pass at the same distance then yes, I think, the sound produced by the shockwaves would arrive at different times. As I pointed out earlier the 'sonic boom' is not normally the shockwave, it's the sound wave that the shockwave has turned into. The sounds would therefore probably arrive very close together because the shockwave doesn't propagate all that far. A better demonstration would be someone with a high power rifle firing bullets close to you, from a very long way away. You would hear the 'crack' from the supersonic bullet before you heard the sound of the rifle firing.

Edited by hairykrishna on Tuesday 5th March 16:17

[Simpo Two]

I thought sound and shock were two different things but couldn't express why. Ta.

[hairykrishna]

No probs.

Here's an article with some simple explanations of shock waves and some seriously impressive photos;

http://www.mne.psu.edu/psgdl/Pubs/2006-Settles-AS....
Settles, G. S., "High-speed imaging of shock waves, explosions and gunshots," American Scientist, 94(1):22-31, 2006.

The joys of enormous amounts of 'homeland security' funding!

[Simpo Two]

PW said:

The "bang and blast" arrived together because the bang WAS the "shockwave", having dissipated into a sound wave, QED it travelled at the speed of sound.

Now, from the tone of your post it clearly isn't worth replying. Oh st, I mean, "I'm off for dinner".

I thought Hairykrishna had explained it to you. Why then return to something posted before, and throw a strop as well?

[Simpo Two]

You claim to know everything, but when challenged no-one is worthy of your infinite knowledge; you prefer to flounce off.

The lack of any qualifications in your profile is hardly an encouragement. Perhaps if it said 'Professor of Physics' you might have more clout, but as it is please go and strut your boundless superiority somewhere else because it's spoiling the thread for everyone else.

[hairykrishna]

PW said:

It seems that this is a case of confused terminology then - the shock wave being a discreet entity at the source that quickly dissipates into a normal pressure wave that affects the wider environment - which is what was described in the original context of the thread...

Just because the shockwaves from planes degenerate very quickly into normal sound waves it doesn't follow that all shockwaves do. How far it travels as a shockwave is largely dependent on the initial energy. From very large explosions like nukes this can be many miles. Things are complicated in airbursts low enough for the shockwave to hit the ground because you also get a reflected wave from the ground.

Various sources suggest the initial big explosion was 90 kilotons. I'd guess that's enough for the shock wave to travel a few miles at least. It might be further - some of the aftermath photos certainly look like buildings that have been hit by a proper shock wave.

[hornet]

One point I heard discussed that I hadn't considered previously - what sort of threat was this event to aircraft? Granted, it was over in 16 seconds and the odds of an aircraft being near at the time were presumably remote, but for the given blast, could the shockwave have taken down planes?