# Supersonic wave in a solid

Gold Member
Is it possible?. I just know it is possible in liquids and gases, but what would be the (structural) effect of a supersonic wave in a solid?. And how can we generate it?

## Answers and Replies

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HallsofIvy
Homework Helper
Is it possible to have a supersonic wave in air? Isn't the speed of sound, by definition, the natural wave speed in whatever medium?

russ_watters
Mentor
You mean a shock wave? Hit a nail with a hammer and a shock wave travels through the nail at the speed of sound.

Supersonic? Not sure what you mean. Objects travel through air at supersonic speeds, but like Halls said, shock waves travel at the speed of sound.

Gold Member
Yea, I am seeing what are you trying to tell me. When I wrote this thread I was thinking in something similar to what occurs in a fluid. But you have reason when you said waves travel at sound speed by definition. So the new situation would be a nail being hit and going inside the solid at supersonic speed. I mean it goes with a velocity faster than the sound one of the solid. It would be similar to a supersonic wing inside a fluid. Does it exists a shock wave inside the solid, Mach cone...etc?.

LURCH
Supersonic waves are generated by high explosives. In fact, I believe that is the definition in between "explosive" and "high explosives". I suppose the shock waves used to pierce armored vehicles or implode condemned buildings could be considered "a wave moving at supersonic speed through a solid", since the force of this pressure wave travels through the metal at a speed greater than the speed of sound through that same metal.

russ_watters
Mentor
LURCH said:
Supersonic waves are generated by high explosives. In fact, I believe that is the definition in between "explosive" and "high explosives". I suppose the shock waves used to pierce armored vehicles or implode condemned buildings could be considered "a wave moving at supersonic speed through a solid", since the force of this pressure wave travels through the metal at a speed greater than the speed of sound through that same metal.
I'm sure I musta learned that at one time. I gotta get my arms around it again.

I guess unlike a regular shock wave, a supersonic one would have a magnitude that is so high the elasticity of the material can't absorb the energy, causing permanent deformation on a molecular level.
So the new situation would be a nail being hit and going inside the solid at supersonic speed. I mean it goes with a velocity faster than the sound one of the solid. It would be similar to a supersonic wing inside a fluid. Does it exists a shock wave inside the solid, Mach cone...etc?.
The nail is analogous to the plane - the shock wave doesn't travel through the plane. I guess a hammer hitting a nail at supersonic speed would simply crush the nail a la Lurch's explanation of high explosives.

More on the definitions: http://www.brainyencyclopedia.com/e...rial.html#Classification by type of explosion

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Moe
By the way, you can actually move faster than light in certain materials. Electrons do that in a nuclear reactor, and emit Cerenkov radiation (that beautiful blue glow you see in pictures).

Gold Member
russ_watters said:
The nail is analogous to the plane - the shock wave doesn't travel through the plane. I guess a hammer hitting a nail at supersonic speed would simply crush the nail a la Lurch's explanation of high explosives.

I've never said the supersonic wave travels trough the plane. I am wondering what occurs with atoms and molecules in the solid that suffers the impact. Sure it suffers plastic deformations, but the question is how would react a solid's particle near of the interface nail-solid when the nail goes trough the solid at a supersonic speed.

Moe said something curious about light velocity. I have never understood (in fluids and solids) why is it said "some parts of the flow field does not suffer any effect due to external boundary conditions (i.e. pressure) if the flow is supersonic. The information has not enough time to alert them of the existence of this boundary conditions". But I wonder, Would not be the light velocity the ultimate information speed also with elastic waves?.

Moe
The trick here is that the speed of light in materials is less than the speed of light in vacuum ($$c_v$$). Nothing can travel faster than $$c_v$$.
The thing with the information not being transmitted faster than light is derived from the same equations. Under certain conditions, the wave speed can be faster than $$c_v$$, whereas the speed at which information is moving will remain at or below c. I can look it up again,if you're interested.

I think that everythime you hit a nail in a wall, the sound battier is broken in the wall, and normal sound of the shockwave is heard by you. This is because sound travels faster in solids. Vsolids>Vliquid>Vgas. This is because on the molecules and their closeness. Therefore, you can break the speed of sound in a solid, simply by emmiting a sound at it. Structurally, there would be no effect, since the molecules of the solid are so tightly packed in and bond energies are high.

Moe
Why would a sound emitted at a solid result in a supersonic wave inside that solid? I couldn't quite follow your reasoning there.

Just a question here...as far as I've read, I don't understand why there exists such a thing as a supersonic wave. I've only heard of supersonic sources, which create shockwaves in fluids, and make a cone-shaped wave front. But, what does that have to do with the wave? The wave still travels at it's normal speed, even in the case of Cheknov radiation, as Moe suggested.
Furthermore...how on Earth do you travel in a solid? Drill through it with a handdrill like thingy? Or melt it down with high temperatures? Whatever the case, it sounds weird.

Nereid
Staff Emeritus
Gold Member
Asteroid impacts on Moon at relative speed of 10 km/sec (or 30, or 100) ... speed of sound in rocks and solid metals ~<5 km/sec. Shock wave travels through the Moonrock (and asteroid rock/ice/metal), at pressures up to 100 GPa near the point of impact (pressures drop considerably further away from that 'point', consider it to be like the propogation of a spherical wave). Youngs modulus (or is it the rigidity modulus?), even for the strongest materials, isn't much above 100 GPa ... result?

Treated classically - and there's really no need for QM or relativity - what set of equations govern these interactions?

BTW, there's a very interesting reason why little if any of the asteroid remains in its original solid form, even it is composed of iron; anyone care to guess why?

Moe
Maybe due to heat? It melts and is scattered?

the reason the sonic wave speeds up is because it travels faster in the solid. I should have said that you emmit a sound from a gas towards a solid. Lets say the sound travels at 300m/s. When the sound hits the solid, the molecules start vibrationg with same oscilations as the gas molecules, but the gas molecules are a lot further appart, making the sound wave go faster in the solid since its molecules are so tightly packed in.

oh, and the asteroid thing, is it because at the pressure of impact, there is enough energy to break metallic bonds, rendering the solid metal into tiny little fragments. Thats my guess.

Moe
the reason the sonic wave speeds up is because it travels faster in the solid. I should have said that you emmit a sound from a gas towards a solid. Lets say the sound travels at 300m/s. When the sound hits the solid, the molecules start vibrationg with same oscilations as the gas molecules, but the gas molecules are a lot further appart, making the sound wave go faster in the solid since its molecules are so tightly packed in.
Well, ok, but that wouldn't make it supersonic, would it?

That wouldn't be supersonic, in a sense. Supersonic, if I'm not mistaken, means "over the sound of speed in that certain medium".
A sound wave in the solid would be faster than the sound wave in air, but that has nothing to do with being supersonic, since the medium is no longer air anyway.

Gold Member
Working out

Well guys, I was just referring to some proyectile penetrating supersonically (i.e. v>>c(solid)) into a solid. What is the difference with an aeroplane flying supersonically in the air?. In both mediums there will be elastic or plastic deformations transported via waves. The problem here is: will a solid's particle be deformed if the source of deformation (proyectile) goes faster than elastic wave speed?. Surely it will be deformed, but not in the classical way. My question is if all physical process involved with discontinuities (shock waves) in fluids are extensible for solids. (I'm sure it will be with another equations).

The other question was: what is the role of light velocity in the propagation of deformations?. We always talk about sound velocity, but the real question is: Is it not curious that light velocity does not appear in the Wave Equation? (for elastic waves in a solid).

HallsofIvy
Homework Helper
Clausius2 said:
Well guys, I was just referring to some proyectile penetrating supersonically (i.e. v>>c(solid)) into a solid. What is the difference with an aeroplane flying supersonically in the air?.
If that was what you were referring to, it certainly was NOT what you said!
You specifically asked about a wave traveling faster than the speed of sound in the solid. That is impossible by definition. It is, of course, possible for an object to move through a solid faster than the speed of sound (i.e. the natural wave speed) in that solid.

In both mediums there will be elastic or plastic deformations transported via waves. The problem here is: will a solid's particle be deformed if the source of deformation (proyectile) goes faster than elastic wave speed?. Surely it will be deformed, but not in the classical way. My question is if all physical process involved with discontinuities (shock waves) in fluids are extensible for solids. (I'm sure it will be with another equations).
Yes, of course. There is a more extensive literature on shock waves (which travel at the speed of sound) in solids than in air.

The other question was: what is the role of light velocity in the propagation of deformations?. We always talk about sound velocity, but the real question is: Is it not curious that light velocity does not appear in the Wave Equation? (for elastic waves in a solid).
Because "sound velocity" is DEFINED as the speed of propagation of deformations. I don't see why you would think that light velocity would have anything to do with it. As for "light velocity does not appear in the Wave Equation", the natural velocity of waves in the solid appears in the wave equation- that is, by definition, the "speed of sound" in that solid. The speed of light is the natural speed of electromagnetic waves in vacuum and has nothing to do with wave in solids.

Gold Member
HallsofIvy said:
If that was what you were referring to, it certainly was NOT what you said!
You specifically asked about a wave traveling faster than the speed of sound in the solid. That is impossible by definition. It is, of course, possible for an object to move through a solid faster than the speed of sound (i.e. the natural wave speed) in that solid.

I don't see why you would think that light velocity would have anything to do with it. The speed of light is the natural speed of electromagnetic waves in vacuum and has nothing to do with wave in solids.

I think it was solved yet that a supersonic wave is impossible (or this is what i've understood). You arrived too late at this discussion I mean.

You don't see what the light velocity of the solid has to do with elastic propagation. But I have just said I think is the ultimate vehicle of information transport. I know this sounds so bad, but perhaps I am wrong. Are we talking about vibration model inside the solid so that it has nothing to do with photons or some particles that travels near light speed?. I'm just not a physics, so those who know (or aparently knows) you have permission to kick me if you want. :rofl:

Moe
Well, at some point you have virtual photons, gluons and all the other stuff interacting between the molecules, the individual atoms, within those atoms... But they aren't really of importance if you are just interested in the propagation of the shock wave.

If you want to calculate the travel time of a car moving from city A to city B, you don't take the speed of light into account, even though it certainly plays a part in your problem, right? It would make everything extremely complicated and then lead to the same results. You could of course start with, say, Newton and Maxwell and then derive the shock wave equations. You'd run across c every now and then, but eventually it would either cancel out or the assorted terms would be so small that you can safely ignore them.

In solids the velocity of sound is not unique, for example a shear wave will travel at a different velocity (higher) than a pure compression wave. As a matter of fact flexural waves do not have a well defined velocity as they are dispersive ... the frequency of the wave changes with displacement. Furthermore in two and three dimensional waves in solids the propagation area changes with distance so that a characteristic impedance cannot be defined and the wavelelength (but not the frequency) change during propagation. $$\therefore$$ you will need to define the type of wave in solid before you can ask if a faster wave is possible.