Some thoughts on sound...
 

I was thinking about the fact that there's no sound in space, as there's no medium (air) for it to pass through. Which led me to ask what it was that produced the sound in the first place - and my answer was 'movement'. Can anyone confirm this...?

So a moving body imparts a bunch of energy onto it's surrounding environment, part of which is sound. Now, say a space craft imparts 100 units of energy, 10 units of which are sound (as the craft goes through the atmosphere and into orbit), and the other 90 units as whatever.

So the craft is in space now (free from the earths atmosphere), I would expect it would still impart 100 units of energy to the imediate environment. But it obviously isn't going to be sound... I thought initially it could be heat, but there's nothing to conduct it too...

So where does the energy go?
Does the craft itself absorb it in some way?
Does it even produce the energy...?

Turbofan

P.S. Amazing the things you think of when you're trying to get to sleep...

Turbofan
Sir Isaac Newton's three laws of motion should explain this. Also it's patently bleeding obvious :-}

The momentum of an object is constant unless an outside force acts on that obejct.

If the spacecraft is in the atmosphere, it requires thrust to maintain it's velocity or to accelerate. Just like an aircraft, in steady level flight, thrust = drag

In space....no one can hear you scream. And of course a body will continue in a straight line and at a constant velocity unless outside forces, (gravity, asteroids, green people) influence it.

I know what you're saying. It will still produce the same amount of energy, and the vibration of the airframe that would have produced sound will clearly not result in a vibration travelling through the surrounding medium (since there is none). This is not particularly significant to the aircraft, because all this would do in air is damp the vibration in the airframe *slightly*, while the rest of the energy would be converted to heat in the process of fatiguing the airframe. I would conclude then that in space, the rest of the energy that would have produced sound would be converted to heat and dissipated by E-M radiation.

Pie

[This message has been edited by Pielander (edited 27 June 2001).]

Strewth!!

How did you guys ever graduate from Starfleet academy?

Space is empty. A vaccuum. There's nothing there. It is a void. Zilch. Nothing. Devoid of matter. Lacking substance. Vacant, Unfilled. Bare. Blank. Unoccupied. It is bleedin.....space.

Therefore there is no friction. No drag. Nothing to conduct sound or vibration. Nothing to absorb energy.

If it is producing thrust it will accelerate. If it isn't it will continue in a straight line until it is influenced by an external force. (see above)

Bally Heck

We're not arguing with Sir Isaac. I realise how a momentum balance works, but the original question was nothing to do with that. It was about what happens to the proportion of 'waste energy' which, within the atmosphere, would normally be dissipated as sound. (From a jet/rocket engine [delete where inappropriate], that is - not from some sort of wind-rush noise, which I realise will be non-existent.)

My answer is that the vibration is damped by internal cyclic stresses of the structure, converted to heat, and radiated out into space.

Please explain to me how this conveys a lack of understanding of Newtonian physics.

Pie

[This message has been edited by Pielander (edited 27 June 2001).]

Ah. Er. Um.

Sorry Pielander. I'm with you now. Yep I guess internal vibration would be damped out by friction in the structure of the vessel which would be dissipated in the form of radiated heat. This is also what happens in the flying machines which you earthlings use.

RTFQ as my old drill sergeant used to say to me. I hope Jean Luc didn't see this posting.

So you're saying that the heat is dissipated in some form of radiation?

I can't quite see it, and am not sure I've understood exactly what you're saying... :rolleyes: :)

Turbofan

Nearly all of the noise from a spacecraft is produced in the vortex sheet between the rocket eflux and the ambient air. This is predominantly a quadripole source of noise from the fluctuating stresses in the vortices.
In space there is no vortex generation and the gases continue in uniform motion, any turbulence source of noise would dissipate in the expanding gas. The acoustic energy is vastly smaller than the kinetic energy of the gas and has no noticeable effect on performance.
Noise propogating through vibrating panels is of little significance, and in space there would just be a change in the panels impedance and the energy would be reflected back and dissipated internally.

Turbofan.
We now reach the first law of thermodynamics which roughly states that the energy in a system remains constant but that the form of the energy may be transformed.

The total energy of the system equals the internal energy, which depends only on the thermodynamic state, plus the kinetic energy, which depends on the system's motion, plus the potential energy, which depends on the system's position with respect to the chosen coordinate frame.

Kinetic energy (ie. vibration) may be converted to heat energy. Strike a piece of metal on an anvil for a few minutes and it's temperature will rise. Likewise a bicycle pumps motion produces heat due to the compression of the air.

Thus the vibration of your spacecraft will convert into a temperature increase as molecules and components move. Next time you destroy a credit card, bend it repeatedly and see how it gets warm at the fold.

If there is a heat gradient between the spacecraft and space. ie one is hot and the other cold, this heat energy will radiate from the spacecraft into space in the same way as it radiates from the bar of an electric fire. The rate of radiation is proportional to the square of the difference in temperatures, so a very hot body will begin cooling very quickly but the rate of cooling will reduce as the temperature difference reduces.

Having said all that....the thing would have to be shaking like a dance floor during an earthquake to produce appreciable amountsd of heat.

That's my boy!

Could I just add to that that any body above absolute zero will radiate heat at a calculable rate, independent of its surroundings. The rate at which heat is radiated depends only on its absolute temperature and the properties of its outer surface. (i.e. it will still radiate heat even if it's the coldest thing around - it just means that it will absorb faster than it radiates until it reaches an equilibrium state).

[This message has been edited by Admiral James T. Kirk (edited 28 June 2001).]

But if you're in space there's nothing to radiate the heat to...

I understand the bar-heater analogy, but there's air present as the medium that transfers the heat.

Surely the heat would stay with the craft...?
Although it doesn't make sense to me that it would stay heated forever, so... how exactly does this heat transfer into nothing/space?


Geez I'm lovin' this!

Turbofan,
heat can be transferred in a number of ways. Convection (air), conduction or... yup, radiation in the EM spectrum. If heat could not be transferred throught empty space through radiation we wouldn't have much use of that sun thing, would we? *grin*

The heat isn't radiated TO anything per se, just radiated. The probabilities dictate that most likely it will hit something or other eventually, otherwise it will just continue forever, helping to expand our universe at the speed of light. :)

Now I'll go back to trying to figure out what a "quadripole source of noise" is. *grin* I'm impressed.

Cheers,
/ft

*slaps forehead*

Thanks ft, I can't believe I didn't think of that! It's like a slap in the face... :)

Opens the door to yet more questions though, but I'll ponder those for a while yet.

Cheers.

A discussion on entropy anyone?

Good discussion, some here may enjoy this:

http://www.secondlaw.com/

Look guys I only got a 3rd but this is really not that subtle.

Sound is simply a disturbance on the fluid medium in which the disturbance is generated. No medium; no sound.

Look at it this way. You descend over the Pacific gently. Everything is fine until you get to 0 amsl. Then the wings fall off.

Why?

Why didn't they fall off before?

Now reverse the argument. You create a sound in the dense medium known as air but as you climb to 3000000000 feet. No sound.

Actually of course Space is not a vacuum so there will be sound. It's just that it is so minimal you don't notice.

What about the engines vibrating? Well they will propagate sound through the vessel but it won't leave the vessel because the vibrations have no surrounding medium to vibrate. Does this mean anything? No. The damping effect of the air on the airframe is negligible by comparison.

Twistedenginestarter.

You fell into the same trap as I did.

RTFQ

twisted,
in addition, above a certain altitude the air will start acting as separate particles rather than a fluid. I e sound waves will not propagate - no sound. They ran into this with some high altitude X series aircraft IIRC...

[Edit] Oh, and this of course meant that, for once, the "lift is created by air molecules bouncing off the bottom of the wing like marbles" finally got to be right. Once. :)

Nothing is ever simple! If it is, it ain't aviation! :)

Cheers,
/ft

[This message has been edited by ft (edited 30 June 2001).]

what what what is all this about..... Sound is simply, very simply brought about by changes in pressure. The rate at which you vary the pressure will determine the frequency of the sound you here. Sound waves are (on this earth) waves of changing air pressure. In space - no air - no sound.

However, when in space firing an engine, matter is diplaced from the engine. If you are behind this engine you will probably hear it. However the sound waves will be waves of the combustion products and not air.

Relevant??

------------------
Doors to Automatic

If a tree falls in the middle of the forest with no one around to hear it-does it make a noise?!?
:) :) :)

Yes, unless it's in a vacuum.

Pielander

Typing error I guess. You obviously meant to say even if it is in a vacuum.

A spaceship flying through space will make a lot of noise. Noise doesn't need air. It is a vibration of any medium.

I think this is what was nagging Turbofan. He was thinking all this commotion - where is it going to? And the answer is of course all the noise generated by smashing into the atmosphere isn't being generated (so no problem there) whilst the noise of the engines etc is propogating around the vessel but not transferring into the medium surrounding the vessel.

Touche.

Although, I have made the point before about the vibration propagating through the vessel/tree and been dissipated as heat. Again, it comes down to the definition of the word noise. Is it a noise unless it is being ransmitted through a fluid? Probably, but I hadn't thouht that far.

Also, what would a tree be doing in a vacuum? I would have to say it wouldn't be in a vacuum to fall down in the first place.

Pie

OK, the picture clears yet more...
Sooo...

Hang on, I need to think a little more.

[please pause here for a few minutes]

So basically the energy that's passed to the air (to create the pressure waves) is non-existent in space. That's the general concensus I'm getting.
OK, well let me put up another hurdle (unless I've missed something obvious, which is very likely at this time of night...). The craft has say 100 units of energy, and loses 10 to the air/sound/pressure waves. So does it just simply hold those 10 units when it's in space? (As say Kinetic Energy for example.) I would asume it would.

Comments...

Turbofan

It's actually much more complicated and much simpler at the the same time, than all of the above.
The reason that aircraft fly is a result of the sound that is produced in the pursuit of entropy. Early aircraft were very loud for not much load or speed, the DoX for example had sixteen sound engines for about 100kts flat out with a dozen pax, but fortunately aerodynamic engineers have worked out how to get a much more efficient use of sound over the years. The new wide chord fans are a great example of this efficiency gain.
The bigger the aircraft the greater level, as distinct from volume of sound is required to "shake" itself between the air molecules and the ground thus producing what we quaintly call 'lift'.
It also explains why the higher the altitude the less actual noise is required, there are less air molecules to pry apart.
See.
Ever noticed how it's actually quite hard to "hear" an aircraft flying overhead at FL350 from the ground.
It is also why you don't see your average space shuttle running its engine as it orbits, no, or very few air molecules, to pry apart, so no sound required.

Just think of the proportion of time taken up in our training for dealing with the eventuality that it becomes quieter than normal or even :rolleyes: totally silent.
Ergo it's all to do with the amount of sound.
Less or no sound is a pilots worst nightmare. :)
Turbofan, there 'tis, simple really, I hope this makes you sleep a little easier. http://www.pprune.org/ubb/NonCGI/cool.gif

hey turbofan, if I gaff on your head, in space, would you hear it......I think not, but by jingoes you'd smell it

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Turbofan......I have no clue???

gaunty:

Eh? Wot u been smokin'?

Sound is not what causes aeroplanes to fly. The principles of aerodynamic flight don't apply in space anyway. The noise comes mostly from the engines, either in the air of out of it, and the noise itself is caused by turbulent flow, whether it be over the wing, in the exhaust stream, or in the engine itself.

The main reaqson why you can't hear an aircraft at FL420 is because it's 8 miles away.

Pie

Very funny, Gaunty!

LOL!

Hey Pielander, I think he just may be taking the p!ss... :) I hope it's that, 'cos Gaunty, you've lost me mate...

Keep in mind folks that this isn't really about sound, it's about energy transference. (I hope that's a word - yes it is, I just looked it up http://www.pprune.org/ubb/NonCGI/cool.gif )

[This message has been edited by Turbofan (edited 06 July 2001).]

[This message has been edited by Turbofan (edited 06 July 2001).]

I hope so too, but alas, it's so difficult to tell these days.

Pielander
Sorry was just taking the p!ss.

But it is true, that when it does go quieter than it should be, the look on the PICs face is a treat.
http://www.stopstart.fsnet.co.uk/smilie/shocker.gif

Which explains how hovercraft, choppers and the Harrier fly - nothing going over the wings, but a bloody great amount of noise jostling those molecules!

Tangential, but it may be youse guys could answer a vexed question which has long been the subject of debate in my place of work, to wit, what is the speed of sound on Mars? (surface level). No, we couldn't find out how many grammes per cubic metre either. Not very many though.

Hi All,

"In space, nobody can hear you scream"... but you can get a cracking suntan.

Let's see if I've understood all this banter:

A rocket is whizzing about space at full throttle. 90% of the energy dissipates in a cloud of nasty, hot, smelly, propulsive particles kicked out at a zillion MPH to keep Newton happy. Stay away from the blunt end, then, huh? However, 10% of the energy boings around the aeroshell (smart arse tech term) causing it to vibrate like an AC-DC concert stage.

Can't hear much of either the vibes, or the propulsion, 'cos the vacuum is unable to conduct sound waves. Also 'cos no one is daft enough to be anywhere near the sodding thing. The wibbly-wobbly panels continue to vibrate like buggery, but ultimately get damped by all those molecules tap dancing together. Wobbling molecules generate heat energy causing said aeroshell to heat up.

Said heat doesn't have the nous to leap into the hot, propulsive gases, so they're not going to make much of a contribution to the going forward thing. So the heat says 'sod it' and simply radiates (slowly) in all directions into the vacuum via the electro-magnetic spectrum whatsit. Contrary to what the Hoover Company says, vacuums propagate photons nicely, but are useless at getting dust out of a corner.

Seeing as the rocket, vacuum, gases, electro-magnetic radiation, Coca-Cola logo, and the astronaut with a suntan, are all within the same Universe, then energy is fully conserved, so we can all sleep at night.

TW

I cant believe you are all so ignorant of the facts.

Its all very simple.

When the craft is in space there is no sound that can be heard externally because there is no air to conduct it. However, if the craft is using its engine to acclerate a tiny fraction of the energy of the exhaust can by conducted back through the nozzle, vibrating the craft and manifest as a faint noise inside the craft where there is (hopefully) some air. Most spacecraft do not run thier engines in space unless they are accelerating or changing orbit. This is because there is no drag (no air) and so the craft can maintain speed almost indefinitely without using any power (conservation of energy). Thus a satellite can remain in space for years without using its rocket motors other than for minor corrections.

Its also seems there is a serious misunderstanding here about heat energy of a spacecraft. The craft can dissipate some energy as heat by infra-red radiation which requires no air as it is an electromagnetic form of energy. However, (to contradist Bally Heck) it cannot conduct heat to its surroundings or convect heat whcich requires the presence of air. Incidentally, Bally Heck mentins the difference between the temperature of space and the craft. What are you on about. As a vacuum cannot contain any heat by defintion, there can be no conduction due to a difference in temperature. Radiation on the other hand is not affected by the difference in temperature between the two mediums, only by the radiating body's temerature.

Is that all clear now?

I preferred my version.

OK, Mister Proach, so then you're saying 100% of what was sound energy is now electro-magnetic radiation...?

I guess you were, and it'll do me.
Cheers folks.

And now for something completely different.

Turbofans sleepless nights are shared by many with useful results.
We are now all more edumakated about matters rocketry.
And.
A fantastic example the application of this science to aviation, is the discovery of the use of the difference in speed of transmission of sound in different media in the investigation of aircraft accidents, specifically bombs or explosions.

Detailed frequency/amplitude analysis of the CVR tape of an aircraft brought down by a bomb showed a very small trace immediately before the sound of the explosion in the cockpit.
A very smart insomniac suspected that it may have been the vibration of the bomb "noise" through the airframe, which being solid transmitted the "noise" at a faster speed than the air.
By measuring the time difference beteween the two and using a little calculus it was possible to calculate the distance of the "noise" from the CVR.

Bingo!!! or Eureka if you are Greek, when they checked the results from this and other accidents it was confirmed to the very seat.

Awesome hey. :cool:

Mr Proach,

I feel I must beg to differ on a point or two. My posting mentioned neither conduction or convection, both of which require a medium to operate in.

Space is not a vaccuum. It is certainly a partial vaccuum but is still rich in matter.

If space contains no heat, then by definition it must be at absolute zero (0 degrees Kelvin) this is the theoretical temperature at which a perfect gas would cease to exist and is theoretical I believe isofar as it cannot be attained. (although scientists have been remarkably close)

I am working from memory here now but back in my engineering days we were taught that the quantity of heat radiated from a body was proportional to the square of the temperature diffference between the body and it's surroundings. Thus if a body is at a temperature of 1000C, and it's surroundings are at 1000C, no heat will be radiated. If the surroundings are close to absolute zero then it will radiate heat with great vigour and perspicacity. The surroundings will consequently increase in temperature. (See the first law of thermodynamics) and the rate of radiation will decrease.

Is that a doughnut.....or a meringue.
(best said with a Scottish accent) :cool: