Why Does Sound Propagate?

[notauser]

3v0, I mentioned gravity because its responsible for the general pressure of the situation.. Apply a vacuum to that air mass, and those intermolecular forces don't do much.. Well thats not exactly true.. Under lower temperature/pressure when molecules get sufficiently adjacent those forces will be less impeded by stray kinetic energy, but they won't be adjacent nearly as often..
This chat seems more about fluid dynamics now than sound propagation.. I've spent MANY hours watching smoke float around a room, but it still baffles me..

 

[crashsite]

Speed vs. Temperature

Papers about temperature talk about how fast they move and time between collisions.

In a mass of air that is of a uniform temperature, are all the molecules moving at the same speed? If not, then what factors determine the speed at which they move?

Is the speed dependent on pressure? In other words, if the air is confined within a vessel and the temperature is changed (let's say, raised), the molecules will move faster and, since the air mass can't expand, I assume that the result is molecules traveling the same distance but, faster and thus striking each other harder, creating the higher pressure.

I know there are factors that complicate the answers but, for this level of things, simple averaging over time and allowing conditions to settle and ignoring the esoteric stuff, generally speaking what are the answers.

 

[notauser]

Palladyne, I don't think you explained that very well.. I'm not sure how right or wrong it is.. I didn't like that bit about repressurization though, I don't think that description fits..

Regarding your timer, start your own thread in a suitable forum, or harvest what you need from another thread.. Just don't hijack somebody else's thread unless its long since dead..

 

[notauser]

The simple gas law states PV=nRT.. That means pressure*volume=quanity_of_gas*(1.10*10^7)*Temperature..
n & R don't change, so if you increase the pressure, the temperature will go up to keep that equation balanced..
I know you don't like math, so I apologize, but here is everything you just asked:
Kinetic Temperature, Thermal Energy
Rydberg Constant -- from Eric Weisstein's World of Physics

 

[3v0]

@notauser (PV=nRT)

Exactly right. It takes energy to compress a gas (reduce the volumn). That machanical energy shows up as a increase in the temperature of the gas. If you expand the volumn of gas it get cooler.... and hey, we just invented the refrigerator! This should not be a mystery to anyone here.

@crashsite
In a mass of air. The temperature is due to the average speed of the molecules. Each has its own speed which changes with each collision. The speed of each molecule is determined by the quanity of kenitic energy it has at that instant.

@Palladyne
Mostly no.

 

[notauser]

I know, but I still don't exactly know what crashsite's background is, so if he's never covered the IGL, I figured I reiterate it..
I hope those probability distributions I posted in that link don't scare him off, I just wanted to illustrate that there isn't a particular particle velocity/energy at any temperature, just a predictable range..
And refrigerators take it beyond the IGL, refrigerants are designed such that they undergo state changes in the process which facilitates alot more energy transfer than if the refigerant remained a gas at all times.. I know most ppl here already know that, but something tells me eventually crashsite will want to know more..
I think its safe to say this is officially a discussion of fluid dynamics now.. EEEK!

 

[crashsite]

I know you don't like math, so I apologize, but here is everything you just asked:
Kinetic Temperature, Thermal Energy
Rydberg Constant -- from Eric Weisstein's World of Physics

It's not that I don't like math. I wish I was better at it. I just don't see the "answers" in it that some do. In Algebra I, in high school, I could always envision where the two trains were (approximately), from the description but, to this day, the equations the teacher wrote on the board are a mystery to me.

But, there are worse. A couple of hippie friends (okay, friend and acquaintence) were hanging a speaker on the wall of their apartment. It was easy to see that their mount (a couple of nails in the wall) was well behind the speaker's CG. But, even after they turned loose of the cabinet and it promptly fell to the floor, they still couldn't conceptualize why it happened. That's how I am with math. I just don't see the answer in the numbers (or the mechanism to manipulate the equations and formulas).

Actually, there are a lot more like me out there than like you. It's just that most of them don't take as much interest in the sorts of things that are generally defined by math.

Sadly, I need to beat these things down to the very lowest common denominator in order to understand them (which oft times pisses people off, royally...take the comment about this whole thread being, "pointless" as an example).

Okay, I've vented. Now back to making a nuisance of myself.

 

[crashsite]

Random vs. Predictable

I just wanted to illustrate that there isn't a particular particle velocity/energy at any temperature, just a predictable range.

Maybe it's premature for one of the $64 questions but, let me pose it.

If the air molecules are moving (and colliding with other molecules) at random rates, what makes the speed of sound so predictable for a given temperature? Must there be some completely unrelated and independent phenomena going on????

I probably shouldn't append this but, if air is a mixture of all the elemental and many of the compound gasses, why aren't there many speeds of sound in the medium of air instead of just one?

 

[crashsite]

Do I understand you correctly?

Your hands are a frequency generator too. When you add force to normal pressure you can create the clapping sound. I might posit that the frequency you want to generate is there already and whatever device you place in its path just "repressurizes" it, only to be taken in by whatever device you have available to receive its measurable rate,.e.g., your ear, which might measure 20hz-20,000 khz, your eye, which measures the higher visible light frequencies, or your scope... radio, etc. So, to answer the "why" part of this thread, one could say that you do something to convert the existing frequency into a measurable format and as long as that condition exists the propagation continues.

So, by that logic the piece of music that the orchestra plans to play tonight at Carneie Hall already exists? It's just necessary for the musicians to place their instruments into the air mass and the music will "repressurize" and become audible to the audience? Or will the musicians still have to painstakingly form each note and vibrato with the instruments?

 

[3v0]

And refrigerators take it beyond the IGL, refrigerants are designed such that they undergo state changes in the process which facilitates alot more energy transfer than if the refigerant remained a gas at all times.. I know most ppl here already know that, but something tells me eventually crashsite will want to know more..

I think its safe to say this is officially a discussion of fluid dynamics now.. EEEK!

A refrigeration device that attracted my attention in high school was the Vortex tube aka the Ranque-Hilsch vortex tube. No moving parts and they can be cascaded to produce cryogenic temperatures.
People do not agree on what makes it work. Could be interesting too look at.

 

[killivolt]

A refrigeration device that attracted my attention in high school was the Vortex tube aka the Ranque-Hilsch vortex tube. No moving parts and they can be cascaded to produce cryogenic temperatures.
People do not agree on what makes it work. Could be interesting too look at.

3v0, I swear you come up with some of the darnedest things.

Cool.


kv

 

[notauser]

I'm not criticizing you at all.. I know there are more like you out there.. I'd be criticizing hard though if you were supposed to learn this for school purposes, and refused to get aquainted with the math, but you aren't..

If the air molecules are moving (and colliding with other molecules) at random rates, what makes the speed of sound so predictable for a given temperature? Must there be some completely unrelated and independent phenomena going on????

I knew you were going to ask that, and the answer has to do with frequency and interference like my cordless phone analogy.. Its funny how energy prefers to react noticably with similar energy than it does with energy that isn't so similar.. The similarity I mean is the frequency at which packets arrive.. Again, wave interference illustrates the concept..
Reading about how a microwave heats water and fat etc but not the plate might help(the plate only gets hot because hot water/fat is on it).. The reason being is the plates molecules resist vibrating at the frequency the MW pumps out, so it rejects almost all of that energy..
Same thing happens in air.. Thermal energy typically causes molecules to vibrate in the MHz/GHz range, not in the kHz range and lower like sound.. Likewise, slow drifting molecules don't impede sound much at all because their frequency/energy is too low to react much with audible waves..
Take the fact that vastly different frequencies don't interact much further, and imagine a molecule having more than one vibrational frequency.. Imagine the high frequency thermal vibrations like the engine vibrations of a car, and imagine the lower sound like frequency vibration to be represented by driving forward then backwards once every 10s.. The car has two frequencies or more just like the air molecules..
As for the random particle action goes, consier this.. In a giant crowd of ppl you can't easily predict what one person will do, but the crowd as a whole is more predictable.. We play the numbers, and they work out..

 

[notauser]

3v0, did Tesla have anything to do with that fridge? Kind of reminds me of his turbine.. Usually when practical application baffles science Tesla's work isn't far away.. More than a few of my physics profs agree that there are still things in Tesla's notes/works, that only he's figured out entirely to this day..
I'll need to read up on that further when I have time to really take it to heart..

 

[crashsite]

This must be digested....

I knew you were going to ask that, and the answer has to do with frequency and interference like my cordless phone analogy.. Its funny how energy prefers to react noticably with similar energy than it does with energy that isn't so similar.. The similarity I mean is the frequency at which packets arrive.. Again, wave interference illustrates the concept..
Reading about how a microwave heats water and fat etc but not the plate might help(the plate only gets hot because hot water/fat is on it).. The reason being is the plates molecules resist vibrating at the frequency the MW pumps out, so it rejects almost all of that energy..
Same thing happens in air.. Thermal energy typically causes molecules to vibrate in the MHz/GHz range, not in the kHz range and lower like sound.. Likewise, slow drifting molecules don't impede sound much at all because their frequency/energy is too low to react much with audible waves..
Take the fact that vastly different frequencies don't interact much further, and imagine a molecule having more than one vibrational frequency.. Imagine the high frequency thermal vibrations like the engine vibrations of a car, and imagine the lower sound like frequency vibration to be represented by driving forward then backwards once every 10s.. The car has two frequencies or more just like the air molecules..
As for the random particle action goes, consier this.. In a giant crowd of ppl you can't easily predict what one person will do, but the crowd as a whole is more predictable.. We play the numbers, and they work out..

Well, I know that in some cases it's very true. Such as the quantum nature of the orgits of electrons and the interchange of energy between electrons and photons (like in pumping a LASER). Again, I know the basics but, not the intricacies.

Before turning loose of the us'ns vs. you'uns thing, there is a quote that has stuck with me for lo these many years. So, I dug out the source:

"Someone told me that each equation I included in the book would halve the sales. I therefore resolved not to have any equations at all. In the end, however, I did put in one equation, E=mc2. I hope that this will not scare off half of my potential readers."

That author is Stephen W. Hawking in the Acknowledgements of his book, A Brief Hist5ory of Time.

Sorry, not sure how to superscript the 2 (although I know it can be done in the message body).

 

[notauser]

Yea, but Stephen Hawking has done more to destroy physics these days than anything else.. His laymans explanations of everything often side-track actual students waay too much.. Its so sad to see ppl flunk out because they couldn't stop reading laymans explanations of cooler things than whats in the curriculum.. Catch-22 is that if you're a physics student, your goal should be to understand the things you want to understand without the layman analogies.. To actually understand physics at any advanced level you REALLY need to first build the toolsets that you need to actually understand physics.. Hawking (like myself in this thread) gives ppl a false sense of understanding when they read his works.. All you can do after reading Hawking, is repeat Hawking.. He doesn't tell you anything you need to know to actually learn..
Einstein once said "I never teach my students, I merely provide an environment in which they can learn..".. When Feynman was growing up he was such a nerd that he modelled everything around him mathematically, just for fun.. He was building the toolset that allowed him to contribute what he did..
Like I said earlier, there is a big difference between ppl like you, and ppl who think like you, yet believe they can hack university physics on conceptual analogies alone.. You aren't in denial, but so many are..

 

[3v0]

3v0, did Tesla have anything to do with that fridge?

I am a Tesla fan but no he did not.

I made a few visits to the Tesla museum in Colorado Springs when it was still in operation. They had a large tesla coil. Visitors were given flourescent bulbs to hold near the coil. The tubes glowed and it transmitted (or seemed to) energy into your body. It made one feel a bit better. A few old people came around now and again for relief from joint aches.

The vortex tube was invented in 1933 by French physicist Georges J. Ranque. German physicist Rudolf Hilsch improved the design and published a widely read paper in 1947 on the device, which he called a Wirbelrohr (literally, whirl pipe).

 

[notauser]

BTW, an easy covention for exponents in font is to write it as E=mc^2.. Usually works nicely unless your eq'n gets complex and needs brackets all over to separate terms.. Thats why I left the units out of the r in pv=nrt (The Rydberg constant is actually 1.10*10^7m^-1) (oh and m^-1 just means 'per metre').. Adding that would have made it waay more confusing..

 

[notauser]

Tesla coils are beyond neat, but I'd never build one.. Doesn't take very much error in design/construction to kill yourself or your friends..
I downloaded a great movie about Tesla's life a while ago.. Seemed pretty old actually, looked late 70's early 80's.. It went over everything with Westburn, Edison, JP Morgan etc..
Apparently Tesla and Westburn were the perfect team.. Who knows where we'd be today if it wasn't for those two other dicks..

 

[3v0]

Tesla coils are neat but the time and money it takes to build one far exceeds the cool factor. It was fun to play with one. My older brother built a vandegraff generator when I was 10. I may have gotten too much of it. Or perhaps it was the time he had me hold the spark plug wire..

Tesla coils are beyond neat, but I'd never build one.. Doesn't take very much error in design/construction to kill yourself or your friends..
I downloaded a great movie about Tesla's life a while ago.. Seemed pretty old actually, looked late 70's early 80's.. It went over everything with Westburn, Edison, JP Morgan etc..
Apparently Tesla and Westburn were the perfect team.. Who knows where we'd be today if it wasn't for those two other dicks..

 

[notauser]

Keep the Leyden Jars away from the Van deGraff, and you're pretty safe.. I'd love to see the VandeGraff generator that defies that rule though..