Why Does Sound Propagate?

[3v0]

Presumably, they do this at some rate that's determined by their inertia and the nature of the elasticity (much like the executive toy has a natural speed of operation dependent on the mass of the balls and length of the strings)

It should come back to F=MA or A=F/M

As the " springs " between the molecules become more tense acceleration increases for any displacement.

I am a little uncomfortable with molecules at rest. Try not displaced by an outside force, or in Equiblium.

 

[notauser]

I know the matrix has pit-falls.. All analogies collapse at some point.. It would be better to picture the elastics like electric/magnetic attractions..
I'm going to go over that more thoroughly to see if I can iron out a medium you can understand..
One thing that jumped out at me was

For example, if air is compressed but, keeps the same temperature, the speed of sound remains the same..

Where did you read this.. Its not true at all.. I'm not sure exactly how the speed of sound varies with temperature compared to pressure, but if the temperature is the same, the speed of sound will be higher in the denser gas..
In the mean-time, this page does a good job explaining what I was trying to.. **broken link removed**

 

[notauser]

I am a little uncomfortable with molecules at rest. Try not displaced by an outside force, or in Equiblium.

Very good point, and well stated..

 

[notauser]

But, there are other effects that suggest that the air molecules are always in motion (and thus presumably continually interacting with their neighbors) due to, if nothing else, thermal effects.

Yea, they move with respect to each other, but an air mass as a whole, reacts to external forces as a whole.. Don't get me wrong, you can also analyze the movements of individual particles, but if you analyze the movement of alot of them you'll see definate patterns..
Besides, in physics you often have to imagine a system as a snapshot in time..
But actually air-masses are IMMENSELY complex.. Fluid dynamics is a GRUELING field.. The air mass as a whole will even have particle like properties, just like a baseball does..

 

[notauser]

For example, if air is compressed but, keeps the same temperature, the speed of sound remains the same...even though the molecules are closer together. By the same token, there's also something that makes the interaction between the molecules happen faster as the air gets hotter.

You know how I was saying an air mass can be thought of as a particle in some sense.. Well as it gets hotter, it acts less like a particle than when its cold.. Hot particles in an air mass have more of a mind of their own than cold ones.. In a way this increases the effective distance between molecules that are behaving themselves in the context of an air mass.. Heat induced motion is kind of like interference to sound induced motion.. The frequencies are vastly different though, so the interference isn't total.. (Remember temperature is a measure of the amount of molecular kinetic energy).
Kind of like how your 60Hz electricity don't interfere with your 2.4GHz phone at all noticably, but 2.45GHz microwave ovens and other phones may..

 

[notauser]

Actually, the sine function is one that I can understand pretty well (once I quit listening to the teacher's gobble-de-gook about adjacent and opposite sides and realized how it's directly related to the circle). I can envision how the molecules would tend to move in a sinusoidal fashion (just as I can envision something like a pendulum doing it) but, I'm not sure how that equates to the propagation of the sound.

My link explains how sine comes in, and explains the danger of confusing it.. The particles move in a longitudinal sense, not transversely like water.. But if you plot the instantaneous particle speed, energy, the pressure, and many other variables you'll see they vary sinusoidally..

 

[notauser]

What I did not quite understand was how the wave can move faster then the disturbance that set it off.

This is tricky to figure out for yourself, but I think you did when you started thinking springs..
When you swat at air at 5m/s the initial row of particles will only move that fast.. But as the wave progresses, the pressure builds up, and its the response to the pressure shift that propagates sound, not really the initial motion..
Free particles (fluids) respond to pressure 'kind of' like electrons respond to voltage..

Edit: Its that response that consitutes the elastic concept.. Particles at equilibrium are analogous to electrons vith no votage applied..

 

[3v0]

The key is that moving the first particle in the chain almost immediatly moves the second because they are connected by the spring. If the spring is so stiff it is not a spring then when you move the 1st you move them all. But they all have some spring to them and it is that amount of spring that determines how long it takes the particle at the other end of the chain to move, aka the speed of sound.

Another view on how the sound travels faster then the movement that creates it. If you push a mile long rod the movement shows up a mile away at the same time as you push it. It takes a second to push the rod... but the energy traveled 1 mile in that second. Fun huh Not air or sound but it demonstrates how energy can travel.

 

[notauser]

Actually that wave with the rod would only propagate at the speed of light (minus any mechanical compression slowdown)..
In a way you've got it, but in a way you're back tracking.. This is where the elastics get damaging to the concept.. The fact that the particles don't respond instantaneously is what allows the pressure buildup.. Kind of like piling a bunch of particles ontop of each other so they're compelled to slide away.. You create the compression and a bunch of particles find themselves too close, so they are forced to respond..
Its funny how lag, and imperfection are what make things actually work in some contexts..

Edit: Consider this.. The more particles you can get on that pile in the time you have before they start sliding (the period for a wave of that frequency) the louder the sound would be..

 

[3v0]

@notauser
I agree with what you are saying.

The rod example illustrated what happens when elasticity reaches one of its limits (none). At the other limit the particles have no effect on each other up to the time they collide. This limits the speed to that of the first particle.

The rod example is contrived but it does a good job of showing energy can travel faster then the actuator. The actual speed is meaningless since we did not send sound through the rod, we push it. But as the elasticity goes to zero the partices start to act more and more like the rod.

 

[crashsite]

Speed of sound vs. temperature/density

One thing that jumped out at me was

For example, if air is compressed but, keeps the same temperature, the speed of sound remains the same..

Where did you read this.. Its not true at all.. I'm not sure exactly how the speed of sound varies with temperature compared to pressure, but if the temperature is the same, the speed of sound will be higher in the denser gas..[/QUOTE]

Speed of sound - Wikipedia, the free encyclopedia

The last sentence in the first paragraph and just about any text on supersonic aircraft.

 

[notauser]

@3v0 I was being pedantic.. In the everyday world we disregard relativistic 'crap'.. Its only not crap when a system is so extreme those effects cannot be ignored..

 

[notauser]

Ahh ok.. The key word there is nearly.. Because pressure shifts on an atmospheric scale are not very extreme as compared to say compressing gas in a canister and letting it cool adiabatically..
In actuality the eq'ns for sound are over simplified.. Kind of like how Newton's gravity is over simplified.. Most important point though is that under normal conditions they're close enough by a long shot, and they save a HECK of alot of work..
For instance, you've probably heard that gravity accelerates at 9.8m/s^2, but I remember back in 1sy year university when they gave us this crazy eq'n that included elevation above sea level, and lattitude etc.. It took alot of lines to calculate g to 6 or so signifigant digits for our location..

 

[crashsite]

Baby Steps

Ahh ok.. The key word there is nearly.

Actually, I feel like this thread is making some progress. I do have a whole bunch of "tiny" questions to try to get a handle on just how the individual parts work. So, let me start:

Obviously, air molecules (lets stick with air for the time being) don't actually 'collide' as they move around. I guess I assume that, in the simplest example, as they get closer to their adjacent molecules, the repulsive effect of their respective electrons is what makes one molecule push on another. With the "elastic" nature of the air being that, as the molecules get closer, the electrons in their atoms apply more of that repulsive force and as they move further apart, less force.

Is that essentially true?

 

[notauser]

They will sometimes collide, other times not.. It depends on their relative position, its not a perfect grid..
And its not all about charge interaction.. Actually very little.. Large systems are funny the way they find equilibriums and want to stick with it.. These questions are quick approaching nitty gritty fluid mechanics aspects that I'm not that up on really..

 

[3v0]

At this point one has to be careful to keep molecules and atoms distinct.

Repulsion due to electrons is mostly an atomic thing. As notauser said molecules collide all the time. In fact molecules are often linked together.

The net has much information on the movement of molecules within a gas. The forces include Debye, London and Chemical bonds. I do not have a clue regarding Debye and London.

For the purposes of the sound in air discussion it is enough to know that the molecules maintain an average distribution.

 

[notauser]

No argument there.. Like hydrogen bonding those interactive forces more often affect rotational orientation than directional movement.. Send that same air through an HV ionizer though and its entirely a different story..

Edit: To clarify though, once air is ionized simple electric interactions take over.. The weaker forces become entirely a moot point, they're thousands of times weaker.. Still though I think gravity itself is more responsible for equilibrium than anything else.. The atmosphere is like a low density all encompassing ocean..

 

[crashsite]

Moving Molecules

At this point one has to be careful to keep molecules and atoms distinct.

Every molecule consists of two or more atoms of some elements bonded together by their electrons. The whole subject of chemistry consists pretty much of the definitions and rules for this and there are plenty of questions about how this relates to the sound propagation thing.

But, for the moment (or should I say 'time being' since moment can have its own meanings), staying in air, when an air mass is at rest (ie: no disturbance creating a sound wave), how do the molecules move?

There are effects that stongly suggest that they are indeed moving. For example, the change in volume or pressure of the air as it's heated or cooled.

Are they moving 'laterally'? In other words, do they move to a different physical location to collide with adjacent molecules (as opposed to say, colliding by expanding and contracting while remaining fixed in space as a balloon will displace air as it's blown up and deflated).

 

[3v0]

Are they moving 'laterally'? In other words, do they move to a different physical location to collide with adjacent molecules (as opposed to say, colliding by expanding and contracting while remaining fixed in space as a balloon will displace air as it's blown up and deflated).

They are not fixed in space.

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

@notauser
To understand what keeps the molecules apart read about the Debye and London forces. They may or may not relate to gravity. I can not say.

 

[Palladyne]

The noob takes a bite out of this thread...

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.

Anyway... hi all.

I have just acquired some (EBAY) UV LED's from Taiwan and will be working on a 12 volt water sterilizer project. I saw someone who had posted a 555 timer tutorial in pdf format but had to register first and now I can't locate it (did the search).

Have a good-o. -later