Why Does Sound Propagate?

[crashsite]

Getting to Mach 1

Well, the nature of harmonic motion is that there is no particular velocity, just a maximum, so even if a molecule had a resonant frequency that made the maximum velocity mach1, the condition would only exist for an instant, just like it would for any frequency that made the maximum speed > mach1.. Only difference is the phase angle where it would occur each period..

Okay. The first question is why is there a need to bring "harmonic motion" into it? What is, harmonic motion as relates to sound propagation? Maybe I'm off base here but, I define harmonic motion as motion in sympathy with the motions of other elements around it. For example, the harmonic balancer on an auto engine crankshaft to smooth out harmonic resonances at particular rpm's.

Second, what makes Mach 1 a constant (for a given set of conditions) if there's no particular relationship between the vibrational velocity of the molecules and Mach number? There's obviously an inherent something that makes it so. If it's not the speed of the molecules, is it the rate at which energy can be transferred from molecule to molecule?

I don't want to gloss over this basic concept because I feel like it's at the core of the answer. But, if it's not, I think it's also important to know then what is.

 

[notauser]

If you know what SHM is, and don't see how it relates to sound then I can't help you..
More important than maximum velocity in most applications of SHM is the period.. Its phase/resonance that determines the effectiveness of transfer..

 

[killivolt]

If you know what SHM is, and don't see how it relates to sound then I can't help you..
More important than maximum velocity in most applications of SHM is the period.. Its phase/resonance that determines the effectiveness of transfer..

crashsite : You have me way out on a limb. I'm not speaking from nothing more than my thick head now. Be critical. I can take it.

I don't understand relevancy I do understand relatively.

My brain is working on watching a flame in front of a speaker or what is the motion that is relative to a dust particle.

If Mach 1 is achieved in that one single moment and just before you push past does that molecular bond exist at rest or is it being accelerated.

kinetic energy release above that point and energy storage below it. Motion is transfered to heat into the surrounding molecules sinusoidally. Therefore motion is not transfered in the direction of motion but perpendicular to it.

Inside the point of axis. Mechanically.

So, the motion is at stasis within the molecule and the molecule itself should not move.

The movement of the molecule before that and after that is at the speed of sound only.

Energy or acceleration is transference kinetically to the surrounding body's. Resulting in Entropy.

No. Not Mach 1.


kv

 

[crashsite]

Yes, I know I'm a dunderhead

If you know what SHM is, and don't see how it relates to sound then I can't help you.

I Googled SHM and got the expected spate of references. The notion of sound being related to waves of various types and characteristics seems to be very much associated with the analysis of sound. But, you don't need a wave to have sound propagation.

I have an old digital tape measure.

**broken link removed**

It basically makes a click, with a piezo-electric plate, and then listens for the echo and measures the time lapse, which is read as feet and tenths. Essentially a simple sonar. But, there is no wave, harmonic or otherwise. Just an impulse-like click and that click propagates at the speed of sound.

Or, is there a wave? Does the "wave" arise within the molecular structure of the medium as part of the process needed to make the sound propagate? This is the point in the explanation at which everything seems to stall.

 

[3v0]

But, there is no wave, harmonic or otherwise. Just an impulse-like click and that click propagates at the speed of sound.

Or, is there a wave? Does the "wave" arise within the molecular structure of the medium as part of the process needed to make the sound propagate? This is the point in the explanation at which everything seems to stall.

A microphone and a recording (digital) scope will show you there is a wave. The microphone does this by causing an electrical pulse as the pressure of each wave pushes the diaphram back.

Even if the element that produced the "click" only produces 1 compresson followed by one rarification it is still a wave.

The compression is caused by moving the molecules closer together.

You can produce the sensation of sound without a wave. Just manipulate the eardrum in the same way as sound wave. Heck you could bypass the drum and manipulate the little hairs connected to the nerves.

May I suggest that sound is energy detected by the ear. In air that is a compression wave. The little gadget makes a wave or six.

It is easy to see that hi pressure on the eardrum will move it in. The faster you vibrate a molecule the warmer it gets. You might transmit sound information by rapidly heating and cooling an object. You need a reciever the rapidly reads the heat, and then a speaker to convert into real sound. Much like the microphone, electricity, speaker setup.

This is as close as I get to understanding the sonic boom: When the airplane flies at the speed of sound "sound" energy does not move fast enough to get away from the front of the aircraft. This energy does travel away from the plane mostly perpendicular to the direction of travel. It is this high pressure/energy that we hear as the boom.

A similar shock wave is generated at the rear of the plane but I am less clear on that end.

Lets see how wrong I am this time
3v0

 

[notauser]

There are different types of waves.. The ones we're familiar with calling waves like the trig functions are called transverse waves.. Sound waves are longitudinal waves.. Periodic compressions, and rarefactions comprise a longitudinal wave.. A mass bouncing like a bungy jumper on a spring/elastic can be modelled as a longitudinal wave.. What makes the types similar is the math that describes them.. SHM, and the basic wave equation are the basis of that math..
Another type of wave, torsional waves are used to describe twisting systems like wound up phone cords you need to unbind.. They follow the same math too, but like transverse waves, they're not applicable for discussing sound..
Sorry, I thought you'd been through all this stuff, and was just arguing what you didn't catch in school.. I can totally understand not knowing how SHM relates if you're pushing yourself to learn things you aren't supposed to know already..
Hyperphysics.org has pretty concise rundowns on this stuff that are good for ppl with limited background..

 

[crashsite]

The baby shoes are not ready for bronzing yet

Even if the element that produced the "click" only produces 1 compresson followed by one rarification it is still a wave.

The compression is caused by moving the molecules closer together.

That seems to be the case with the supersonic shock wave you bring up later. It's called a wave even though it seems to be more of an impulse of energy.

May I suggest that sound is energy detected by the ear.

Of course you can get as, "Zen" as you want.

It's been said that there is no such thing as sound. Just the movement of air molecules at specific rates that, when detected by our auditory organs (ears), is interpreted, by our brains, as sound. When a tree falls in the forest and there's no one to hear it that it truly does not make a sound. Of course, it still moves the air molecules.

Those same air movements, when detected as vibrations on our skin are felt as tingles or warming or pulsations.

It is easy to see that hi pressure on the eardrum will move it in. The faster you vibrate a molecule the warmer it gets. You might transmit sound information by rapidly heating and cooling an object.....

This is as close as I get to understanding the sonic boom: When the airplane flies at the speed of sound "sound" energy does not move fast enough to get away from the front of the aircraft. This energy does travel away from the plane mostly perpendicular to the direction of travel. It is this high pressure/energy that we hear as the boom.

A similar shock wave is generated at the rear of the plane but I am less clear on that end.

I've thought of this but, see it as a special case. I'm thinking that any phenomena that can produce a disturbance in the air will create a source of energy that will radiate away from that point at the speed of sound and, at some point needs to be considered. But, in the framework of "baby steps", after figuring out how the disturbance propagates in the first place and I don't think that's been addressed yet (I'm sure others would disagree).

I've seen the Schleren (sp?) photographs of supersonic shock waves in wind tunnels and do not understand a LOT of it. For example, how you can have subsonnic air behind the shock wave in a supersonic environment. But, that's waaaaay beyond this baby step.

Actually, I have a whole laundry list of factors and phenomena related to sound that never seem to get answered (at least on the elemetary level needed by us lay folks). The supersonic shock wave is just one. Why there's just one speed of sound for air when air is a mixture of gasses. How temperature and Mach number relate at the molecular level. The mechanism of sound propagation in liquids and solids (and, perhaps plasmas). How acoustical impedance converting/matching devices work. And, many, many more.

Lets see how wrong I am this time
3v0

Actually, I personally think that this line of thinking is getting you closer to getting to the root of this sound propagation thing. And, again, I suspect that others would disagree.

In fact, I think many here (perhaps most) will find my pathetic attempts to learn how this works by asking the questions I do and making the comments I do to be sort of beneath them. I do know that once this thread is going that I have no choice except to continue or give up. Trying to relaunch the topic to try to get renewed interest will, with certainty, lead to heavy censure for trying to revive a tipic that's already been "answered".

 

[crashsite]

Sound waves are longitudinal waves.. Periodic compressions, and rarefactions comprise a longitudinal wave.. A mass bouncing like a bungy jumper on a spring/elastic can be modelled as a longitudinal wave

Sorry, I thought you'd been through all this stuff, and was just arguing what you didn't catch in school.. I can totally understand not knowing how SHM relates if you're pushing yourself to learn things you aren't supposed to know already.

So, what you seem to be saying is that there is no elemetary explanation for any of the phenomena related to sound and if you're not into it at an advanced engineering level, you're out of luck. I guess I'm not convinced of that.

I know how a car engine works. I know about operation of the carburetor and how the crankshaft moves the pistons to draw in and compress the mixture and how the ignition expands the mixture to move the piston to creat the power. I also know about the crankshaft and camshaft and valves, etc. Actually, I have the FAA A&P License and we learned about this stuff, a lot of which I already knew.

What I don't know is the math behind Bernoulli or the airflow past the valves or how to calculate the rate of expansion of the mixture in the cylinders for a given octane rating, and so on. In other words we learned it in a way to be able to analyze engine operation, to understand the instrumentation and to troubleshoot problems.

I'm trying to gain a similar level of understanding about sound propagation and what's being taught has me (and a lot of people like me...who I suspect are in the vast majority) confused.

If longitudinal waves are the key to understanding the propagation of sound (the actual physics of why the sound propagates away from a point of disturbance rather than the usual analysis of the compression and rarefaction of the air molecules in a space), please add to my understanding. If you are convinced that the next level of that understanding is the mathematical model...well, maybe I have no chance of understanding it since I just don't see the elegance of the workd, through math, like some folks do.

 

[crashsite]

The air is awfully thin up here in the stratosphere

crashsite : You have me way out on a limb. I'm not speaking from nothing more than my thick head now. Be critical. I can take it.

Okay...rather than simplifying the explanation you seem to be moving closer and closer to the esoteric possibilities. I almost expect the introduction of pixie dust.

I don't know how to properly reply since I'm not sure what you are saying in the first place.

Can you use that intellect to drive your explanation down to the level of us average Joes?

 

[Nigel Goodwin]

I just can't believe such a pointless useless thread has gone to five pages?

 

[3v0]

I can see where you might view "May I suggest that sound is energy detected by the ear." as Zen, it was not intended that way.

If one accepts that somewhat narrow definiton of sound, ultrasound is not sound. Unfortunatly the english language is no help because ultra has two meaning: one is extreme and the other is beyond. No help.

We are simply dealing with semantics. Rather then talk about sound one should be more specific. For now lets take sound to mean compression waves moving through the atmospher. To start with lets further restrict it to the non fringe cases (speeds to fast or too slow to allow compression, if they exist). We can talk about solids another time.

In that contest :

A single inpulse or compresson traveling through the a simplistic wave. Several inpules would be called waves.

In regards to how sound propagates. The document I refered to earlier (a page or two ago) is a good place to start. What I originaly missed is that in addition to distance between molecules the is a stiffness/springy-ness constant associated with each material. The articl may contain the entire picture if one took the time to fully understand it. I have not.

The shock wave (relative to the ground) has to move at the same speed as the plane. The movement of the plane forces it to. The compression waves (sound) ahead and behind the shock wave do not have the plane forcing them and move at the speed of sound.

The region that is the shock wave exists in a narrow band that moves with the craft producing it. This high energy compression wave travels faster then the speed of sound because it is driven/pushed/pulled? by the craft itself. As compared to the sound leaving a drum or speaker. Neither the drum or the speaker need to move at the speed of sound to produce the compression waves we know as sound. But yet the compression wave move at the speed of sound.

The "subsonic air" behind the shockwave is just run of the mill sound. Outside of the shockwave that is all it can be.

3v0

 

[killivolt]

I just can't believe such a pointless useless thread has gone to five pages?

My BIG Mouth.


kv

 

[killivolt]

Okay...rather than simplifying the explanation you seem to be moving closer and closer to the esoteric possibilities. I almost expect the introduction of pixie dust.

I don't know how to properly reply since I'm not sure what you are saying in the first place.

Can you use that intellect to drive your explanation down to the level of us average Joes?

crashsite, (Touché) Moch 1

I guess you forgot my dust particle.

kv

 

[crashsite]

Getting to Mach 1

I can see where you might view "May I suggest that sound is energy detected by the ear." as Zen, it was not intended that way.

I didn't mean to imply that your analysis was, "Zen". Just that sound is only sound when it's heard. Otherwise it's air movements that happen to follow some laws of physics. I'm not quite sure why it's so difficult to get a coherent, simplified version of it.

In regards to how sound propagates. The document I refered to earlier (a page or two ago) is a good place to start. What I originaly missed is that in addition to distance between molecules the is a stiffness/springy-ness constant associated with each material. The articl may contain the entire picture if one took the time to fully understand it. I have not.

It was a good place to start and it had some good information but, I didn't feel like it told the whole story (at least my simple-minded brain didn't get a 'whole story' from it).

Certainly, conventional logic would suggest that the closer together the molecules are, the faster sound would propagate through the material. But, it tuns out to be untrue. Ergo, there's something else that determines the speed.

The region that is the shock wave exists in a narrow band that moves with the craft producing it. This high energy compression wave travels faster then the speed of sound because it is driven/pushed/pulled? by the craft itself. As compared to the sound leaving a drum or speaker. Neither the drum or the speaker need to move at the speed of sound to produce the compression waves we know as sound. But yet the compression wave move at the speed of sound.

The "subsonic air" behind the shockwave is just run of the mill sound. Outside of the shockwave that is all it can be.

...but, it's subsonic air that's zipping along at a supersonic speed. High speed aircraft are designed so that the intake for the jet engines and the engines themselves, for example, are behind the shock wave and operate subsonically...even when the jet is traveling at Mach 2. I know there's been some thought and research done on supersonic turbojet engines but, I don't know if there's ever been a practical one built or not.

Yes, the speaker cone moves at relatively slow speeds even though the sound created propagates away from the speaker cone at the speed of sound. That, in a nutshell is what I've been trying to find out about in this thread. The basic mechanics of how that happens without having to resort to the complex mathematical modeling that I just don't understand.

 

[crashsite]

Nigel...you have the power (per Darth Vader's illegitamate brother)

I just can't believe such a pointless useless thread has gone to five pages?

I fully agree that you just can't believe that such a pointless thread has gone on for five pages.

Perhaps you'd like to step in and resolve the issue by providing the simple, coherent, essentially non-mathematicaly modeled answer so we can stop here?

 

[crashsite]

Dust in the Wind

crashsite, (Touché) Moch 1

I guess you forgot my dust particle.

Actually, I do remember your dust particle. I just never quite figured out how it fit into things....

 

[killivolt]

Dust in the Wind.

Actually, I do remember your dust particle. I just never quite figured out how it fit into things....

**broken link removed**


Ok, but it's just cool.

While your reaching Pressure temp correlation nothing more happening. So is the restriction of Mach 1 is met and if it were a cylinder it would make sense. But, in this case it's a cone force inward to compression then release.


kv

 

[notauser]

So, what you seem to be saying is that there is no elemetary explanation for any of the phenomena related to sound and if you're not into it at an advanced engineering level, you're out of luck. I guess I'm not convinced of that.

I know how a car engine works. I know about operation of the carburetor and how the crankshaft moves the pistons to draw in and compress the mixture and how the ignition expands the mixture to move the piston to creat the power. I also know about the crankshaft and camshaft and valves, etc. Actually, I have the FAA A&P License and we learned about this stuff, a lot of which I already knew.

Well then picture all air molecules as a lattice/grid attached on all sides by elastics.. The elastics represent the fact that these particles cannot continue to move in one direction unimpeded because of surrounding molecules, so they will eventually return.. Imagine then being shorter and under more tesion in solids/liquids etc..
Sound is produced by causing local molecules to move and colide with neighbours.. The collisions provide means of propagation chain reaction like, while the elastics represent the returning force.. Think of the elastics as the desire of the substance to retain its current uniformly spread-out state.. In a way it works kind of like those executive clicking pendulum balls..
At any given time velocity, kinetic/potential energy, and momentum of a particle, its frequency of oscillation, and maximum displacement from where it began can be calculated with the wave function.. The wave function doesn't really help explain the concept here at all really, rather it facilitates information gathering if you already understand the concept.. And believe it or not, it all boils down to simple trig functions, because that is the basis of the math for SHM and the wave equation..
Trig functions often act as simple multiplier functions to determine the relevant portion of a maximum value at any point in the cycle..
For instance:
sin(0°) = 0 and sin(90°) = 1 (and between these two extremes it smoothly changes from 0 to 1)
If an imaginary system was modeled like:
y = 10*sin(x) (BTW, in this 10 is analogous to the maximum amplitude)
then at x=0° y=0.. At x=90° y=10.. And at x=45° y=7.071..

So the math doesn't really help explain the system.. Actually when you get familiar with the math, and you need to apply similar math to absurd scenarios that are tough to picture, similarities with easy to understand sound propagation etc can be analogies to the tougher to picture absurd system..

 

[3v0]

@notauser. I like the idea of the matrix.

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

EDIT: I think I have it. As A moves to B the tension between them goes down, that causes B to move to C long before A reaches B (which it never does).

Very Cool.

 

[crashsite]

Has the door opened a crack?

Well then picture all air molecules as a lattice/grid attached on all sides by elastics.. The elastics represent the fact that these particles cannot continue to move in one direction unimpeded because of surrounding molecules, so they will eventually return..

Thanks. This does give a graphic picture. But, it also raises some questions. It sort of seems to assume that, in a medium that's undisturbed, the molecules are at rest. When they are moved, they deflect and move the adjacent molecules. 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). In air, that rate would be Mach 1 at a nominal speed of 1100 feet per second.

Imagine then being shorter and under more tesion in solids/liquids etc..

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.

Here's where it gets tricky to make a verbal descrioption, not so much because the verbage to do it is lacking but, because in order to be precise and have it make sense, it requires careful and rather protracted wording.

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.

I guess I'm sort of mentally envisioning (perhaps incorrectly) that the elastic nature of the medium is due to the interaction of the electrons around the atoms in the molecules.

I don't feel like I'm being nit-picky here because these are just the sorts of things that are needed to undertand, even on an elementary level, what's going on.

Sound is produced by causing local molecules to move and colide with neighbours.. The collisions provide means of propagation chain reaction like, while the elastics represent the returning force.. Think of the elastics as the desire of the substance to retain its current uniformly spread-out state.. In a way it works kind of like those executive clicking pendulum balls..

At any given time velocity, kinetic/potential energy, and momentum of a particle, its frequency of oscillation, and maximum displacement from where it began can be calculated with the wave function.. The wave function doesn't really help explain the concept here at all really, rather it facilitates information gathering if you already understand the concept.. And believe it or not, it all boils down to simple trig functions, because that is the basis of the math for SHM and the wave equation..

Trig functions often act as simple multiplier functions to determine the relevant portion of a maximum value at any point in the cycle..
For instance:
sin(0°) = 0 and sin(90°) = 1 (and between these two extremes it smoothly changes from 0 to 1)

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.

So the math doesn't really help explain the system..

I can't really disagree with that.