1. Welcome to our site! Electro Tech is an online community (with over 100,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Why Does Sound Propagate?

Discussion in 'Mathematics and Physics' started by crashsite, Dec 30, 2008.

Thread Status:
Not open for further replies.
  1. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    I know that seems like a really stupid question but...I was thinking of something else, related to sound and suddenly realized that I have no idea why sound speeds away from an emitter at the speed of sound.

    If I stand some distance from a rock face and loudly yell a "hello", that rather complex packet of information zips out to the wall and reflects back (delayed) as an echoed "hello". Okay, fine. The sound propagates at the speed of sound.

    The usual reason given is that the adjacent air molecules vibrate and impinge on neighboring air molecules and the sound travels by that mechanism. And I have no quarrel with that...as a way to define why the air disturbances interact or the speed with which they do so in a given medium.

    But, why does a "packet" of those disturbances propagate away from the emitter, intact? If the emitter is some sort of diaphragm that vibrates and moves the air back and forth just slightly, why doesn't the sound just stay there and become a more and more complex wveform with each cycle...with that cacaphony propagating,, by the interchange of energy between adjacent molecules, as basically a damped waveform as it loses energy with distance?

    In other words...why does the sound packet speed away from the emitter? What propels it?

    More important, is there a simple, 8th grade science class explanation of that reason?
     
    Last edited: Dec 30, 2008
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

    Joined:
    Jan 4, 2007
    Messages:
    19,943
    Likes:
    536
    Location:
    England.
    ONLINE

    Attached Files:

  3. Hayato

    Hayato New Member

    Joined:
    Jun 21, 2006
    Messages:
    694
    Likes:
    14
    Location:
    BR
    Well, you do not like engineers but you are asking an engineering question. ;)

    Why does "sound packet" speeds away from the source?
    -> What propells a wave is energy. When you generate sound waves, the particles around the source will be more energized, and the adjacent ones will not.
    The rule of any system is to stabilize. So the energy will propagate through the adjacent particles to distribute energy, so that all particles in the system will have the same amount of energy.

    Why does "sound packet" propagates intact?
    -> You have this doubt because you are definig the sound as a packet. But it is not. When you say a word, for example, each sound of each letter, will be generated in different positions on the time. If you say "car", the first sound generated will be the "c", so the first waves to hit the target will be the ones that forms "c", and so on.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    A Good Rehash of the Mechanism

    Unfortunately, that pdf, too does not even attempt to address my question. It assumes (obviously correctly since sound does propagate) that the sound will propagate and gives the classical reason why (as I did, based on molecular interaction). He goes on to talk about how different variations in the medium through which the sound travels affects the propagation but, not why a sound propagates away from the emitter in the first place...just that it does.

    I, too had just always taken it at face value that it happens and really didn't give it much thought until I was recently thinking about the related item (which I'm pretty sure it going to end up as another "sticky" question here in the future).
     
  6. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    I don't have anything against engineers and rather pride myself on having done a little engineering, myself. But, my math skills (lack thereof) force me to beat things down to the very lowest common denominator (yikes, a math analogy) to be able to understand them. Where engineering types are perfectly satisfied with an equation that "explains" it, I must actually figure out, in almost a mechanical way, how things work to "get it".

    Of course the sound vibrations are energy and that energy moves away from the emitter at the speed of sound. But that doesn't explain why the each disturbance propagates away from the emitter on an instant by instant basis.

    I believe the "packet" analogy is valid...and, for the very reason you give (right after saying that the sound doesn't travel as a packet).

    Let me ask the question anew. If the emitter is moving only a small amount to disturb the air that is adjacent to it, why don't the repeated oscillations simply build up some level of energy at that node, which is then propagated outward as a damped wave, with the energy falling off with distance.

    We get a clue from the old "rock in the water" effect but, even that assumes a simple wave that's generated and does indeed travel outward as a damped wave (intensity falling off with distance). What's more, it travels very slowly. If you make the wave more complex, you're back to the propagation of sound, as opposed to simply a wave, question again except now through water rather than air...complete with the "packet" concept.

    How is that darn "C" generated one moment and is suddenly the same "C" some 1100 feet away a second later?
     
    Last edited: Dec 30, 2008
  7. Papabravo

    Papabravo New Member

    Joined:
    Mar 9, 2006
    Messages:
    2,889
    Likes:
    25
    Location:
    Michigan, USA (GMT-5)
    The answer to your question is rooted in the mathematical description of physical systems. Starting with Newton's three laws of motion, some geometry, and some properties of materials like air and water, the mathematics allows only certain classes of solutions, and no others. It is the existance and uniqueness of solutions to differential equations that determines how things like sound waves propagate.

    Feynman has a really good exposition on waves that you might find interesting. It starts with the intuition before springing the math on you.
     
    Last edited: Dec 30, 2008
  8. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,589
    Likes:
    60
    Sounds doesn't travel as packets nor on an "instant by instant basis" ...just think about pool balls. If you hit some balls and they hit other balls and so forth, you can't really divide it into discrete parts. It's more of a moving gradient of air pressure (you can't just take the areas of highest pressure and call that a packet and ignore everything else in between).

    I think a sort of explanation to your question about why the air that is originally moved doesn't just stay there and get all chaotic is that:
    1. some of the energy is lost when the particles collide with other particles (thus propogating the sound)
    2. I think a problem lies in an oversimplification. Your description almost seems to imply perfect propogation of all sounds. It seems to leave out the consideration that sounds of higher frequencies don't propogate nearly as well through air. It may lie in why the higher frequencies losing their energy faster (the cacophony that you speak of causing it to lose all it's energy and not go anywhere very far).

    If your question is why sounds travels away from the emitter, and not backwards, I think it's because of this:
    1. THe sound does travel backwards a bit, but for the most part I think these are a bit like "echoes" of the original particles that are moving backwards a bit after colliding with other particles. So they are weaker.
    2. If most of the energy is imparted to the next particle in the collision, it leaves a lot less energy remaining in the original particle to travel backwards. I would think that the faster the molecule is moving in the first place, the harder it hits, and the more it rebounds after the collision dissipating energy away from the direction of propogation. That would explain why higher frequencies don't propogate as well as lower ones.
     
    Last edited: Dec 30, 2008
  9. 3v0

    3v0 Coop Build Coordinator Forum Supporter

    Joined:
    Jul 14, 2006
    Messages:
    9,372
    Likes:
    215
    Location:
    OKLAHOMA USA
    This question has been cooking in my brain since you posted it. Dominoes explain the compression wave. The air molecules like the dominoes do not move very far, they just transfer energy by wacking into the next guy in line.


    The speaker cone pushes the first batch, they push the next, etc. This explains how the energy moves but the air does not. The nature of a compression wave.

    Now think of the air molecules like dominoes attached to the table by springs that return them to the upright position. When we wack the first one the energy travels but the dominoes all return to the neutral position. Air is much like this in that it attempts to equalize pressure.

    Instead of a speaker cone use a linear actuator to wack the first domino. We have a system that is very close to sound execpt the mass of the dominoes and the air resistance causes it to move much slower.

    The NASA SCIence Files™: Kids: Dr. D's Lab: Sound: A Domino Effect
     
  10. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,589
    Likes:
    60
    It doesn't...it's quieter (obviously). But are you asking why the frequency doesn't get lower from a distance because presumably the particles slow down (decreasing frequency) as they lose energy slow down as they lose energy? Because you'd think that it does...but maybe the effect is negligible compared to the dampening that occurs so by the time it's noticeably lower it's almost died out. Maybe that relationship is somehow fixed that way by physics no matter what medium you choose.

    EDIT: BUt I forgot about the speed of propogation being the same regardless of frequency so it doesn't make too much sense.
     
    Last edited: Dec 30, 2008
  11. Hayato

    Hayato New Member

    Joined:
    Jun 21, 2006
    Messages:
    694
    Likes:
    14
    Location:
    BR
    That's not true, when the emitter is moving only a small amount to disturb the air, there is propagation, but the intensity and speed of the propagation depends upon the way that the emitter moves.

    If the emitter moves smoothly, in a frequency of 0.5 Hz, then a 0.5 Hz wave will be generated and propagated smoothly.
    If the emitter moves with strengh, then the intensity of the wave will be high, but the frequency remains 0.5 Hz.

    And, in a real system, the waves are aways damped, when there is movement there is friction, so the energy will be converted in heat.

    Well, I didn't get this part much, but as far I understood, you are comparing the speeds of propagation?

    Well that's an interesting topic.

    Let's imagine the following situation.

    An emitter, emitts a "C", but it goes as fast as the sound, and 1 second after that "C" is 1000 feet away.

    But when you touch the emitter on the water surface, you are able to see the waves travelling in low speed.

    Good comparison?

    Well, the first thing to state is that in the both the frequencies are the same. Just the speed of propagation changes.
    Every time that you have a wave travelling along different mediums there is a fenomena called "Refraction", I believe you studied that.
    In a gross way, we can calculate that with the Snell's Law:
    v1.n1 = v2.n2

    Where n1 and n2 are the refraction coefficient, and v1 and v2 are the wave speeds at the mediums.

    For example, n1 = 1 (air), n2 = 590 (water).
    If sound travels at 340 m/s at air, then, a soundwave that is generate outside the water that enters the waters suffers refraction, its speeds goes down to 0.58 m/s.

    Then you are able to see it propagating very slowly. You sacrifice speed, but gain amplitude, and the frequency stays the same.


    But there is more and more to analyse.
     
  12. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,589
    Likes:
    60
    Ahhh, now I'm confused about the speed of propogation vs the frequency of compression waves lol. I was thinking just about the speed particles collided at as frequency only, and forgot about the speed of propogation. Probably because frequency is an oscillation of the particle and not a forward movement like the propogation of the wave.
     
    Last edited: Dec 30, 2008
  13. Hayato

    Hayato New Member

    Joined:
    Jun 21, 2006
    Messages:
    694
    Likes:
    14
    Location:
    BR
    Frequency never changes. Just the speed or amplitude.

    You can have Doppler effect, but thats for a moving "emitter", and even so you do not change frequency, just "create" some "relative frequency" between the source and receptor.

    But those crashsite questions are impossible to answer for 8th grade. For example wave propagation in a circular surface (Bessel) is different from linear propagation (Dirichlet), those are more advanced than Fourier topics.
     
  14. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    Sorry for the "shotgun" approach here guys but...

    ...but, you guys have giving me a lot to respond to.

    Dominos and pool balls do a fair job of describeing the "rock-in-the-water" propagation but, that's not the propagation of sound and it's easy to describe and envision. The rock displaces some water and raises it. As the water falls it displaces the adjacent water and the ripple spreads. But, that is not sound propagation. Maybe part of the problem with understanding this thing is that elementary science tries to equate the ripples with sound waves and they are really completely different phenomena.

    Again, the dominos only propagate the "ripple". The sound of the dominoes falling is the sound waves and it is the thing that somehow zips off at some 1100 feet per second. I say, "somehow" because that's the thing I don't understand.

    I would like to think that if I could glean a reasonable understanding of how an airplane flies by the time I was about 13, there should be a corresponding explanation as to why sound propagates. I would hate to think that something so elementary and common is so frought with technical and mathematical gibberish that only the eggy-ist eggheads can possibly understand it.
     
  15. 3v0

    3v0 Coop Build Coordinator Forum Supporter

    Joined:
    Jul 14, 2006
    Messages:
    9,372
    Likes:
    215
    Location:
    OKLAHOMA USA
    The SOUND of the dominoes has nothing to do with anything.

    The speed of sound is due to the space between molecules. Explains why sound is faster in desnser air. Denser air allows the molecules to collide faster thus increasing the speed of sound.
     
  16. Hayato

    Hayato New Member

    Joined:
    Jun 21, 2006
    Messages:
    694
    Likes:
    14
    Location:
    BR
    Perfect one, 3v0.
     
  17. Papabravo

    Papabravo New Member

    Joined:
    Mar 9, 2006
    Messages:
    2,889
    Likes:
    25
    Location:
    Michigan, USA (GMT-5)
    Since the mathematical argument appears to be unpersuasive, I'll offer some additional observations which may trigger some introspective thought.

    1. Sound does not propagate in a vacuum, unlike EM waves.
    2. The speed of sound in a fluid medium like air or water depends on the properties of the fluid.
    3. Vibrations are transmitted by a fluid, because the fluid lacks the ability to contain the motions produced by the vibrating source.
    4. The vibrations are attenuated as a function of distance and are eventually reduced to unmeasureable levels.
    5. A continuous sound depends on the constant addition of energy from the source to the fluid medium.

    Did anyone bother to read Feynman on the subject of sound? I think it would help.
     
  18. dknguyen

    dknguyen Well-Known Member

    Joined:
    May 6, 2006
    Messages:
    6,589
    Likes:
    60
    I wouldn't be so sure of that. Almost all of those elementary explanations for flight are wrong. And a lot of it seems to be working from the math up, rather than down to the math (don't ask me how the person who came up with the math did it originally). I thought I did too, until I actually dug into it and you realize how much of it is wrong. It's supposed to have something to do with any curved fluid flow requires some inward acceleration to form that curved flow which means there is a centripital force acting on it somehow. Also something to do with the presence of fluid friction making it all possible. I don't entirely understand it myself. It seems to be one of those things where it's very easy to convince you know how it works when you really don't. THis maybe be one of those things.

    Huh? How does dropping a rock in water have anything to do with the analogy for sound? Those are transverse waves and sound is a longitudal wave. Dominoes and pool balls are analogies for longitudal waves, not transverse waves, since the direction of the moving particle is the same direction that the wave is propogating. Sound itself is a gradient of pressure that is moving on a particular direction. It seems that you've gotten locked in a certain mindset thinking about this since you've misinterpreted more than one analogy from more than one person (especially if you've been trying to use the water ripple analogy since that's a completely different type of wave). Let's try and clear things up.

    There is a difference between the propogation of the wave (the speed that the particles are moving at up to the point when they collide) and the frequency at which it is oscillating forward and backwards (relative to the direction of propogation) which makes the frequency. Every time the particle reaches the peak of it's oscillatory motion, it collides with another particle which also then oscillates and collides with another particle, etc. So this transfer of oscillations seems to be the propogation of the wave that travels at ~343m/s, while the oscillations itself is what makes the sound. See this picture I found. Maybe it will help:
    [​IMG]

    This graphical part of this image from the website (the part with molecules and spacings) may seem to not account for how sound works in liquids and solids becayse are no spaces between the particles. But keep in mind pressure is a force. It's just that in gasses, because there is free space between molecules, they tend to be farther apart when the pressure/force. THe sinusoidal graph maps out the PRESSURE NOT THE MOTION OF THE PARTICLES (the particules do not move up/down, side-to-side, or otherwise perpindicular to the direction of propogation. THey move along the same axis as the wave (but oscillate forward and backward, whereas the wave itself just moves forward).
    [​IMG]

    More explanation on the website where these two images came from:
    Sound is a Pressure Wave

    It should also be noted that sound does propogate through solids except it would probably be called a vibration until the vibration is transferred from the solid to a surrounding fluid. Perhaps the analogy should just be
    sound<->fluids
    vibration<->solids

    Looking at those images, it seems to me that how a sound wave (or any other longitudal wave) work is very obvious. On the other hand, your rock-dropped-in-the-water analogy for transverse waves has me wondering how a transverse wave travels in the first place. Obvious, it is not!
     
    Last edited: Dec 30, 2008
  19. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    Focus

    I agree. As I said, the domino effect and the sound effect are two completely different phenomena.;

    I have no argument with that either. But, what it doesn't do is explain why the sound (what word should I use since, "packets" seems to be unpopular?) propagates away from the emitter at some rate. The rate is about 1100 feet per second in air and different in different mediums (as noted above and for the reasons noted). We keep rehashing that but are not getting any closer to why the...okay, let me -pause here for clarification...

    Tell me the word (or words) you guys would like to use that we can all agree on to mean the concept that the instant a disturbance is generated and coupled to the air (or other medium), it begins to propagate away from the emitter at the speed of sound. And, in this manner, propagates the sound so that the structure of the waveform, over time, remains intact (ie: the "hello" echo comes back as, "hello". Of course, it's assumed that there will be external forces that will modify that sound such as friction (attenuation), temperature and pressure gradients (filtering effects), echoes (reverberation effects), etc.

    ...but, we're not getting any closer to the reason why the (insert chosen acceptable word(s)) propagates away from the emitter.
     
    Last edited: Dec 30, 2008
  20. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    Not Obvious

    It's not obvious at all. The tuning fork moves back and forth but somehow the waves keep moving outward. Shouldn't the "logic" of it be that the waves will also move back and forth in sympathy with the fork?
     
    Last edited: Dec 30, 2008
  21. crashsite

    crashsite Banned

    Joined:
    Nov 20, 2007
    Messages:
    725
    Likes:
    2
    All that makes sense...but, it does nothing to explain why the sound propagates...waveform structure intact...away from the emitter.

    This is not the first time I've seen this Mr. Feynman touted as being the trusted "final word" on a physics subject in these forums. If Mr. Feynman has answered the question, can you copy and paste that simple and understandable text into one of your posts? I fear that Googling for "Feynman" and "sound propagation" will not very likely take us to anything useful.
     
Thread Status:
Not open for further replies.

Share This Page