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I just began learning about the topic of sound energy in physics and I already have lots of question that is not being explained with a simple google search. In my research so far, I have learned that in the mechanics of the propagation of sound, it is not the molecules/particles that travel but the energy that is transferred from molecule to molecule. So now with this fact in mind, my question is then that in propagation of sound, why do molecules/particles not travel and keep on traveling in one direction, until its reaches somewhere like the ears of some living being or something. Why does it vibrates, back and forth in stead? Like, what is causing the molecules to move back after it has moved forward where it is carrying the energy of sound?

The reason why I am asking this is that so far from my understanding of sound energy, I tend to conceptualize sound energy as something that originated with some kind of mechanical or contact based force. So like, when I tap/touch on a molecule , that molecule travels a certain distance, which in turn taps/touches another molecule in its path that in turn travels a certain distance, so on and so forth. This situation, in my understanding, is similar to me pushing a macroscopic object like a table by hand, and in this case we know from everyday experience when you push a table, the table as a whole together moves a certain distance. It doesn't travel back and forth. So, thus, unless there is a flaw in my understanding of sound energy so far, my question then becomes that why do molecules/particles in sound energy vibrate in stead of traveling a certain distance? If anyone can add to this question, it will be greatly appreciated. Thank you.

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3 Answers 3

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So like, when I tap/touch on a molecule , that molecule travels a certain distance, which in turn taps/touches another molecule in its path that in turn travels a certain distance, so on and so forth

It is right. It is the same principle of a row of coins. When the first is hit by a moving coin, the last of the row moves. The intermediate coins also moved, but it was an very small longitudinal vibration. The energy of the first coin propagated to the last one by these mechanic vibrations.

In the sound waves in the air, the short longitudinal path of each molecule corresponds to the coin vibration.

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  • $\begingroup$ Thanks for your comment.I get your point of the last coin moving a distance. But my question is that in the animation of propagation of sound energy I have seen so far, I see the "intermediate" molecules move a certain distance and then come right back to the original position, in a coherent and not random way (like a whole bunch of molecules move forth and right back to the original position in unison)? Why do they vibrate around a mean position this way? Why not just get displaced a certain distance, stop and not vibrate (other than thermal motion which is every present of course) at all? $\endgroup$
    – TLo
    Commented Nov 8, 2020 at 20:34
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    $\begingroup$ The model for a gas is kinematic. Some description as you mentioned can lead to the idea of oscillation around an equilibrium position, what is appropriate for liquids and solids, where potential energy plays a greater role. But thinking only on recoils after collisions the effect is similar. $\endgroup$ Commented Nov 8, 2020 at 21:01
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The situation get messier than that but you are on the right track. Whatever acts as the source of sound in a fluid pushes on fluid particles and what is propagating is an increase in local density (and/or pressure) of the fluid. If the particles are pack up against each other then rather than the particles you pushes moving to someone's ear they push neighboring particles etc. At the location of the source at the instant that it is turned on it either makes a pocket of "more" dense air in front of it or "less" dense, depending on whether it pushes forward or pulls back. This is all it takes to create a single pulse. This density change is what travels and that may or may not correspond to particle movement. The reason I say it's messier than that is because molecules in air or water and not bound to a specific location as they are in a solid. So once they start moving they can in fact keep moving in the direction of the sound. This is how fluids work, they are random and chaotic. The picturesque descriptions in some science books do not do justice to what is happening at the particle level. We try to develop a description of sound as small disturbances in the density, pressure, and other thermodynamic state variables of the fluid. But in fact making sound can also make "wind", or steady flow. Our assumptions that these two things can be separated and each described by a distinct separate set of equations is often violated in real life. Acoustics is really a small piece of fluid dynamics and that makes it very exciting to study.

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  • $\begingroup$ Hi ggcg. Thanks for confirming my doubts and providing a more complex view of what is actually happening at the molecular level. If i could ask you to elaborate on one further question, I am still stuck on the model/simulations I have seen of sound molecules where right after the first pulse is created where the first "batch" of molecule get packed together, bunch of these molecules actually move in a backward direction after wards. Why do they do that? Is it because they losing momentum/energy after they collide with the second batch of molecules that later gets packed? Thanks. $\endgroup$
    – TLo
    Commented Dec 5, 2020 at 4:45
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    $\begingroup$ Can you cite one of these "sims"? I think what is happening is that after the first push there is a pull of the mechanism back and that exposes a region of lower molecular density, and pressure. The particles are inclined to fill that space. In a fluid one cannot say that the particles remain at some equilibrium location moving back and forth about it since at a given temperature all the molecules are moving about. This is an attempt to describe some bulk movement in the mean. $\endgroup$
    – user196418
    Commented Dec 7, 2020 at 17:38
  • $\begingroup$ dosits.org/science/sound/what-is-sound $\endgroup$
    – TLo
    Commented Dec 9, 2020 at 20:19
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    $\begingroup$ Oh I see... I have attached a link above that shows the GIF of the back of forth movement of particles in sound wave. When I first began learning of sound, I got confused of exactly what was "locking" the particles in a fluid to move in same patterned, regular back and forth way, knowing well before that fluid particles are free to zip off and move around in any random way, into any location. I guess I see your point of average, bulk movement rather than focusing on single particle at a time. $\endgroup$
    – TLo
    Commented Dec 9, 2020 at 20:49
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    $\begingroup$ That's right. If the mean movement is 0 then particles will move back and forth though I wouldn't call them "locked" in place. I think that is deceptive. But we can't really track individual atom motion in the bulk movement of fluid. The "particle" they are showing is conceptual, a virtual particle like a cell in an elastic solid, not a virtual particle state in a quantum field theory. The classical movement of this cell is what is being shown. Who knows what things inside are doing. $\endgroup$
    – user196418
    Commented Dec 9, 2020 at 20:58
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U see energy is related to the the amplitude and until the energy is diminished the particles oscillate I'll make this clear a object is pushing a particle so that particle hits the neighbouring particle and the energy is passed on..ur other q why does it come back? U see normally particles tend to move to a region of low pressure from a region of high pressure they do not accurately move back to their mean position but move back and equally spread..hope this helps

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