Transverse waves, such as upper-surface of pond-water, or in shaking-rope; the Transverse-Wave can be easily understood and drawn (for different times such as at t second, t + 0.25 second, t+0.5 second (for-say) ), where, each particle can move in both direction (UP and DOWN) from its original position.

Now, my question-is;

(1) in case of Longitudinal-waves; does a point (of medium) move in 2 ways (FORWARD AND BACKWARD) with-respect to its ORIGINAL POSITION ?

The spring waves are fast, and not clear, and also could-not be well-remembered (for me) the actual spatial-position of each individual turns, so I could't compare the conditions t1, t2, t3, etc. so, cant understand movement of individual-particle. Just the overall movement of compressions and rarifications could be well-watched

(From this animation of wikipedia, it seems to me that the particles go to forward direction and comes-back to the original position, but not seem to go further backward from original position )


or https://upload.wikimedia.org/wikipedia/commons/6/62/Onde_compression_impulsion_1d_30_petit.gif

(failed to upload)


(2) How to draw (on notebook) the different steps of longitudinal-waves with respect to time? How to draw that, if the particle really move in both-direction from original position. In different textbooks, images are given, with vertical-bars or hemicircles, such as:

t1=> |.|||..|...|....|.......|.......|...|..|.|||.

t2=> .|.....|....|.....|..|.|||..|...|....|.......|.

t3=> ||..|...|....|........|....|......|....|.|.|||..

But in-such drawings , it is not clear to me, in-which ways individial-particles are oscillating. Because it is not clear, among so-many vertical-bars, which-one bar at t1 is same with which-one vertical-bars on t2 or t3.


I've solved this just as a guess, but requires a verification.

  1. The layers could shift both in forward or backward direction

  2. I've rendered one.

Longitudinal wave

The upper diagram is a sine type graphical representation. The lower diagram is actual longitudinal wave. The colors indicate 3 separate timely-position (t1, t2, t3 ; respectively which are towards future). The thick dots indicate a crest, and box-sign indicate a trough.

Before rendering in this way, every time I was assuming that, the original position of layers (neutral-point) (node?) in the sine-graph would be comparable with the layer on actual wave that shows no shift from its original position. (Just similar to transverse wave on rope).

that gave me a cumbersome drawing each time.

Then it seemed to me, the sine-graph is only applicable for pressure and it is is not applicable on shift of layers. It seemed to me, if from the zones of rarification some layers go-out (from both sides), and to the zones of compression if some layer comes-in from both-side, on the intermediate portion (the original position or neutral points) (nodes?) of the sine-curve, there would shift of layer from original position. Rather, the layer on the centre of rarification (considered as trough here) and the layer at centre of compression (considered as crest here), would stay at their original positions.

Now the rendering looks Ok, but needs verification.


1 Answer 1


You have the correct conclusion, and I think you have the correct analysis, but I don't fully understand your presentation.

Think of it this way: If the displacement occurs only between "neutral" and "forward", then the average density (air molecules per volume, or spring coils per length) over the entire system (air chamber or spring) must increase. But this is not possible! We have a fixed number of molecules and a fixed number of coils of spring, so we cannot increase the average density.

In order to satisfy the condition that we do not add any more molecules or spring coils (closed system) we must have it that the displacement goes both "forward" and "backwards".

  • $\begingroup$ Which "Presentation"? could you tell me which-part is not clear? then I could try. I guess you're telling about the diagram. Because, To avoid clumsy-diagram, I had not added some detail labelling. Such as The dots as "Crest" on both sine-like and realistic diagrams. The boxes mean the trough for both cases . Another cause of unclearity I'm not good in English, and I 've made the text too long. $\endgroup$
    – user115953
    Commented Jul 21, 2016 at 18:12
  • $\begingroup$ Your logic seems satisfactory and simple. Btw what may happen if the vibrating source move only between original position to forward direction (and not in backward direction) ? and the opposite (ie. if vibrate between original position to backward direction?) $\endgroup$
    – user115953
    Commented Jul 21, 2016 at 18:20
  • $\begingroup$ By "Presentation" I meant what you presented to us. :) The figure is fine. The text below it was a little hard to follow, but the figure helped. I put up a warning about my understanding in case I answered a question that you didn't ask, which would have made me sound like a fool. :) $\endgroup$
    – garyp
    Commented Jul 21, 2016 at 18:38
  • $\begingroup$ No no I did not complained anything to your understanding :) If I've wrote something foolish , I'm sorry for that :( . Thanks :) $\endgroup$
    – user115953
    Commented Jul 21, 2016 at 18:43

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