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I Can a sound wave be transverse?

  1. Jan 12, 2017 #1
    Hello,
    Sound waves are always introduced as longitudinal mechanical waves: the medium particles oscillate in a direction parallel to the direction of motion of the sound wave. We can only hear sound frequencies between 20Hz and 20KHz. For us to hear these mechanical sound waves, the waves need to be longitudinal to be able to push on our ears' eardrums. A transverse mechanical wave is just a mechanical wave but cannot be called a sound wave. Is that the case?

    As mentioned, there are transverse mechanical waves. For instance, if we hit the surface of a metal rod (not the end) with a hammer we generate transverse mechanical waves in the rod. These transverse waves produce an oscillation of the rod surface which generates longitudinal sound waves in the neighboring air. Is that correct?

    When we hear a train that is far away by placing our ears on the train tracks, it is because a mechanical wave ( longitudinal or transverse?) travels inside the metal track and generates a longitudinal sound wave in the air where the observer is.

    Sound, as a longitudinal wave, can propagate in a liquid, a gas (like air) or inside a solid but the wave needs to be longitudinal to be called sound. That is what I believe.

    Thanks!
     
  2. jcsd
  3. Jan 12, 2017 #2

    davenn

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    .correct

    longitudinal ... it propagates along the track in a longitudinal direction

    again, correct


    one of the few traverse wave sources is from an earthquake rupture ..... longitudinal and traverse waves are produced


    Dave
     
  4. Jan 13, 2017 #3

    Ibix

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    Isn't the point just that you can't get transverse waves in air? There's no mechanism to propagate shear forces in any strength. This is unlike a solid, which has internal structure secured by forces that do propagate shear (e.g. earthquakes, as davenn points out, but also stringed instruments and the like). Of course, we don't really hear these things directly (unless you literally keep your ear to the ground). We hear the longitudinal oscillations excited in the air by the motion of the solid.
     
  5. Jan 13, 2017 #4

    Saw

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    Just to give the answer a little more background:

    To get wave motion in a medium, you need two ingredients: inertia and (it is usually said) "elasticity", although we could also call it "rigidity", since it means that the material can be deformed, but internal forces restore it to its original shape and the material shows more of such property the more "rigid" it is, the more it resists deformation (= the more strongly that it is restored). In turn this has to do with the internal cohesion among its molecules. This quality is measured by the Young modulus (resistance to longitudinal deformation), shear modulus (to transverse deformation) and bulk modulus (to volume deformation) of the material. As you probably know, fluids have less inter-molecular cohesion than solids. This does not deprive them of the capacity to resist longitudinal deformation, but try to imagine deformation and restoration perpendicular to the direction of propagation of the wave: no way, you would get deformation but no restoration... Thus a fluid can be defined as a material having zero shear modulus. So it cannot sustain transverse waves.
     
  6. Jan 13, 2017 #5
    I don't know about defining a fluid in this way. What about a polar fluid for instance?
    I think that is the point also. To go into a bit further, shouldn't it basically boil down to whether the internal contact-type forces (i.e. those which are connected to the stress tensor and permit wave propagation) can be expressed as the gradient of some scalar field or not? For instance, in the case of air the contact forces are associated with local pressure differences and as a consequence there is no way for the system to support rotational force fields, i.e no curl.
    On the other hand if the force fields are the result of vector fields, tensors or spinors etc.. then the system can support transverse force fields and propagate transverse waves. Does this sound reasonable?
     
  7. Jan 14, 2017 #6

    Saw

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    That definition (a material that does not resist shear stress and so changes its shape) is the classical definition of a "fluid" (see wiki here for example). Like any definition, however, I suppose that this assumes some ideal conditions and that certain real fluids show some sort of resistance to shear stress (viscosity, isn't it?) and then there are the cases where that "anomaly" is stronger, like the polar fluids that you mention, so that they live close to the border between fluids and solids.
     
  8. Jan 16, 2017 #7

    Orodruin

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    Viscosity is resistance to strain rate, i.e., changes in strain.

    With the risk of oversimplification: In a viscous fluid, strain rate leads to stress. In a solid, strain leads to stress.

    All waves propagate in the longitudinal direction. This is the definition of the longitudinal direction.
     
  9. Jan 16, 2017 #8

    davenn

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    yes they all propagate forward but the particle motion for a shear/traverse wave is perpendicular to the direction of propagation
    But I'm sure you already knew that :wink::-p

    longitudinal -- compressional wave

    upload_2017-1-16_19-30-37.png


    shear / traverse wave ... one type

    upload_2017-1-16_19-31-14.png

    another type

    upload_2017-1-16_19-31-56.png

    unfortunately they wont do their animation after uploaded

    Dave
     
  10. Jan 16, 2017 #9

    Orodruin

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    The reason I mentioned it was that your post may be read as "transverse waves do not propagate in the longitudinal direction".
     
  11. Jan 16, 2017 #10

    davenn

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    yes, I could have worded it better :smile:

    thanks
     
  12. Jan 16, 2017 #11

    Saw

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    Thanks, I understand that you mean that under shear stress solids deform a given amount, where viscous fluids deform continuously over time, but what I don't grasp is the idea of "strain leading to stress". Isn't it the other way round: stress (force) leads to / causes strain (deformation)?
     
  13. Jan 16, 2017 #12

    Orodruin

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    I understand how you are thinking. If you apply an external force, this will lead to deformation and therefore strains. However, the internal forces are the result of the material being strained (or of a strain rate in the case of a viscous fluid). Consider the non-equilibrium situation (for a solid), the internal forces will depend on the strains, but the strains will not be given directly by the stresses. However, stresses will lead to changes in the strains based on Newton's second law.

    It is the same with a spring. The force in a spring is directly dependent on the extension, but the extension usually follows a second order differential equation, e.g., if you fix a mass to the string. So in a way you are correct, applying a force generally leads to changes in the strain - but the strain directly gives the internal forces.
     
  14. Jan 26, 2017 #13
    Mechanical waves can be generated and propagated in a medium as transverse waves. But sound (a mechanical wave) is interpreted by the receptor as a longitudinal wave.
     
  15. Jan 26, 2017 #14

    Ibix

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    Depends on your medium. Some - e.g. gases - only support longitudinal waves.
    Not really, I think. I hear what I hear without interpreting it as anything. But the word "sound" usually refers to waves in air - which are longitudinal.
     
  16. Jan 26, 2017 #15
    Thanks. Do you mean the longitudinal nature is what makes it sound
     
  17. Jan 26, 2017 #16

    Ibix

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    No. Both longitudinal and transverse oscillations (in mediums which support them) are called sound. But the original question was why sound is always introduced as a longitudinal wave. I think that is because that's what it is in air, which is most familiar type of sound.

    The other point I was making is this. Look at the diagrams @davenn posted in #8. Imagine you have a solenoid (i.e., a microphone) pressed against the square end of the rod. It'll be very insensitive to transverse motion because the end of the rod would slide across the solenoid rather than push it in and out. Similarly, a solenoid pressed to the side would be more or less insensitive to longitudinal oscillation but pick up transverse movement. And combining three solenoids along the three axes gives you sensitivity to both transverse polarisations and the longitudinal mode. So the point I was making was that a sensor can detect longitudinal or transverse waves depending on how it is set up; it doesn't need to "interpret it as a longitudinal wave".

    There's also the philosophical point that you don't actually need to interpret your sensor input as any kind of physical model. We've had working ears for far longer than we've had a physical model of sound. I just hear the music without thinking about the mechanical process by which it's reached me.
     
  18. Jan 26, 2017 #17
    Hi Dave,

    thanks for the reply. How did you create your simulations? Really cool.
     
  19. Jan 26, 2017 #18

    davenn

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    I cant take credit for them
    they are commonly used for explaining seismic waves ... a big interest of mine
     
  20. Jan 26, 2017 #19
    The LOVE wave seems to be just a transverse wave where the plane of oscillation is horizontal Is there more to it? Where are these types of waves generated?

    Besides transverse and longitudinal waves, there are also hybrid waves, like water waves, which are a mixture of transverse and longitudinal motions. The water particles consequently move in a circular fashion CW. Raleigh surface waves are the same but the particles rotate CCW.

    Any other type of wave to add to the list?

    Talking about traveling and standing waves, I have seen waves that move around anphidromic points. Are these standing waves? They seem to be traveling waves trying to catch their tails...
     
  21. Jan 27, 2017 #20
    Do you just want waves that result from transformations of your degrees of freedom through space, e.g. moving a degree of freedom in space and seeing how this propagates; or do we also look at internal transformations if the degrees of freedom have internal structure e.g. spin waves?
     
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