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Waves on a string, ripples on a pond are transverse waves generated by mechanical energy and in the simplest form oscillations. Sound also is a form of energy and basically oscillations. What makes sound unique in the fact that it is longitudinal ?

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    $\begingroup$ Sound, in a solid, can be transverse as well. $\endgroup$ – Jon Custer Jan 23 at 23:58
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    $\begingroup$ In line with Jon Custer's comment, I think the question should be "why can waves not be transverse in a gas"? $\endgroup$ – BioPhysicist Jan 23 at 23:59
  • $\begingroup$ I think I would add that waves in a gas are pressure waves. Their transport is longitudinal as they are propagated through collisions. Look at how a speaker is moving, it is moving back and forth pushing and pulling the air. The pressure wave will be in the propagation direction. $\endgroup$ – Gwanguy Jan 29 at 22:59
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In order for mechanical waves to propagate there needs to be some form of "restoring force" that tries to bring the system back to equilibrium. For longitudinal waves in gases this restoring force is supplied by pressure in the medium. However, there is no restoring force in a gas for bulk shear movement of the gas particles. Therefore, there is no transverse wave propagation in gases.

However, as mentioned in the comments, sound can also propagate transversely through solids, where there is a restoring force due to shear movement. The fact that both bulk transverse and longitudinal waves can propagate through solids, but only bulk longitudinal waves can propagate through fluids, is important in studying seismic waves, since the Earth is made up of both solid and fluid components.

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    $\begingroup$ To add to this: surface waves on water appear to be transverse not longitudinal, even though water and air are both fluids. In this case the "restoring force" is gravity, which causes the pressure in the water to change with depth. Surface waves are significantly more complicated to model than waves in a single solid or fluid medium. $\endgroup$ – alephzero Jan 24 at 0:40
  • $\begingroup$ @alephzero Right. I didn't really want to touch surface waves, which is why I used the word "bulk" when appropriate. $\endgroup$ – BioPhysicist Jan 24 at 0:45
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    $\begingroup$ @alephzero: …and, indeed, transverse gravity waves (not to be confused with gravitational waves!) can occur even without a liquid surface, e.g. in the Earth's atmosphere. So you can have a kind of bulk transverse wave in a fluid medium, as long as the medium isn't homogeneous and there's some restoring force like gravity maintaining the inhomogeneity. And of course we haven't even got to weird things like magnetohydrodynamic waves yet. $\endgroup$ – Ilmari Karonen Jan 24 at 14:41

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