Polarization of sound

Sound can't be polarized because the vibration of such type can't be polarized i.e, it can't be limited or controlled by any barriers and so polarization is not possible in them.

This is what my teacher answered me when i asked the question. But i didn't understand what did he mean by "the vibration can't be controlled or limited." Does the word cant be limited or controlled make sense here? Moreover can anybody explain in details and more clearly to me?

It sounds like your teacher's explanation might have been a little misleading. The reason sound can't be polarised is that it is a longitudinal wave, unlike light which is a transverse wave. (Those links have some animated diagrams that should help to make the difference clear.)

"Transverse" means that if a beam of light is coming towards you, the electromagnetic field is vibrating either from side to side or up and down. Unpolarised light is doing a mixture of those two things, but a polarising filter puts it into a more "pure" state, so that it's only going side to side, or only going up and down. (Or diagonally or whatever. There's also a third possibility, called circular polarisation, which is a special combination of the two.) On the other hand, "Longitudinal" means that if a sound wave is coming towards you, the air molecules are vibrating forwards and backwards, not side to side or up and down. Sound waves cannot be polarised because because they don't have any side-to-side or up-and-down motion, only front-to-back.

• could it be summed in: in transverse waves the direction of propagation of energy and the direction of the electromagnetic "vibration" carrying that energy are perpendicular to each other, allowing for an extra degree of freedom. Longitudinal waves the direction of vibration and the direction of propagation of energy are the same, and cannot be unscrambled. Apr 30 '13 at 8:13

I am not sure exactly what your teacher meant, but to me the simple explanation is simply that sound is a pressure wave. Pressure has no direction, only a single value (scalar) and sound waves are fast periodic modulations of the pressure.

A wave on a string can have polarization because the string can be distorted in more than one "direction". It can have an amplitude and a direction of distortion. Similarly, light has polarization because it is a distortion (or variation) of a vector-field (electric and magnetic fields are vector-fields).

Perhaps your teacher meant, that you can choose a direction for a wave on a string for example, thereby constraining it. So in this sense, if there is enough freedom for the wave (i.e. the wave is disturbing more than one direction) such that it can be constrained, it can carry polarization.

It would be difficult to imagine a polarisation of a longitudinal wave, but this certainly is true for transverse waves. The air couples weakly to transverse waves and so does your ear! Hence, we often consider sound waves (in the air) as longitudinal only, that would not be polarised.

Generally, though, sound waves can certainly be transverse and polarised. See for example a shear-stress mode in quartz.

That happens because electromagnetic waves are consist of electric and magnetic parts

which are running in orthogonal planes. And usually they run in all directions. When You polarize light - You let waves go just in one plane.

But air waves are just vibration of the same matter. You can not take air waves just in one plane.

the general modern consensus is that sound does not have polarization possible due to lack of shear forces in air required for transverse wave propagation. however here is an old forgotten study that reports measuring a weak effect by a professor of physics at Ohio State university and published in Science magazine. wonder if there is any more modern analysis of this? could there be more modern/ lowcost versions/setups or reformulations of this experiment that measure the effect?

It is simple to understand geometrically: light waves vibrations are 3D which, can be confined to a 2D plane. But sound waves vibration are 1D and they cannot be confined to particular 2D plane.

The statement,

Sound can't be polarized because the vibration of such type can't be polarized i.e, it can't be limited or controlled by any barriers and so polarization is not possible in them.

is not correct. Of course sound can be limited or controlled by barriers. Sound can be absorbed and reflected, and even refracted. But it's true that sound is not (typically) polarized when propagating in air. The reason is, as stated by others answering this question, that air cannot transmit shear forces effectively: all it can transmit is pressure forces.

Sound (that is, acoustic waves, which are elastic deformations of a medium) can be polarized in a solid, in a viscous liquid, or on a surface, because those media are able to transmit shear forces in addition to pressure forces.