Why can't longitudinal waves be polarised? I understand why transverse waves can be polarised because their oscillations can be blocked by a polarizer. But, why can't longitudinal waves be polarised? Are there no polarizers, or something similar to that available for longitudinal waves?
 A: With transverse waves, there is a choice in which direction (in which plane) the oscillations occur. For instance, let the transverse wave move in $z$-direction. Then the oscillations could be for instance in the $x-z$-plane, or they could be in the $y-z$-plane or they could be anywhere inbetween. In order to distinguish between these different waves (i.e. waves with oscillations in different directions), physicists introduce a parameter called "polarization" which describes the geometrical orientation of oscillations.
With longitudinal waves on the other hand, the oscillations always occur in only one direction, namely along the wave. There is no need to distinguish different oscillations direction, because there is only one oscillation direction. Therefore it does not make much sense to speak of "polarization" of longitudinal waves, because those waves are fully described by wavelength/frequency/velocity.
A: Longitudinal waves aren't polar. The oscillation of the medium is the same direction as the motion of the wave. There aren't any other oscillations to 'filter out.'
A: The particles in a longitudinal wave vibrate in the same direction that the wave travels in. Hence there is no possibility to isolate a particular direction of vibration from it. Thus polarisation is not possible in longitudinal waves. 
