# 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?

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.