fluids have bulk

The waves considered above, transverse or
longitudinal, are travelling or progressive waves
since they travel from one part of the medium
to another. The material medium as a whole
does not move, as already noted. A stream, for
example, constitutes motion of water as a whole.
In a water wave, it is the disturbance that moves,
not water as a whole. Likewise a wind (motion
of air as a whole) should not be confused with a
sound wave which is a propagation of
disturbance (in pressure density) in air, without
the motion of air medium as a whole.

Mechanical waves are related to the elastic
properties of the medium. In transverse waves,
the constituents of the medium oscillate
perpendicular to wave motion causing change
is shape. That is, each element of the medium
in subject to shearing stress. Solids and strings
have shear modulus, that is they sustain
shearing stress. Fluids have no shape of their
own - they yield to shearing stress. This is why
transverse waves are possible in solids and
strings (under tension) but not in fluids.

However, solids as well as fluids have bulk
modulus, that is, they can sustain compressive
strain. Since longitudinal waves involve
compressive stress (pressure), they can be
propagated through solids and fluids. Thus a
steel bar possessing both bulk and sheer elastic
moduli can propagate longitudinal as well as
transverse waves.