  Using Green's function of semi-infinite Weyl semimetals we find that the
collective charge excitations of Fermi arcs in undoped Weyl semi-metals are
linearly dispersing gapless plasmon modes. The gaplessness comes from proper
consideration of the deep penetration of surface states at the end of Fermi
arcs into the interior of Weyl semimetal. The linear dispersion -- rather than
square root dispersion of {\it pure 2D} electron systems with extended Fermi
surface -- arises from the strong anisotropy introduced by the Fermi arc
itself, due to which the continuum of surface particle-hole excitations in this
system will have strong resemblence to the one-dimensional electron systems.
This places the Fermi arc electron liquid in between the 1D and 2D electron
liquids. Contamination with the particle-hole excitations of the bulk gives
rise to the damping of the Fermi arc sounds.
