  We study the effect of two metallic slabs on the collective dynamics of
electrons in graphene positioned between the two slabs. We show that if the
slabs are perfect conductors the plasmons of graphene display a linear
dispersion relation. The velocity of these acoustic plasmons crucially depends
on the distance between the two metal gates and the graphene sheet. In the case
of generic slabs, the dispersion relation of graphene plasmons is much more
complicated but we find that acoustic plasmons can still be obtained under
specific conditions.
