  We study the formation of Faraday waves in an elongated Bose-Einstein
condensate in presence of a one-dimensional optical lattice, where phonons are
parametrically excited by modulating the radial confinement of the condensate.
For very shallow optical lattices, phonons with a well-defined wave vector
propagate along the condensate, as in the absence of the lattice, and we
observe the formation of a Faraday pattern. By increasing the potential depth,
the local sound velocity decreases and when it equals the condensate local
phase velocity, the condensate becomes dynamically unstable and the parametric
excitation of Faraday waves is suppressed.
