  We investigate experimentally a Bose Einstein condensate placed in a 1D
optical lattice whose phase or amplitude is modulated in a frequency range
resonant with the first bands of the band structure. We study the combined
effect of the strength of interactions and external confinement on the 1 and
2-phonon transitions. We identify lines immune or sensitive to atom-atom
interactions. Experimental results are in good agreement with numerical
simulations. Using the band mapping technique, we get a direct access to the
populations that have undergone $n$-phonon transitions for each modulation
frequency.
