Using high resolution inelastic x-ray scattering we studied the collective dynamics of the Pd77Si16.5Cu6.5 metallic glass, focusing on the energy-momentum region where the boson peak appears. The dispersion relation and the width of the acoustic excitations are determined showing how the longitudinal acoustic modes maintain their dispersive character for frequencies well above the boson peak frequencies. Moreover, we prove that close to these frequencies there is a softening of the apparent sound speed indicating a failure of the Debye continuum approximation at the boson peak frequencies and challenging previous results on other metallic glasses.
