The rate of structure formation in the Universe is different in homogeneous and clustered dark energy models. The degree of dark energy clustering depends on the magnitude of its and fordark energy clusters in a similar fashion to dark effective sound speed c effmatter while for c eff = 1 it stays (approximately) homogeneous. In this paper we consider two distinct equations of state for the dark energy component, w d = const and w d = w 0 + w 1 1+z with as a free parameter and we try to constrain the dark energy effective sound speed using current available data including Type Ia supernovae, baryon acoustic oscillation, cosmic microwave background shift parameter (Planck and WMAP), Hubble parameter, big bang nucleosynthesis and the growth rate of structures fσ 8 (z). At first we derive the most general form of the equations governing dark matter and dark energy clustering under the assumption const. Finally, performing an overall likelihood analysis we find that the likelihood that however, the dark energy sound speed is degenerate with respect to .function peaks at the cosmological parameters, namely m and w d .
