  We introduce two new effective quantities for the study of comoving curvature
perturbations $\zeta$: the space dependent effective sound speed (SESS) and the
momentum dependent effective sound speed (MESS) . We use the SESS and the MESS
to derive a new set of equations which can be applied to any system described
by an effective stress-energy-momentum tensor (EST), including multi-fields
systems, supergravity and modified gravity theories. We show that this approach
is completely equivalent to the standard one and it has the advantage of
requiring to solve only one differential equation for $\zeta$ instead of a
system, without the need of explicitly computing the evolution of entropy
perturbations. The equations are valid for perturbations respect to any
arbitrary flat spatially homogeneous background, including any inflationary and
bounce model.
  As an application we derive the equation for $\zeta$ for multi-fields $KGB$
models and show that observed features of the primordial curvature perturbation
spectrum are compatible with the effects of an appropriate local variation of
the MESS in momentum space. The MESS is the natural quantity to parametrize in
a model independent way the effects produced on curvature perturbations by
multi-fields systems, particle production and modified gravity theories and
could be conveniently used in the analysis of LSS observations, such as the
ones from the upcoming EUCLID mission or CMB radiation measurements.
