  We study experimentally the impulse response of a cylinder wake below the
critical Reynolds number of the B\'enard-von K\'arm\'an instability. In this
subcritical regime, a localized inhomogeneous region of convective instability
exists which causes initial perturbations to be transiently amplified. The aim
of this work is to quantify the evolution resulting from this convective
instability using two-dimensional particle image velocimetry in a hydrodynamic
tunnel experiment. The velocity fields allow us to describe the evolution of
wave packets in terms of two control parameters: the Reynolds number and the
magnitude of the imposed perturbation. The temporal evolution of energy
exhibits a transient algebraic growth at short times followed by an exponential
decay.
