  We consider production of additional relativistic particles coupled to the
inflaton. We show that the imprints of these particles on the spectrum of
primordial perturbations can be used for the direct measurement of the speed of
sound of scalar perturbations, regardless of the mechanism of the production of
this species. We study a model where these relativistic localized sources are
decay products of heavier particles generated via a resonance mechanism. These
particles emit in-phase inflaton particles which interfere constructively on
the the so-called sound boom, leading to an "inflationary Cherenkov effect".
The resulting shock waves lead to distinctive patterns on the temperature
anisotropies of the cosmic microwave background. Moreover, we show that the
model predicts unique features on the power spectrum of curvature perturbation
and sizeable flattened non-gaussianity for a suitable range of parameters.
