  We have investigated the energy loss of hot electrons in metallic graphene by
means of GHz noise thermometry at liquid helium temperature. We observe the
electronic temperature T / V at low bias in agreement with the heat diffusion
to the leads described by the Wiedemann-Franz law. We report on
$T\propto\sqrt{V}$ behavior at high bias, which corresponds to a T4 dependence
of the cooling power. This is the signature of a 2D acoustic phonon cooling
mechanism. From a heat equation analysis of the two regimes we extract accurate
values of the electron-acoustic phonon coupling constant $\Sigma$ in monolayer
graphene. Our measurements point to an important effect of lattice disorder in
the reduction of $\Sigma$, not yet considered by theory. Moreover, our study
provides a strong and firm support to the rising field of graphene bolometric
detectors.
