  The induced momentum spectrum of soft gluons radiated from a high energy
quark propagating through a QCD medium is derived in the BDMPS formalism. A
calorimetric measurement for the medium dependent energy lost by a jet with
opening angle $\theta_{{\rm cone}}$ is proposed.The fraction of this energy
loss with respect to the integrated one appears to be the relevant
observable.It exhibits a universal behaviour in terms of the variable
$\theta^2_{{\rm cone}} L^3 \hat q$ where $L$ is the size of the medium and
$\hat q$ the transport coefficient. Phenomenological implications for the
differences between cold and hot QCD matter are discussed.
