  We consider in this paper some class of perturbation for the semilinear wave
equation with subcritical (in the conformal transform sense) power
nonlinearity. We first derive a Lyapunov functional in similarity variables and
then use it to derive the blow-up rate. Though the result is similar to the
unperturbed case in its statements, this is not the case of our method, which
is new up to our knowledge.
