  In this short paper we consider a semi-linear, energy sub-critical,
defocusing wave equation $\partial_t^2 u - \Delta u = - |u|^{p -1} u$ in the
3-dimensional space with $p \in (3,5)$. We prove that if the energy of radial
initial data $(u_0, u_1)$ outside a ball of radius $r$ centred at the origin
decays faster than a certain rate $r^{-\kappa(p)}$, then the corresponding
solution $u$ must scatter in both two time directions. The main tool of our
proof is a more detailed version of the classic Morawetz estimate.
