  This article investigates the interaction of a spherically symmetric massless
scalar field with a strong gravitational field. It focuses on the propagation
of waves in regions outside any horizons. The two factors acting on the waves
can be identified as a redshift and a backscattering. The influence of
backscattering on the intensity of the outgoing radiation is studied and
rigorous quantitative upper bounds obtained. These show that the total flux may
be decreased if the sources are placed in a region adjoining an apparent
horizon. Backscattering can be neglected in the case $2m_0 /R<< 1$, that is
when the emitter is located at a distance from a black hole much larger than
the Schwarzschild radius. This backscattering may have noticeable astrophysical
consequences.
