Propagation of scalar field waves interacting with a strong gravitational field exhibits effects of backscattering. Due to this the total flux of radiation can diminish if sources are placed in a region adjoining a compact body. Backscattering can be neglected in the case when the emitter is located at a distance much larger than the Schwarzschild radius. The effect should be detectable in some astrophysical sources of electromagnetic or gravitational radiation.

PACS numbers: 04.30.Nk