  The low-energy part of the vibration spectrum in KNbO$_3$ was studied by cold
neutron inelastic scattering in the cubic phase. In addition to acoustic
phonons, we observe strong diffuse scattering, which consists of two
components. The first one is quasi-static and has a temperature-independent
intensity. The second component appears as quasi-elastic scattering in the
neutron spectrum indicating a dynamic origin. From analysis of the inelastic
data we conclude that the quasi-elastic component and the acoustic phonon are
mutually coupled. The susceptibility associated with the quasi-elastic
component grows as the temperature approaches T$_C$.
