The dispersion relation of interfacial waves is calculated for a layer
ed almost pure He-3 phase above a He-3-He-4 mixture. Both the real and
imaginary parts of the ripplon frequencies are evaluated by taking in
to account the essential process of viscous attenuation for various va
lues of the He-3-enriched film thickness. We have also considered the
van der Waals coupling between the phases. Starting from the spectrum
in the case of ideal fluids, the evolution of the spectrum of coupled
oscillations of both liquid interfaces is studied by increasing step b
y step the liquid-helium viscosity up to its experimental value. We sh
owed that ripplon excitations are low damped in a wide range of wave n
umbers in contrast to the sound like branch, which is strongly attenua
ted due to viscous forces.