Harrison, Sykes and Matins article «Wave effects in Insulation Mounts» in JASA from feb 1952 give models and measurement results for vibration insulation transfer curves showing more than one resonance.

I think I saw that reference in a more recent paper by Charlotte Crispin on floating floor models along with with more modern calculation methodes.

Harrison et al use two terms for viscosity, longtudinal viscosity and a shear viscosity of the vibration insulating material, assumed to be a kind of rubber. They appear to have high expertise in material properties and may have measured these in their lab on their own.

However, regular «googling» only have lead to data for extrusion of rubber, that is as hot melted material and not as dry cold dampers

I am used to work with the loss factor, or any damping parameter, as a part of the vibration insulation, and assume the viscosities relates to damping or losses in some form. It seem as if the «damping» is frequency dependant. For a standard vicous damping model the used damping term is usually a frequency independant constant.

I would be very interested in sources for shear viscosity data for damping materials, or a guide to theory or literature that shed light on this. Or a method to measure it.