Propagation of Sound in Strongly Viscous Liquids (1966) [pdf]

>"Experimental data on anomalous absorption and dispersion of sonic, shear, and electromagnetic waves in strongly viscous liquids show that the relaxation theory of wave propagation is not applicable to these liquids."

This is quite the statement -- if it is true!

Related:

General Wikipedia article on relaxation in Physics:

https://en.wikipedia.org/wiki/Relaxation_(physics)

Relaxation of Fields and Skin Depth:

https://phys.libretexts.org/Bookshelves/Electricity_and_Magn...