An impact absorber employs dilatant (shear responsive) fluid that is subjected to a controlled, low amplitude, high frequency oscillatory stress which controls the stiffness of the fluid. Piezoelectric transducers, voice coils, and other forms of transducers may be used to apply controlled vibratory stress to the fluid. The energy absorber may be used in protective body armor, medical devices such as splints and casts, vehicle safety absorbers and many other products which benefit from ability to control the stiffness of the energy absorber.