Vibration generation by meshing gear pairs is a significant source of vibration and cabin noise in rotorcraft transmissions. This tonal, high-frequency gearbox noise (500 Hz - 2000 Hz) is primarily transmitted to the fuselage through rigid connections, which do not appreciably attenuate vibratory energy. Because periodically-layered elastomer and metal isolators exhibit transmissibility "stop bands", or frequency ranges in which there is very low transmissibility, they may provide an elegant passive vibration control solution. Realistic design constraints associated with helicopter gearbox isolator mass, axial stiffness, and elastomeric fatigue are estimated. An optimization routine is then used in concert with design constraints and an axisymmetric isolator model to determine layered isolator passive performance limits. The optimization results suggest that layered isolators cannot always be designed to meet target frequencies given a certain set of constraints. Therefore, passive performance enhancements to layered isolators are considered. The use of embedded fluid elements in the metal layers results in a combination of advantageous performance benefits, including motion amplification and vibration absorber effects. The enhanced layered isolators are capable of passively providing broadband noise attenuation, as well as dramatic attenuation at discrete problematic tones.