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. Design optimization studies results suggest that layered isolators cannot always be designed to meet target frequencies given a certain set of gearbox isolation constraints. The addition 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. The overall objective of this research effort is to reduce the vibration and noise transmitted into a Bell Model 427 helicopter cabin by meshing transmission gears via high-frequency gearbox isolation. Existing attempts to solve the gearbox noise problem have focused on active add-on, or retrofitted configurations, and do not directly interrupt the vibratory load path. The major supposition of the current effort is that a choke-point vibration control methodology can be employed, wherein all flight loads are transmitted through layered elastomeric and metal isolators before they enter the cabin.