Arthur Young was the developer of the two blade, semi-rigid, teetering, rotor system and had an exclusive rights agreement with Bell whereas the company could not use any other type of rotor system as long as Arthur was alive. As told to me by a former Bell Helicopter engineer, Arthur was paid a "royalty fee" for every Bell helicopter that was built using his rotor system. When Arthur passed away, Bell immediately went forward with fully articulating rotor systems.
Nodal beam and relatively soft Lord type isolator mounts are two examples that Bell used to help absorb the 2/rev vibration.
Watch the following video on how Wagtails answer to help absorb the 2/rev vibration of their rotor system on the Kriek II tractor gyroplane. As the gyroplane is taxing over uneven ground, one can see (starting at 0:25 seconds) and during flight (starting at 0:45, and again at 1:25 seconds) how the rotor mast is allowed to move about and track to absorb those 2/rev vibrations. Focus one's vision at the rotor mast in between both instrument panels.
With fully articulated rotor systems. the vibration is from a mismatch of individual rotor blade weights and blade tracking.
Wayne