an apparatus includes a hub , a first disk , a second disk , and a spacer . the hub is supported for relative rotation about a stationary component . the first disk is mounted to the hub with a first dynamic resonance mode associated therewith . the second disk is mounted to the hub with a second dynamic resonance mode associated therewith . the spacer is positioned between the first disk and the second disk , wherein the spacer is operable to cause the first dynamic resonance mode to be different from the second dynamic resonance mode .