In ideal terms, the vertical vibration response of an optical table on isolators is defined as a single degree-of-freedom system. The system is symmetrical about the vertical (gravity) axis, and a vertical force through the c.g. of the table produces only translation motion. Only one linear deflection parameter is required to define the position of the table in space.
For horizontal forces and motion, since the isolators are beneath the table there is no symmetry with respect to the c.g., and a horizontal force through the table's c.g. will simultaneously cause rotation (tilt) plus translation. Two deflection parameters are required, therefore the horizontal system (in one plane) has two degrees-of-freedom and two resonant frequencies.
The rotation and translation occur simultaneously, therefore it can be shown that these are "coupled" to the vertical resonance with one slightly higher (upper rocking mode) and one slightly lower (lower rocking mode).
These coupled resonances are a function of the inertia properties of the table and its load. There are an infinite variety of possible loads in service, therefore horizontal isolation test data can vary slightly from one application to the next. KSI horizontal data reflect typical service conditions and are presented without editing. We make our vertical natural frequency as low as practical so that the inherent horizontal coupling has negligible effect on high frequency isolation.