The metal used to construct modern optical tables has a higher speed of sound than granite and therefore a higher frequency of the first eigenmode. Any vibration produced on the table below this frequency does not produce a resonant response, making the setup less sensitive to vibrations from motorized optics, cooling water pumps, etc. Vibration damping may be added to tables during their construction. As with granite's composite structure, the combination of several stiff materials with different speeds of sound produces a table for which a wide range of vibrations are critically damped. Viscous fluids are used in between the stiff materials, to aid in damping.