A riser sleeve apparatus for preparing and insulating a passage within which molten metal is accumulated prior to its movement into a mold cavity which includes an inner surface which longitudinally extends between a first end and a second end wherein the inner surface bounds the passage. The ultrasound generator comprises a regulation means which has a regulation member connected upstream of the ultrasound generator to receive a feedback signal from the resonant system and to generate a regulation variable which is supplied to the ultrasound generator. The visor is capable of being disposed such that at least a portion of the visor is located outside a field of view of the operator. In addition, when the optical system seeks tracks for reading required data under a constant angular velocity modulation mode, the PLL circuit will increase the locked frequency gradually as the pick-up head moves towards the outer tracks of the optical disc, and decrease the locked frequency gradually as the pick-up head moves towards the inner tracks of the optical disc. A carriage bracket is slidably attached to each track and has a foot support extending therefrom. A mitral ring may have a higher anterior saddle relative to a posterior saddle, with the relative heights varying across the ring sizes.