A low-alloy heat-resistant steel may be used to manufacturing a large element which has uniform superior high temperature properties through a surface layer to a center part. In addition, the closed-control loop design provides stabilization and isolation of the end effector from external disturbances. The spacers are disposed between the conductive substrate and the electret diaphragm to define an acoustic projecting portion and two acoustic receiving portions on the electret diaphragm. The lever defines a cam channel for guiding the cam projection therein to mate the internal and external connector housings with each other.