A brake dischub connection for connecting a brake disc to a brake disc hub includes supporting elements on the inner circumference of the brake disc. In the invention, second-stage reaction zone of mainly producing unsaturated acids by the catalytic vapor phase oxidation of an unsaturated aldehyde-containing gas mixture produced in a first-stage reaction zone with molecular oxygen is divided into two or more shell spaces by at least one partition. The acoustic tuning member defines a back volume chamber of the driver and includes an acoustic output port for outputting sound from the back volume chamber of the driver to improve an acoustic performance of the earphone. A rotary force need only be applied to the insert to rotate it in the handpiece.