Systems, methods and apparatus are disclosed for amplifiers for wireless power transfer. A pressure equilibrium means includes a side air passage through which air within a tube or air within a speaker unit in the canal type earphone is discharged to a side surface of a gasket or a side surface of the speaker unit, and a rear air passage through which air in the side surface of the speaker unit is discharged to the rear of the speaker unit. The data is tabulated and indexed in a function data table according to the program segments.