The fixed electrostatic earphone is also called electret earphone. Its diaphragm itself is polarized or the electrostatic field is polarized by the polarized material outside the diaphragm, so it does not need special equipment to provide polarization voltage. Electret earphones have most of the characteristics of electrostatic earphones, but the electret will gradually depolarize and need to be replaced. Its service life is about 5-10 years.