An earphone has a housing with a signal process unit to output sound. The housing has a chamber and a speaker mounted inside the chamber. The chamber is defined as a front volume and a rear volume by the speaker, and the rear volume having a port. Wherein the speaker in the volume outputting a chamber frequency response curve is at least a portion higher than the earphone's ideal frequency response curve at the bandwidth ranging from 20 Hz to 1000 Hz, where creates a gain difference between the chamber frequency response curve and the ideal frequency response curve to be decreased the (superfluous) positive value by a signal process unit at the bandwidth ranging from 20 Hz to 1000 Hz. Therefore, the total acoustic energy is maintained, the earphone acoustic quality is preserved, and the earphone volume may output even higher power to be deduced by the signal process unit for lowering down the circuit power consumption.