There are many kinds of distortion of loudspeakers, such as harmonic distortion (caused by uneven magnetic field of loudspeaker and distortion of vibration system, often produced at low frequency), Intermodulation distortion (deterioration of sound quality caused by simultaneous addition of two different frequency signals and mutual modulation) and transient distortion (due to the fact that the inertia of vibration system can not keep up with the change of signal, resulting in signal distortion). Harmonic distortion refers to the addition of harmonic components not found in the original signal when replayed. The harmonic distortion of loudspeakers comes from nonlinear distortion such as uneven magnetic field of magnet, characteristics of vibrating film, sound coil displacement and so on. At present, the harmonic distortion index of the better loudspeaker is not more than 5%.