The speaker voice coil in the magnetic gap back up and down vibration due to the effect of the voice coil inductance when the audio signal input to the speaker.Then is induced in the voice coil out a reverse with the audio signal of induction electromotive force. The reverse with the audio signal of induction electromotive force will weaken the current in the voice coil. So that the impedance of the voice coil, as audio signal rise the effect will be more and more big.This law that causes the impedance of the speaker unit to vary with frequency is called the impedance characteristic of the speaker.
