Intermodulation distortion

Far worse than normal distortion is intermodulation distortion, which is also caused by non-linear movement of the voice coil. If the voice coil is required to propagate frequencies of e.g. 50 Hz and 1000 Hz simultaneously, the higher frequency will be distorted because the 50 Hz sound causes a chopped diaphragm movement as a result of the larger excursion. Intermodulation distortion only occurs, however, when sounds of different frequencies are to be produced and the diaphragm is unable to follow the coil current in a linear direction. In contrast to this, the Doppler effect can occur even the signal is linear. IM distortion is highly distracting because it creates frequencies not normally associated with music. Splitting the range of frequencies among several drivers is often a good way to solve this.