Basic Training for Sound System Operators
Articles from Pro Comm's Quarterly Newsletter

A crossover is a device used to separate an audio signal into two or more component signals above and below a certain frequency, said to be the crossover frequency or crossover point. Crossovers may be active or passive.

A passive crossover is built into most speaker cabinets in order to separate bands of frequencies from the full range speaker level signal, and routing those bands of frequencies to the proper speaker or driver.  Most use iron in the inductors to decrease their size, which can be a source of distortion due to the non-linearities in the coil from core saturation. Another potential problem with passive crossovers is that the signal going to the high frequency driver must be reduced, or attenuated, due to the increased efficiency of the high frequency driver as compared to a bass driver. This power must go somewhere.  Usually it is converted into heat through the use of resistors. 

Electronic crossovers do not have this problem because only the power needed by the driver must be generated by the amplifier.  An active crossover is employed when bi-amping a system.  The active crossover separates the audio spectrum (full range) into bands of frequencies above (high pass) and below (low pass) a certain frequency (x-over point.)  The low pass is rolled off (attenuated) so many dB per octave above the crossover frequency.  The high pass is rolled off (attenuated) below a certain crossover frequency at a rate of so many dB per octave.  The high pass and low pass outputs of the electronic (active) crossover are connected to the inputs of two separate power amplifiers whose respective outputs are used to drive the high end (horns) or low end (woofers) of a sound system.