In order for a sound source to be reproduced correctly, different size and types of speakers are used together. Each speaker driver in a system is responsible for a range of frequencies. The larger the speaker, the longer the wavelength and so the lower the frequency it is capable of producing. Smaller diaphragm speakers (tweeters) produce a very short wavelength and as such produce higher frequencies. In order for each speaker to receive the correct signal, a crossover is used.
The term ‘crossover’ is used as it describes what happens to the original audio source before reaching the speakers. As the signal passes through the crossover, the frequencies are filtered to effectively ‘crossover’ into each driver. So, as the bass frequencies are fed to the woofers, the next set of frequencies crossover into the next driver and so on until the full audio spectrum is accounted for.
A passive crossover is common in domestic Hi-Fi speaker systems. It is an electronic device that usually sits on the back wall of the speaker cabinet. The signal comes into the crossover from one amplifier and is filtered before being sent to each driver. Each driver connects to this crossover with short cables within the enclosure.
More common in high-quality home systems and pro audio systems, the active crossover splits and filters the signal for each speaker driver BEFORE it enters the amplifiers. This way, more precise control with low voltage electronics is possible. The signal from the pre-amp is fed into the active crossover where it then outputs one signal to each amplifier for each individual speaker driver. So, for example, a stereo 3 way (woofer, mid, tweeter) speaker system will require 3 stereo amplifiers. One amplifier drives the woofers, one drives the mid range, one drives the tweeters. Unlike multi-channel home theatre setups where passive crossovers are generally used and one cable is connected to each speaker box, one active speaker enclosure can sometimes require 3 or 4 sets of cables coming from the amplifiers!