Audio Crossover
Audio crossovers are a class of electronic filter used in audio applications. Most individual loudspeaker drivers are incapable of covering the entire audio spectrum from low frequencies to high frequencies with acceptable relative volume and lack of distortion so most hi-fi speaker systems use a combination of multiple loudspeakers drivers, each catering to a different frequency band. Crossovers split the audio signal into separate frequency bands that can be separately routed to loudspeakers optimized for those bands.
Active crossovers allow drivers covering different frequency ranges to be powered by separate amplifiers, a configuration known as bi-amping.
Signal crossovers allow the audio signal to be split into bands that are adjusted (equalized, compressed, echoed, etc.) separately before they are mixed together again. Some examples are: multiband dynamics (compression, limiting, de-essing), multiband distortion, bass enhancement, high frequency exciters, and noise reduction (for example: Dolby A noise reduction).
Read more about Audio Crossover: Overview