Loudspeaker system design

Crossover

Used in multi-driver speaker systems, the crossover is a device that separates the input signal into different frequency ranges suited to each driver. Each driver, therefore, receives the frequency range it was designed for, so the distortion in each driver, and interference between the drivers, is reduced.

Crossovers can be passive or active. A passive crossover is an electronic circuit using a combination of one or more resistors, inductors and non-polar capacitors. These parts are formed into carefully designed networks, and placed between the amplifier and the loudspeaker drivers to divide the amplifier's signal into the necessary frequency bands before being delivered to the individual drivers. Passive crossover circuits need no external power beyond the audio signal itself. An active crossover is an electronic filter circuit which divides the complete signal into individual frequency bands before amplification, thus requiring one amplifier for each bandpass. The active crossover requires an external power supply.

Passive crossovers are generally installed inside speaker boxes and are by far the most common type of crossover for home and low power use. In car audio systems, passive crossovers may be in a small separate box, necessary to accommodate the size of the components used. Passive crossovers may be simple, or quite elaborate, although steep slopes such as 24dB per octave require components of unusually close tolerances. Passive crossovers, like the driver units that they feed, have power handling limits, and have a modest amount of insertion loss as they convert a small portion of the amplifier power into heat. So, when the highest output levels are required, active crossovers may be preferable. Active crossovers may be simple circuits which emulate the response of a passive network, or may be more complex allowing audio adjustments. Active crossovers called Digital Loudspeaker management systems may include facilities for precise alignment of phase and time between frequency bands, equalization, and dynamics (compression and/or limiting) control.

Some hi-fi and professional loudspeaker systems now include an active crossover circuit as part of an onboard amplifier system. These designs are identifiable by their need for AC power in addition to a signal cable. This 'active' topology may also include driver protection circuits, and other features of a digital loudspeaker management system. Powered speaker systems are common in computer sound (for a single listener) and, at the other end of the size spectrum, in concert sound systems. Powered speaker systems for concert sound, by virtue of no external adjustments, have the potential to provide predictabile, if not necessarily good, sound quality by removing control of crossover, delay and limiter settings from the concert sound engineer.