Subwoofer crossovers are the settings, whether fixed or variable, that determine which sounds are sent to the subwoofers and which are sent to the main speakers. While settings can be tweaked, the best option can depend on the subwoofer itself, the set-up of the room, and personal taste. There can be potential for confusion if both a subwoofer and a surround sound receiver have separate options for subwoofer crossovers.
The subwoofer is a special form of loudspeaker designed to handle very low frequency sounds, commonly known as bass. Although originally introduced for home stereo systems, they are today most commonly used by consumers in surround sound systems. Subwoofers can appear in car audio set-ups as well, though usually the user has little control over the settings.
A subwoofer crossover is a component in a subwoofer or the stereo system itself, and controls which sounds, or parts of sound, are played by the subwoofer. There are three main types of crossover. A low-pass filter blocks any signal above a defined frequency; high-pass filter blocks any signal below a defined frequency; and a band-pass filter blocks all signals except those that fit into a defined range of frequencies. As there is usually no call to divert any signals with a lower frequency than those handled by a subwoofer, subwoofer crossovers are virtually always a low-pass frequency.
Most surround sound systems have a user setting for the frequency used for subwoofer crossovers. This is often accessible through the settings menu and allows the user to set the crossover of their choice. The most common setting is 80 Hz, and users may find they can improve the sound by altering this setting, for example if they have unusually-sized speakers or are in an echoey room. As a very rough rule of thumb, the higher the crossover frequency, the greater the chance that the listener will be able to detect that the bass sound is coming specifically from the direction of the subwoofer, which can be distracting and spoil the "realism" of the sound mix.
Some subwoofers also have a built-in control to set the crossover frequency, usually as a manual dial. With most set-ups there is no need to use this, and changing it from the default setting may even affect sound quality. The only time to use this is when the subwoofer is connected by standard speaker cables like other speakers, rather than through a dedicated subwoofer cable that is plugged into a specific socket on the surround sound receiver, usually labeled "Sub Out."