Foldback (sound engineering)

Foldback is the use of performer-facing loudspeaker cabinets known as monitor speakers or stage monitors on stage during live music performances in which a PA system or sound reinforcement system is used to amplify the performers' singing, music, speech and other sounds for the audience. Monitor speakers often include a single full-range loudspeaker and a horn in a cabinet. Monitor speakers have numerous features which facilitate their transportation and protection, including handles, metal corner protectors, sturdy felt covering or paint and a metal grille to protect the speaker. There are two types of monitors: passive monitors consist of a loudspeaker and horn in a cabinet (they must be plugged into an external power amplifier); active monitors have a loudspeaker, horn and a power amplifier in a single cabinet, which means the signal from the mixing board can be plugged straight into the monitor speaker.

The sound at popular music and rock music concerts is amplified with power amplifiers through a PA system or sound reinforcement system. With the exception of the smallest venues, such as coffeehouses, most mid- to large-sized venues use two sound systems. The main or "front of house" (FOH) system is a PA system/sound reinforcement system which amplifies the onstage sounds for the main audience. The monitor system consists of monitor speakers aimed at the on-stage performers rather than the audience and power amplifiers. The sound signal for the monitor speakers may be produced on the same mixing console as the main mix for the audience (called the "front of house" mix), which is often the case in small venues, such as pubs where singer/guitarists perform. Monitor systems have a huge range of sizes and complexity. A small pub or nightclub may have a single, 100 watt powered monitor speaker onstage so that the lead vocalist can hear her/his singing, with the "aux send" signal from a small powered mixer plugged straight into the monitor cabinet, and the singer setting her/his own levels with the onstage mixer. On the other hand, a stadium rock concert may use a large number of monitor wedges, big racks of power amplifiers with thousands of watts of power, and a separate mixing board and sound engineer for the monitors.

...
Wikipedia