Foldback is the use of rear-facing loudspeakers known as monitor speakers or stage monitors on stage during live music performances. The sound is amplified with power amplifiers or a public address system and the speakers are aimed at the on-stage performers rather than the audience. This sound signal may be produced on the same mixing console as the main mix for the audience (called the "front of house" mix), or there may be a separate sound engineer and mixing console on or beside the stage creating a separate mix for the monitor system.
Without a foldback system, the sound that onstage performers would hear from front of house would be the reverberated reflections bouncing from the rear wall of the venue. The naturally reflected sound is delayed and distorted, which could, for example, cause the singer to sing out of time with the band. A separate mixed signal is often routed to the foldback speakers, because the performers may also need to hear a mix without electronic effects such as echo and reverb (this is called a "dry mix") to stay in time and in tune with each other. In situations with poor or absent foldback mixes, vocalists may end up singing off-tune or out of time with the band.
For live sound reproduction during popular music concerts there are typically two complete Loudspeaker systems: the "main" system and the "monitor" system. Each system consists of a mixing board, sound processing equipment, amplifiers, and speakers. The two systems usually share microphones and direct inputs using a splitter microphone snake. There is disagreement over when to call these audio systems Sound Reinforcement (SR) systems or Public Address (PA) systems. This distinction is important in some regions or markets, while in other regions or markets the terms are interchangeable.
The "main" system (also known as "Front of House", commonly abbreviated FOH), which provides the amplified sound for the audience, will typically use a number of powerful amplifiers driving a range of large, heavy-duty loudspeakers including low-frequency speaker cabinets called subwoofers, full-range speaker cabinets, and high-range horns. A coffeehouse or small bar where singers perform while accompanying themselves on acoustic guitar may have a relatively small, low-powered PA system for the "mains", such as a pair of two 200 watt powered speakers. A large club may use several power amplifiers to provide 1000 to 2000 watts of power to the "main" speakers. An outdoor rock concert may use racks of a number of power amplifiers to provide 10,000 or more watts.
The "monitor" system reproduces the sounds of the performance and directs them towards the onstage performers (typically using wedge-shaped monitor speaker cabinets), to help them hear the instruments and vocals. The monitor system in a coffeehouse or singer-songwriter stage for a small bar may be a single 100 watt powered monitor wedge. In the smallest PA systems, the performer may set their own "main" and "monitor" sound levels with a simple powered mixing board. The simplest monitor systems consist of a single monitor speaker for the lead vocalist which amplifies their singing voice, so that they can hear it clearly.
In a large club where rock or metal bands play, the monitor system may use racks of power amplifiers and four to six monitor speakers to provide 500 to 1000 watts of power to the "monitor" speakers. In most clubs and larger venues, sound engineers and technicians control the mixing boards for the "main" and "monitor" systems, adjusting the tone, levels, and overall volume of the performance.
Larger clubs and concert venues typically use a more complex type of monitor system which has two or three different monitor mixes for the different performers. Each monitor mix contains a blend of different vocal and instruments, and an amplified speaker is placed in front of the performer. This way the lead vocalist can have a mix which forefronts their vocals, the backup singers can have a mix which emphasizes their backup vocals, and the rhythm section members can have a mix which emphasizes the bass and drums. At an outdoor rock concert, there may be several thousand watts of power going to a complex monitor system that includes wedge-shaped cabinets for vocalists and larger cabinets called "sidefill" cabinets to help the musicians to hear their playing.
In the early 1960s, many professional live sound engineers were wrestling with the problem of giving singers enough of their own voice to stay in tune during a performance. The employment of loudspeakers used for foldback may have been developed independently by sound engineers in different cities who were trying to resolve this problem. The first recorded time that a loudspeaker was used specifically for foldback was for Judy Garland at the San Francisco Civic Auditorium on September 13, 1961; provided by McCune Sound Service. Bob Cavin, then an engineer at McCune Sound, designed the first monitor mixer designed expressly for foldback duties. He also designed the first stage monitor loudspeaker that had two different listening angles, for performers standing at the loudspeaker and for performers further away.
From the 1960s to the 1980s, most monitor speaker cabinets used an external power amplifier. In the 1990s and 2000s, clubs increasingly used powered monitors, which contain an integrated power amplifier. Most monitor speakers include an L pad for controlling the volume of the horn. Another trend of the 2000s was the blurring of the lines between monitor speaker cabinets and regular speaker cabinets; many companies began selling wedge-shaped full-range speakers intended to be used for either monitors or main public address purposes.
Hardshell headphones are typically used by the sound board operator to listen to specific channels or to listen to the entire mix. While an amplified monitor speaker can also be used for this purpose, the high sound volumes in many club settings make hardshell headphones a better choice, because the hard plastic shell and foam cushions help to block the room noise. Some performers may use headphones as monitors, such as drummers in pop music bands.
In the 2000s, some bands and singers have begun using small "in ear"-style headphone monitors. In-ear monitors allow musicians to hear their voice and the other instruments with a clearer, more intelligible sound, because the molded in-ear headphone design blocks out on-stage noise. While some in-ear monitors are "universal fit" designs, some companies also sell custom-made in-ear monitors, which require a fitting by an audiologist. Custom-made in-ear monitors provide an exact fit for a performer's ear.
Drummers typically use a monitor speaker that is capable of loud bass reproduction, so that they can monitor their bass drum. However, having a 15" or even 18" subwoofer producing a high sound pressure level can raise the overall stage volumes to uncomfortable levels for the drummer, since the drums are already very loud. Since much very low bass is felt, some drummers use tactile transducers called "bass shakers", "butt shakers" and "throne shakers" to monitor the timing of their bass drum. The tactile transducers are attached to the drummer's stool ("throne") and the vibrations of the driver are transmitted to the body and then on to the ear in a manner similar to bone conduction. They connect to an amplifier like a normal subwoofer. They can be attached to a large flat surface (for instance a floor or platform) to create a large low frequency conduction area, although the transmission of low frequencies through the feet isn't as efficient as the seat. This helps the concert drummer to monitor his or her kick drum performance without "polluting" the stage with powerful low frequency waves from a subwoofer monitor.
The term "foldback" is sometimes applied to in-ear monitoring systems, also described as artist's cue-mixes, as they are generally set up for individual performers. "Foldback" may less frequently refer to current limiting protection in audio electronic amplifiers.
The term foldback has been used when referring to one or more video monitors facing a stage, in the same manner as an audio foldback monitor. The video monitor allows a person on stage to see what is behind them on screen, to see distant parties during a video conference, or to read notes or sing lyrics to a song. Other terms for this usage are confidence monitor and kicker monitor.