Why Stage monitors, and public address system

In Gears, Public address, Sonorisation on January 18, 2011 at 12:06 am

Simply to familiarize ourselves with those strange accronisms in sound engineering. This article is about onstage monitor speakers for live performances.

Foldback (also called “stage monitors”) is the use of rear-facing heavy-duty loudspeakers known as monitor speaker cabinets 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 FOH  ‘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 would 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.

This small venue's stage shows an example of a typical monitor speaker set-up: there are three "wedge" monitors directed towards the area of the stage where singers will be performing. The drummer has both a subwoofer cabinet (for monitoring the bass drum and the electric bass) and a "wedge"-style cabinet for monitoring vocals and mid- or high-frequency sounds

For live sound reproduction during popular music concerts there are typically two complete PA 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. In British English, the monitor system is referred to as the “fold back“. 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.



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