Editing Sound reinforcement system (section)

===Signal processors===
Small PA systems for venues such as bars and clubs are now available with features that were formerly only available on professional-level equipment, such as digital [[reverb]] effects, [[graphic equalizer]]s, and, in some models, [[feedback prevention]] circuits which electronically sense and prevent audio feedback when it becomes a problem. Digital effects units may offer multiple pre-set and variable reverb, [[Delay (audio effect)|echo and related effects]]. Digital loudspeaker management systems offer sound engineers digital delay (to ensure speakers are in sync with each other), limiting, crossover functions, EQ filters, compression and other functions in a single rack-mountable unit. In previous decades, sound engineers typically had to transport a substantial number of rack-mounted analog [[effects unit]] devices to accomplish these tasks.

====Equalizers====
[[Image:Graphic equalizer.jpg|thumb|200px|Graphic equalizer]]
[[Equalization (audio)|Equalizers]] are electronic devices that allow audio engineers to control the tone and frequencies of the sound in a channel, group (e.g., all the mics on a drumkit) or an entire stage's mix. The bass and treble controls on a [[home stereo]] are a simple type of equalizer. Equalizers exist in professional sound reinforcement systems in three forms: shelving equalizers (typically for a whole range of bass and treble frequencies), graphic equalizers and [[parametric equalizer]]s. Graphic equalizers have [[fade (audio engineering)|faders]] (vertical slide controls) which together resemble a frequency response curve plotted on a graph. The faders can be used to boost or cut specific frequency bands.

Using equalizers, frequencies that are too weak, such as a singer with modest projection in their lower register, can be boosted. Frequencies that are too loud, such as a "boomy" sounding [[bass drum]], or an overly resonant [[dreadnought guitar]] can be cut. Sound reinforcement systems typically use graphic equalizers with one-third [[octave]] frequency centers. These are typically used to equalize output signals going to the main loudspeaker system or the monitor speakers on stage. [[Parametric equalization|Parametric equalizers]] are often built into each channel in mixing consoles, typically for the mid-range frequencies. They are also available as separate rack-mount units that can be connected to a mixing board. Parametric equalizers typically use knobs and sometimes buttons. The audio engineer can select which frequency band to cut or boost, and then use additional knobs to adjust how much to cut or boost this frequency range. Parametric equalizers first became popular in the 1970s and have remained the program equalizer of choice for many engineers since then.

A [[high-pass]] (low-cut) and/or [[low-pass]] (high-cut) filter may also be included on equalizers or audio consoles. High-pass and low-pass filters restrict a given channel's [[Bandwidth (signal processing)|bandwidth]] extremes. Cutting very low-frequency sound signals (termed ''[[infrasonic]]'', or ''subsonic'') reduces the waste of amplifier power which does not produce audible sound and which moreover can be hard on the subwoofer drivers. A low-pass filter to cut [[ultrasonic]] energy is useful to prevent interference from radio frequencies, lighting control, or digital circuitry creeping into the power amplifiers. Such filters are often paired with graphic and parametric equalizers to give the audio engineer full control of the frequency range. High-pass filters and low-pass filters used together function as a [[band-pass]] filter, eliminating undesirable frequencies both above and below the auditory spectrum. A [[band-stop filter]], does the opposite. It allows all frequencies to pass except for one band in the middle. A feedback suppressor, using an [[embedded system|microprocessor]], automatically detects the onset of feedback and applies a narrow band-stop filter (a [[notch filter]]) at specific frequency or frequencies pertaining to the feedback.

====Compressors====
[[File:Comp. rack (Supernatural).jpg|thumb|150px|A rack of electronic audio compressors]]
[[Dynamic range compression]] is designed to help the audio engineer to manage the dynamic range of audio signals. Prior to the invention of automatic compressors, audio engineers accomplished the same goal by "riding the faders", listening carefully to the mix and lowering the faders of any singer or instrument which was getting too loud. A compressor accomplishes this by reducing the gain of a signal that is above a defined level (the threshold) by a defined amount determined by the ratio setting. Most compressors available are designed to allow the operator to select a ratio within a range typically between 1:1 and 20:1, with some allowing settings of up to ∞:1. A compressor with high compression ratio is typically referred to as a [[limiter]]. The speed that the compressor adjusts the gain of the signal ([[Envelope (music)|attack and release]]) is typically adjustable as is the final output or ''make-up gain'' of the device.

Compressor applications vary widely. Some applications use limiters for component protection and gain structure control. Artistic signal manipulation using a compressor is a subjective technique widely utilized by mix engineers to improve clarity or to creatively alter the signal in relation to the program material. An example of artistic compression is the typical heavy compression used on the various components of a modern rock drum kit. The drums are processed to be perceived as sounding more punchy and full.

[[File:FOH Racks.jpg|thumb|right|250px|Effect processing rack-mounted units at the FOH position at an outdoor concert.]]

====Noise gates====
A [[noise gate]] mutes signals below a set threshold level. A noise gate's function is in, a sense, opposite to that of a compressor. Noise gates are useful for microphones which will pick up noise that is not relevant to the program, such as the hum of a miked electric guitar amplifier or the rustling of papers on a minister's lectern. Noise gates are also used to process the microphones placed near the drums of a drum kit in many hard rock and metal bands. Without a noise gate, the microphone for a specific instrument such as the floor tom will also pick up signals from nearby drums or cymbals. With a noise gate, the threshold of sensitivity for each microphone on the drum kit can be set so that only the direct strike and subsequent decay of the drum will be heard, not the nearby sounds.

====Effects====
[[Reverberation]] and [[Delay (audio effect)|delay]] effects are widely used in sound reinforcement systems to enhance the sound of the mix and create a desired artistic effect. Reverb and delay add a sense of spaciousness to the sound. Reverb can give the effect of singing voice or instrument being present in anything from a small room to a massive hall, or even in a space that does not exist in the physical world. The use of reverb often goes unnoticed by the audience, as it often sounds more natural than if the signal was left "dry" (without effects).<ref>[http://www.harmony-central.com/Effects/Articles/Reverb/ Reverberation]. Harmony-Central. Retrieved on January 23, 2009.</ref>  Many modern mixing boards designed for live sound include on-board reverb effects.

Other effects include modulation effects such as [[Flanger]], [[Phaser (effect)|phaser]], and [[chorus effect|chorus]] and spectral manipulation or harmonic effects such as the [[Exciter (effect)|exciter]] and [[harmonizer]]. The use of effects in the reproduction of 2010-era pop music is often in an attempt to mimic the sound of the studio version of the artist's music in a live concert setting. For example, an audio engineer may use an [[Auto Tune]] effect to produce unusual vocal sound effects that a singer used on their recordings.

The appropriate type, variation, and level of effects is quite subjective and is often collectively determined by a production's audio engineer, artists, [[bandleader]], [[music producer]], or musical director.

====Feedback suppressor====
A [[feedback suppressor]] detects unwanted [[audio feedback]] and suppresses it, typically by automatically inserting a [[notch filter]] into the signal path of the system. Audio feedback can create unwanted loud, screaming noises that are disruptive to the performance, and can damage speakers and performers' and audience members' ears. Audio feedback from microphones occurs when a microphone is too near a monitor or main speaker and the sound reinforcement system amplifies itself. Audio feedback through a microphone is almost universally regarded as a negative phenomenon, many [[electric guitar]]ists use guitar feedback as part of their performance. This type of feedback is intentional, so the sound engineer does not try to prevent it.