Stage monitor system
A stage monitor system is performer-facing loudspeakers 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. In Britain the term foldback is often used to describe the system. Monitor speakers are useful when amplified instruments are used with acoustic instruments and voice. 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. They are driven by a separate mix from the main front of house system. This mix typically highlights the vocals and acoustic instruments so they can be heard over the electronic instruments and drums. 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.
In most mid- to large-size venues, there is a separate sound engineer and mixing console on or beside the stage creating a mix for the monitor system. The monitor mix is often different from the "front of house" mix, because performers may request to hear more of certain accompaniment or rhythm section instruments. In the most sophisticated and expensive monitor set-ups, each onstage performer can ask the sound engineer for a separate monitor mix for separate monitors. For example, the lead singer can ask to hear mostly her/his voice in the monitor in front of her/him and the guitarist can ask to hear mostly the bassist and drummer in her/his monitor.
Without a foldback system, the sound that on-stage 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 in mid- to large-size venues, there are typically two complete loudspeaker systems and PA systems (also called sound reinforcement systems): the "main" or "front of house" system and the "monitor" system. Each system consists of a mixing board, sound processing equipment, power 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 loudspeaker cabinets 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 large 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 on-stage 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 large venues, there are often separate monitors for the vocalists and instrumentalists. In most clubs and larger venues, sound engineers and technicians control the mixing boards for the "main" and "monitor" systems, adjusting the tone, sound 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 and singing. In the 2000s, major professional bands and singers often use small "in ear"-style headphone monitors rather than onstage monitor speakers.
In the early 1960s, many pop and rock concerts were performed without monitor speakers. In the early 1960s, PA systems were typically low-powered units which could only be used for the vocals. The PA systems during this era were not used to amplify the electric instruments on stage; each performer was expected to bring a powerful amplifier and speaker system to make their electric guitar, electric bass, Hammond organ or electric piano loud enough to hear onstage and to fill the venue with sound. Since many early 1960s PA systems did not have monitors, singers could only hear their vocals by listening to the reflected sound from the audience-facing "front of house" speakers after it bounced back from the rear wall of the venue. This was not an effective way to hear one's vocals, though, because the sound you hear bouncing back from the rear wall of the venue is late. Because singers could not hear their vocals, it made it hard to sing in rhythm with the band and in tune.
At this time, many professional live sound engineers were wrestling with the problem of how to give singers enough volume of their own voice to stay in tune and in time during a performance. The use of performer-facing loudspeakers for foldback or monitoring 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 for foldback (monitoring) was for Judy Garland at the San Francisco Civic Auditorium on September 13, 1961; provided by McCune Sound Service.
Early stage monitors were simply speakers on each side of the stage pointed at the performers driven by the same mix as the FOH because PA mixers rarely had auxiliary send mixes. Today these would be called "side fill" monitors. Bill Hanley working with Neil Young of the Buffalo Springfield was the first person to combine putting the speaker on the floor angled up at the performer with directional microphones to allow louder volumes with less feedback.
Bob Cavin, a former engineer at McCune Sound, designed the first monitor mixer designed expressly for stage monitoring. He also designed the first stage monitor loudspeaker that had two different listening angles.
With the introduction of monitor speakers, it made it much easier for performers to hear their singing and playing on stage, which helped to improve the quality of live performances. Singers who can hear their singing clearly through monitor speakers can sing more in tune and more in rhythm with the backup band. As well, a singer who has a good monitor system does not have to "oversing" or strain her/his voice to try be heard. Monitor systems also helped rhythm section instrumentalists to improve their playing. With a good onstage monitoring system, even if there is a huge stage (e.g., at a stadium rock concert), and the musicians are far apart, the keyboardist can have a monitor speaker which reproduces the bass, rhythm guitar and drums, which helps her to play more in rhythm with the other band members.
From the late 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, a potentiometer (knob) 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. The introduction of generic stage speakers provides sound engineers with more flexibility to make changes to the sound system set-up, because if a venue has 10 generic "front of house"/monitor speakers, if the sound check reveals that the three monitor speakers are not providing enough monitoring volume (even at their maximum level), then the sound engineer can take one of the "front of house" speakers and put it on stage as an additional monitor.
The stage monitoring systemEdit
The monitor system consists of the monitor mix, equalization or other signal processing, amplifiers, and monitor speakers on stage pointing at the performers. Microphones and direct inputs are shared with the front of house or "FOH" system.
Front of House auxiliary speakerEdit
The simplest monitor system is a speaker pointed at the performer powered by the FOH mix. This might be used by one or two performers in a coffee house, small club, or small house of worship. In this setting a two channel powered mixer might be used with one channel powering the main speakers and one channel powering the monitor speaker. The mixer would be on stage with the performers setting their own levels.
Monitors mixed from Front of HouseEdit
The more common monitor system is one that uses one or more separate auxiliary mixes or sub-mixes on the FOH mixing board. These mixes are pre-fader and pre-input equalization so that changes to the FOH levels and equalization do not affect what the performers hear on stage. These mixes then drive dedicated monitor equalizers and signal processors which in turn drive dedicated monitor amplifiers which power the monitor speakers. The FOH mixer is operated by a sound engineer located in the audience who also adjusts the monitor mix for the performers.
Separate monitor mixerEdit
Larger clubs, concerts and houses of worship will use a completely separate system for monitors with its own mixer and monitor sound engineer. In this case a splitter is used to split the signal from the microphones and direct inputs between the monitor mixer and the FOH mixer. This splitter may be part of the microphone snake or it may be built into the monitor mixer. With a separate monitor system there may be 8, 12, or more separate monitor mixes, typically one per performer. Each monitor mix contains a blend of different vocal and instruments, and a floor monitor speaker or "wedge" 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. In addition there may be "side-fill" monitors to provide sound for areas on stage not covered by the floor wedges.
An innovation first used in recording studios is to use small mixers placed next to the performer so that they can adjust their own mix. The simplest of these are four-channel mini-mixers like the Furman HR-6 Personal Mixing Station The mixers are driven by four sub-mixes on the FOH console with each sub-mix having a subset of the inputs on stage. For example, mix 1 vocals, mix 2 guitars, mix 3 keyboards, and mix 4 drums and bass. The performers could then adjust these four groups to their own preference. If the balance between several vocals or the balance between bass and drums needed to be changed, the sound engineer would have to change it at the main mixing board.
A recent variation on this is to add an additional input to each mixer which is the performer's instrument or vocal microphone so that each performer can add more of their performance to the other sub-mixes. This approach has been called "more me" in the monitors. A simple example of this is the Rolls PM50s Personal Monitor Amplifier, which combines a monitor mix with a local input to drive a headphone amplifier. A more sophisticated example is the "hear back" system by Hear Technologies.
With advances in digital technology, it is now possible to transmit multiple audio channels over a single Ethernet cable. This allows the distribution of most or all of the input sources to each performer's mixer, giving them complete control over their mix. An example of this is the Aviom A-16II personal mixer.
Monitor speakers come in two types: floor monitors and side fill monitors. Monitor speakers are normally heavy duty speakers that can accept high input power to create high volumes and withstand extreme electrical and physical abuse.
Floor monitors are compact speakers with an angled back that is laid on the floor. This angled shape gives the floor monitor its other name of wedge. The angle is typically 30 degrees which points the speaker back and up towards the performer. These speakers may be single small speakers such as the "hot spot" monitor by Galaxy Audio which are sometimes mounted on a microphone stand to get them closer to the performers' ears. More often they are heavy duty two way systems with a woofer and a high frequency horn. A small floor monitor might use a 12" woofer with an integrated high frequency horn/driver combination such as the JBL 4602A floor monitor. A large floor monitor might use one or two 15" woofers and a high frequency driver attached to a high frequency horn such as the EAW SM159zi stage monitor The speaker might use a passive crossover or might be bi-amped with an active crossover and separate amplifiers for the woofer and high frequency driver.
A recent trend has been to build the amplifier and associated sound processing equipment into the monitor itself. These monitors are called "active" or "powered" monitors. This design allows amplifiers with the right amount of power to be custom made for the speakers. Active monitors are typically bi-amped and have an active crossover with custom equalization to tune the monitor to have a flat frequency response. One of the first examples of this type of monitor is the Meyer Sound Laboratories UM-1P.
Side-fill monitors are monitors that sit upright on the side of the stage and are used to provide sound to the areas of the stage not covered by the floor monitors. Side fill monitors are typically standard FOH speakers. A special case of a sidefill monitor is a "drum fill". Drum fills are typically large 2- or 3-way speakers with one or more large woofers capable of extremely high volumes.
Monitor amplifiers are heavy-duty amplifiers, typically 200 watts per channel or more. If the monitor speakers are using passive crossovers, then each channel is connected to one or more monitor speakers. In simple monitor configurations, a single monitor mix might drive both channels of the amplifier so that the single mix can be heard by most of the band by using multiple speakers. In more complex monitor configurations where each performer has a separate mix, then each mix will drive a separate channel on the amplifier. If the monitors are bi-amped, then two or more amplifier channels are needed for each speaker. This may be a single channel of two different amplifiers, a high-power amp for the woofer and a medium-power amp for the high-frequency driver, or it may be two channels of a high-power amp. A key criteria of a monitor amp is reliability. They must be able to survive the abuse of their output being shorted or speakers being plugged and un-plugged while the amp is turned on. They must continue to function while being overloaded for extended periods of time.
Equalization and signal processingEdit
Monitor speakers need their own equalization primarily to reduce or eliminate acoustic feedback. One of the main problems affecting monitors is acoustic feedback or "ringing". Acoustic feedback occurs when the time delay between the acoustic input of a microphone and the output of a monitor speaker is a multiple of the period of a frequency. When this occurs the acoustic output of the speaker is picked up by the microphone and amplified again by the monitor speaker. This is a positive feedback loop that reinforces the specific frequency, causing the speaker to howl or squeal. Equalization is used to attenuate the specific frequency that is feeding back.
The process of eliminating feedback in the monitor is called ringing out the monitors. To eliminate feedback, the monitor's level is increased until it starts to feed back. The feedback frequency is identified either by ear or by a frequency analyzer. Equalization is used to reduce that frequency. The monitor level is again increased until the next frequency starts to feed back and that frequency is eliminated. Normally, eliminating the first four or five feedback frequencies is all that is needed. If multiple monitor mixes are being used, the process has to be repeated for each separate monitor mix.
The most common equalizer used in monitor systems are graphic equalizers. They get their name from the slide potentiometers or "sliders" used to adjust the level of each frequency band. Graphic equalizers are fixed frequency equalizers. The center frequency of each band can not be adjusted. The bandwidth or Q of each band can either be 1/3, 2/3 or one octave, giving a 31-band, 15-band, or 10-band graphic equalizer. The narrower the band the more precisely the feedback frequency can be isolated. Normally 31-band equalizers are used, such as a Klark Teknik DN360 or a DBX 1231. Taller units are preferred since the longer slider length gives more precise control over level adjustments.
A variation on the graphic equalizer is a "cut only" graphic equalizer. Since most of the time, monitor equalization involves the removing of frequencies, a cut only equalizer can give you more precise level adjustments since the entire travel of the slider is used for reducing the level rather than wasting half the travel for boost. An example of this is the UREI 539.
One of the advantages of graphic equalizers is their simplicity of use. When ringing the monitors, a person can boost then restore each frequency band until the ringing starts.  This helps you identify the feedback frequency. A drawback of graphic equalizers is the fixed frequency bands. Feedback rarely occurs on the exact center of the frequency band so two adjacent frequency bands may have to be reduced in parallel to eliminate the feedback.
1/3 octave RTA
A second type of equalizer used in monitor systems are parametric equalizers. A parametric equalizer does not use fixed frequency bands. Instead each frequency band can be adjusted. The center frequency can be adjusted over a several-octave range. The bandwidth of each band can be adjusted from a wide Q affecting several octaves to a narrow Q affecting 1/3rd, 1/6th, or less of an octave, and the level of the band can be adjusted. Each band may have a different frequency sweep range, with the left or lower bands sweeping the lower octaves, the middle bands sweeping the middle octaves, and the right or higher bands sweeping the higher octaves. There is normally a lot of overlap between bands. Parametric equalizers typically have 3 to 5 filtering bands per channel. Examples of parametric equalizers are Rane PE 55 and Klark Teknik DN410.
The advantage of using parametric equalizers in a monitor system is that the filter can be exactly adjusted to the specific feedback frequency, and the bandwidth of the filter can be set to be very narrow so the adjustment affects as little of the frequency band as possible. This leads to more precise feedback elimination with less coloring of the sound. For this reason many professionals recommend using parametric equalizers over graphic equalizers for monitors.
The process of using a parametric equalizer is different from when using a graphic equalizer. When using a parametric equalizer the first step is to choose the band to use. Normally the first feedback frequency is in the lower mid-range so the second band would be a good choice. If the feedback frequency is in the upper mid-range, then the 3rd or 4th band would be a good choice. Next adjust the Q of the filter to be as narrow as possible and boost the frequency by 6 to 9 db. Raise the level of the monitor until it just begins to feedback, lower by 3 db or so. Now sweep the frequency of the filter until the monitor feeds back. Sweep it back and forth over the feedback frequency to find the center frequency by finding the lower and upper frequency of the ring and setting it to the middle between these two frequencies. You may need to drop the gain on the frequency if the feedback is to loud. You repeat the process for the next and the next feedback frequencies. You may discover that the order of the frequencies does not increase left to right. For example the sequence might be 250 Hz, 800 Hz, 500 Hz, 2.6 kHz, and 1.7 kHz. While this may look confusing, it is OK. You can reorder them if you like at the cost of extra work on your part.
A notch filter is a semi-parametric equalizer where the bandwidth is set very narrow, a 1/6 an octave or less and is a cut only filter. An example is a UREI 562 Feedback Suppressor and the Ashly SC-68 Parametric Notch Filter.
Automatic digital notch filterEdit
Digital signal processing has allowed the integration of a graphic equalizer, a parametric equalizer, automatic notch filter and active crossover into a single unit called a digital signal processor or a speaker processor.
Other signal processingEdit
Monitor mixers allow musicians to be provided with a stage mix. This can be controlled by the sound engineer or by the band, depending on the exact mix and the amount of control required. The stage mix will be a mixture of whatever vocal and instruments are connected to the PA system.
Some musicians may prefer their own bespoke In Ear monitor's sub-mix when performing on stage. This provides a more musician controllable mix and provides them exactly what they want. This can be achieved, in a few ways, one is by using another, usually smaller, mixing desk (the monitor mixer) and using either a split PA snake cable, or mic 'Y' splitter cables. Both of these allow the required instrument or vocal inputs, to feed both the FOH mixer and monitor mixer. These inputs can then be mixed on the monitor mixer, setting whatever level is required for each separate input e.g. more guitar, less bass, more lead vocals, less backing vocals, thus providing a bespoke mix for whomever is connected to the sub-mixer. The number of inputs on the sub-mixer will determine the number of instruments and vocals that can be sub-mixed and the number of outputs determines how many musicians can be provided with a bespoke monitor mix.
Sub-Mixes On FOH MixerEdit
Aux level sets overall volume
May not have faders
Cue mix for engineer
Monitor mixes may be mono or stereo
Placement of the monitor mixer
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, typically touring professionals, began 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. In-ear monitors greatly reduce on-stage volume by eliminating the need for on-stage monitor wedges. This reduced on-stage volume makes it easier for the Front of House audio engineer to get a good sound for the audience. In-ear monitors also make audio feedback "howls" much less likely, since there are no monitor speakers. The lower on-stage volume may lead to less hearing damage for performers. One drawback of in-ear monitors is that the singers and musicians cannot hear on-stage comments spoken away from a microphone (e.g., the bandleader turning away from the vocal mic and looking at the band and calling for an impromptu repetition of the chorus) or sounds from the audience. This issue can be rectified by placing microphones in front of the stage so that the band can hear the audience.
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.
- "Taming The Wild Sage Monitor" (PDF). Archived from the original (PDF) on 2011-06-13. Retrieved 2009-01-12.
- http://www.sweetwater.com/expert-center/glossary/t--StageMonitor Stage Monitor
- Borgerson, Bruce. "Is it P.A. or SR?." Sound & Video Contractor. 1 Nov. 2003. Prism Business Media. 18 Feb. 2007 <"Archived copy". Archived from the original on 2008-12-01. Retrieved 2007-02-18.CS1 maint: Archived copy as title (link)>.
- Local Crew: McCune Sound Archived 2008-07-18 at the Wayback Machine
- Another Sensational Evening With Judy!(Reference only for date of concert) Archived 2008-10-15 at the Wayback Machine
- http://www.billhanley.org/innovation/ Innovations credited to Bill include the following...
- Bob Cavin. Inventions Archived 2005-10-01 at the Wayback Machine
- http://www.furmansound.com/product.php?div=01&id=HR-6 HR-6 PERSONAL MIXING STATION
- http://www.rolls.com/product.php?pid=PM50s PM50s Personal Monitor Amplifier
- http://www.heartechnologies.com/hb/hearbacksystem.htm hear back system
- http://www.aviom.com/Product-Categories-1/Mixers-1/A-16II-Personal-Mixer Aviom A-16II personal mixer
- http://www.sweetwater.com/shop/live-sound/monitors/buying-guide.php Live Sound Monitors: Buying Guide
- http://www.galaxyaudio.com/HSRG.jsp Galaxy HSRG Hot Spot Monitor
- http://www.jblpro.com/pub/obsolete/Cabaret_Series1.pdf JBL Cabaret Series
- http://www.eaw.com/products/SM159zi.html EAW SM159zi stage monitor speaker
- http://www.meyersound.com/products/ultraseries/um-1p/ Meyer Sound Laboratories UM-1P Narrow Coverage Stage Monitor
- The Sound Reinforcement Handbook, Gary Davis, Yamaha, 1988
- http://www.klarkteknik.com/dn360.php DN360.
- http://www.dbxpro.com/1231/1231.php 1231 Dual 31-Band Graphic Equalizer
- http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-539.pdf model 539 ROOM EQUALIZER
- http://www.tcelectronic.com/media/536e827526484a09b2066cb8eaa2e332.pdf TC Electronic TC1140/2240 PARAMETRIC EQUALIZER PREAMP
- http://www.rane.com/peq55.html PEQ 55 Parametric Equalizer
- http://www.klarkteknik.com/dn410.html Klark Teknik DN410
- http://www.eqmag.com/article/stage-monitor-systems/may-06/19972 STAGE MONITOR SYSTEMS
- http://www.rane.com/pdf/old/pe17man2.pdf RANE PE 17 Operators Manual
- http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-562.pdf UREI 562 Feedback Suppressor
- http://www.ashly.com/images/relics/man-sc68.pdf Ashly SC-68 Parametric Notch Filter Operating Instructions
- O'Reilly, 2004. Brett McLaughlin. Home Theater Hacks: 100 Industrial-Strength Tips & Tools
- Home Theater. Robert Silva. Subwoofers: What You Need To Know Page 3: Subwoofer Alternatives.
- Home Theater Hi-Fi, June 2002. Evan Upchurch. Product Review - ButtKicker 2 Low Frequency Shaker
- ExtremeTech, September 8, 2005. Jeremy Atkinson. Get Your Butt Kicked