Stage monitor system

A stage monitor system is a set of performer-facing loudspeakers called monitor speakers, stage monitors, floor monitors, wedges, or foldbacks on stage during live music performances in which a sound reinforcement system is used to amplify a performance for the audience. The monitor system allows musicians to hear themselves and fellow band members clearly.

A JBL floor monitor speaker cabinet with a 12" woofer and a "bullet" tweeter. Typically, the speaker would be covered with a metal grille to protect it.

The sound at popular music and rock music concerts is amplified with power amplifiers through a 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 amplifies the onstage sounds for the main audience. The monitor system is driven by a mix separate from the front-of-house system. This mix typically highlights the vocals and acoustic instruments so they can be heard over the electronic instruments and drums.[1][2]

Monitor systems have a range of sizes and complexity. A small pub or nightclub may have a single monitor speaker on stage so that the lead vocalist can hear their singing and the signal for the monitor may be produced on the same mixing console and audio engineer as the front-of-house mix. A stadium rock concert may use a large number of monitor wedges and a separate mixing board and engineer on or beside the stage for the monitors. 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 their voice in the monitor in front of them and the guitarist can ask to hear mostly the bassist and drummer in their monitor.

Role

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 and instrumentalists 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 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 or foldback system. Each system consists of a mixing board, sound processing equipment, power amplifiers, and speakers.

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. In situations with poor or absent foldback mixes, vocalists may end up singing off-tune or out of time with the band.

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. A separately mixed signal is often routed to the foldback speaker to allow musicians to hear their performance as the audience hears it or in a way that helps improve their performance. More frequently, major professional bands and singers often use small in-ear monitors rather than onstage monitor speakers. The two systems usually share microphones and direct inputs using a splitter microphone snake.

The front-of-house system, 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, 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 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 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. 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.

Larger clubs and concert venues typically use a more complex type of monitor system which has two or three different monitor speakers and mixes for the different performers, e.g., vocalists and instrumentalists. 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 that forefronts their vocals, the backup singers can have a mix that emphasizes their backup vocals and the rhythm section members can have a mix which emphasizes the bass and drums. 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.

History

A rock band stage clearly shows the stage monitors.

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 that 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 on stage and to fill the venue with sound.

With these systems, singers could only hear their vocals by listening to the reflected sound from the audience-facing front-of-house speakers. This was not an effective way to hear one's vocals because of the associated delay which made it hard to sing in rhythm with the band and in tune.

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 earliest recorded instance 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.[3][4]

Early stage monitors were simply speakers on each side of the stage pointed at the performers driven by the same mix as the FOH; audio mixers used in PAs at the time rarely had auxiliary send mixes. Today these would be called sidefill monitors. F.B. "Duke" Mewborn of Atlanta's Baker Audio used left and right arrays of Altec loudspeakers to cover the audience and to serve sidefill duties for the Beatles at Atlanta Stadium on August 18, 1965.[5] Bill Hanley working with Neil Young of Buffalo Springfield pioneered the concept of a speaker on the floor angled up at the performer with directional microphones to allow louder volumes with less feedback.[6]

In the 1970s, Bob Cavin, chief 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.[7]

The introduction of monitor speakers made it much easier for performers to hear their singing and playing on stage, which helped to improve the quality of live performances. A singer who has a good monitor system does not have to strain their voice to try to be heard. Monitor systems also helped rhythm section instrumentalists hear each other and thus improve their playing together even on a huge stage (e.g., at a stadium rock concert) with the musicians are far apart.

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. 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 stage monitoring system

The monitor system consists of the monitor mixer, 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 system.

Front of house auxiliary speaker

The simplest monitor system is a speaker pointed at the performer fed from 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 house

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 that 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 mixer

Larger venues 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 that forefronts their vocals, the backup singers can have a mix that 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.

Distributed monitoring

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[8] 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 preferences. 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,[9] 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.[10]

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.[11]

Distributed monitor mixers are most successful with headphones or in-ear monitors. If monitor speakers are used, feedback problems are common when the performer turns their microphone up too loud.

Monitor equipment

Monitor speakers

Monitor speakers often include a single full-range loudspeaker and a horn in a cabinet. Monitor speakers have numerous features that 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.

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.[12]

Floor monitors

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 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.[13] 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[14] 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.

Active monitors

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.[15]

Side-fill monitors

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 side fill 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

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 criterion of a monitor amp is reliability. They must be able to survive the abuse of their output being shorted or speakers being plugged and unplugged while the amp is turned on. They must continue to function while being overloaded for extended periods of time.

Equalization and signal processing

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.

Eliminating feedback

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.[16] 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.

Graphic equalizer

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[17] or a DBX 1231.[18] 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 removal 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.[19]

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. [2] 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.

Parametric equalizer

A second type of equalizer used in monitor systems are parametric equalizers.[20] 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[21] and Klark Teknik DN410.[22]

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.[23]

The process of using a parametric equalizer is different from when using a graphic equalizer.[24] 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 too 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.

Notch filter

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[25] and the Ashly SC-68 Parametric Notch Filter.[26]

Speaker processors

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.

Monitor mixer

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 whoever 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 Mixer

Pre-fader, pre-EQ

Aux level sets the overall volume

Monitor Mixer

May not have faders

Cue mix for an engineer

Monitor mixes may be mono or stereo

Built-in splitter

Placement of the monitor mixer

Monitorland

Related products

A picture of in-ear monitors, also known as canalphones, which are used by on-stage performers. This particular model is the Etymotic ER-4S

Headphones

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-ear monitors

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.

Bass shakers

Drummers typically use a monitor speaker that is capable of loud bass reproduction, so that they can monitor their bass drum. Since the drums are already very loud, 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 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.[27][28] 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.[29] 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.[30]

Other meanings

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.

See also

• Sidetone

References

1. http://www.sweetwater.com/expert-center/glossary/t--StageMonitor Stage Monitor
2. "Taming The Wild Sage Monitor" (PDF). Archived from the original (PDF) on 2011-06-13. Retrieved 2009-01-12.
3. Local Crew: McCune Sound Archived 2008-07-18 at the Wayback Machine
4. Another Sensational Evening With Judy!(Reference only for date of concert) Archived 2008-10-15 at the Wayback Machine
5. Montgomery, Elizabeth (February 7, 2014). "When Beatlemania Came to Atlanta". Atlanta Journal-Constitution.
6. Innovations credited to Bill include the following...
7. Bob Cavin. Inventions Archived 2005-10-01 at the Wayback Machine
8. http://www.furmansound.com/product.php?div=01&id=HR-6 HR-6 PERSONAL MIXING STATION
9. http://www.rolls.com/product.php?pid=PM50s PM50s Personal Monitor Amplifier
10. http://www.heartechnologies.com/hb/hearbacksystem.htm hear back system
11. http://www.aviom.com/Product-Categories-1/Mixers-1/A-16II-Personal-Mixer Aviom A-16II personal mixer
12. http://www.sweetwater.com/shop/live-sound/monitors/buying-guide.php Live Sound Monitors: Buying Guide
13. http://www.jblpro.com/pub/obsolete/Cabaret_Series1.pdf JBL Cabaret Series
14. http://www.eaw.com/products/SM159zi.html EAW SM159zi stage monitor speaker
15. http://www.meyersound.com/products/ultraseries/um-1p/ Meyer Sound Laboratories UM-1P Narrow Coverage Stage Monitor
16. The Sound Reinforcement Handbook, Gary Davis, Yamaha, 1988
17. http://www.klarkteknik.com/dn360.php DN360.
18. http://www.dbxpro.com/1231/1231.php 1231 Dual 31-Band Graphic Equalizer
19. http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-539.pdf model 539 ROOM EQUALIZER
20. http://www.tcelectronic.com/media/536e827526484a09b2066cb8eaa2e332.pdf TC Electronic TC1140/2240 PARAMETRIC EQUALIZER PREAMP
21. http://www.rane.com/peq55.html PEQ 55 Parametric Equalizer
22. http://www.klarkteknik.com/dn410.html Klark Teknik DN410
23. http://www.eqmag.com/article/stage-monitor-systems/may-06/19972 STAGE MONITOR SYSTEMS
24. http://www.rane.com/pdf/old/pe17man2.pdf RANE PE 17 Operators Manual
25. http://www.jblproservice.com/pdf/Vintage JBL-UREI Electronics/UREI-562.pdf UREI 562 Feedback Suppressor
26. http://www.ashly.com/images/relics/man-sc68.pdf Ashly SC-68 Parametric Notch Filter Operating Instructions
27. O'Reilly, 2004. Brett McLaughlin. Home Theater Hacks: 100 Industrial-Strength Tips & Tools
28. Home Theater. Robert Silva. Subwoofers: What You Need To Know Page 3: Subwoofer Alternatives.
29. Home Theater Hi-Fi, June 2002. Evan Upchurch. Product Review - ButtKicker 2 Low Frequency Shaker
30. ExtremeTech, September 8, 2005. Jeremy Atkinson. Get Your Butt Kicked