The PAforMusic glossary of PA and related terminology

The glossary pages provide definitions for over 1900 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

The definitions for these terms are given on the assumption of their use in the context of PA systems; many of the terms have more general meanings when used in a wider context. Where more than one definition is given for a term, the definitions are numbered (1), (2) etc.

Some of the definitions themselves use terms (such as "signal") in a specific way − most of these are links (just the first time they are used, in each definition), so just click on them to see the meanings that are intended.

Back-emf
An abbreviation for 'back electromotive force', the opposing voltage generated in an inductor when a change occurs in the current flowing through it. It is this behaviour which causes the effect known as 'inductance'.

Backing
Describes an instrument or vocal part whose sound is intended to be non-dominant in the overall sound of the band, as perceived by the audience. For example, backing vocals (often abbreviated to 'BV') provide vocals support to the lead vocalist(s); backing guitar plays (mostly) background chords (rather than single notes) and does not normally have any solo parts. See also BGV, Rhythm guitar, Chorus (2), Harmony and the next definition. Compare Lead (2).

Backing chorus
A backing vocal part of a song, often sung while at the same time the lead vocalist(s) are singing a different lyric. The backing chorus lyric and/or melody may be the same as used in the main chorus of the song, or may be different.

Backline
The combos, and/or the speakers and their heads, that are provided for each individual musician (especially guitarists and bassists), to enable them to hear what they are playing and to enable them to make their own adjustments to the type of sound produced (in conjunction with adjustments on the instrument itself and on any effects they are using). This is in contrast to stage monitors, which are under the control of a sound engineer. The backline equipment is typically placed in a row across the back of the stage behind the relevant musician, hence the name. See also Pedal.

Baffle
The surface on which a driver is mounted, forming a part of the enclosure for a speaker. Its primary purpose is to prevent sound from the rear of the driver being heard from the front of the speaker, and so dramatically improve the bass response of the speaker. See also Port, Direct radiator and Sealed box.

Balance control
A control used to adjust the relative level of two signals, most usually the Left and Right channels in a stereo configuration. Such an adjustment is made either to make the two signals equal in level, so compensating for an unwanted difference that was introduced earlier (e.g. differing Left and Right playback levels from a tape), or to intentionally make the two signals different in level, so compensating for a difference occurring later (such as a non-central listening position between the Left and Right speakers).

The control consists of two potentiometers mechanically linked such that, as the control is moved in one direction from the mid-point, the level of one channel is reduced without affecting the other channel − moving it in the opposite direction from the mid-point reduces the level of only the other channel. This is called an 'MN taper'. A balance control is often fitted with a central detent.

Balanced
Describes an interconnection between two items of equipment, in which two signal-related conductors are provided through the connectors and cable, separately from the signal earth or 'screen' of the interconnection (making three conductors in total), and in which the destination equipment registers the difference in voltage between that pair of signal-related conductors. Frequently (but not always), these two conductors carry signals of equal signal voltage but opposite polarity. They are usually referred to as the 'hot' (or '+', or 'in-phase') and 'cold' (or '−', or 'anti-phase') legs, and their wires are normally twisted around each other along the length of the cable, rather than being laid parallel.

Or, describes an input, an output or a line that operates (or is intended to operate, or is capable of operating) with such an interconnection.

When good quality cable is used, this arrangement (properly termed a 'balanced line') is highly immune to the effect of earth loops and to the pick-up of interference, and so is commonly employed for low-level signals (such as between a microphone and a mixer) and/or when long distances (up to 200 metres) are involved. Picked-up interference originates primarily from RFI which succeeds in penetrating the screen of cables and connectors and from capacitively and inductively coupled signals from other nearby cables and equipment.

It achieves this immunity not (as commonly supposed) because of the opposite polarity of the hot and cold signals of the interconnection, but by the ability of a balanced input to reject (or 'cancel') interference that arrives identically on its hot and cold terminals, termed common mode interference. The extent of this ability is the input’s common mode rejection ratio. Such inputs are sometimes referred to as 'differential inputs', because (ideally) the input registers only the difference in voltage between its hot and cold terminals. In order for the cancellation to occur satisfactorily, it is essential that the hot and cold legs have the same impedance to signal earth (strictly, it is in this sense that the interconnection is 'balanced').

In fact, in some interconnections that are referred to as 'balanced', no output drive signal is provided on the cold leg of the interconnection. This has no effect on the input's ability to cancel the effects of earth loops and picked-up interference, provided that the hot and cold legs are presented with the same value of impedance by the output. Examples of this type of output are the semi-balanced and ground-compensated types. However, the case where both legs are driven has the added benefits of reduced capacitive coupling into other circuits when not individually screened (such as inside stageboxes and multiway connectors), reduced signal current in the screen, and increased dynamic range; we shall refer to this case as 'fully balanced'.

A further advantage of a balanced interconnection is its ability to carry phantom power, useful for powering condenser microphones and active DI boxes. Caution: Phantom power may cause damage to equipment not intended to handle it.

The 3-pole (3-pin) XLR connector is almost always used for balanced audio interconnections − see the diagram. (A notable exception to this is on some patch bays, such as are frequently used in studios, where 3-pole jacks are often used.) However, an interconnection should not be assumed to be balanced solely because it uses an XLR connector (nor because it uses a 3-pole jack).

In a fully balanced interconnection there are two scenarios that can be encountered, depending on how the balanced operation is achieved by the equipment at each end of link. We can refer to these scenarios as 'isolated loop' and 'fully electronically balanced'.

Type	No. of O/P poles	O/P poles driven	Cable connections (see Key below)	No. of I/P poles	+6 dB headroom (or SNR)	Earth loop rejection	Noise rejection
Balanced (fully)	3	2	2T->---hot---2T 3R->---cold--3R 1S---screen--1S	3	Yes	Good	Good
			2T->---hot----T 3R->---cold--X# 1S---screen---S	2	No	None	None
			2T->---hot----T 3RY 1S---screen---S	2	No	None	None
Semi-balanced / Impedance- balanced / Quasi-balanced	3	1	2T->---hot---2T 3R-----cold--3R 1S---screen--1S	3	No	Good	Good
			2T->---hot----T 3R-----cold--X% 1S---screen---S	2	No	None	None
			2T->---hot----T 3RZ 1S---screen---S	2	No	None	None
Quasi-floating	3	2	2T->-<-hot---2T 3R->-<-cold--3R 1S---screen--1S	3	Yes	Good	Good
			2T->-<-hot----T 3R->-<-cold--¦* 1S---screen--¦S	2	No	Fair to Good	Bad
			2T->-<-hot----T 3R-<¦ 1S--¦screen---S	2	No	None	None
Ground- compensated	3	1	2T->---hot---2T 3R-<---cold--3R 1S---screen--1S	3	No	Good	Good
			2T->---hot----T 3R-<---cold--¦* 1S---screen--¦S	2	No	Fair to Good	Bad
			2T->---hot----T 3R-<¦ 1S--¦screen---S	2	No	None	None
Pseudo-balanced	2	1	T-->---hot----T S------cold--¦ X---screen--¦S	2	No	None	Poor

Key to 'Cable connections' column:

Red text = balanced to unbalanced interconnection; see paragraph prior to the table.
1,2,3 = XLR pin numbers.
T,R,S = tip, ring and sleeve jack terminals (see TRS and TS).
> = driven output pole.
< = voltage sensing via output pole.
X = cable conductor not connected.
Y = plug terminal should not be connected; do not use a 2-pole plug.
Z = plug terminal may be left disconnected or linked to S; or a 2-pole plug may be used.
# = to avoid shorting driven output, 3-pole TRS plug should not be inserted in 2-pole socket.
% = not driven, so no problem if 3-pole TRS plug is inserted in 2-pole socket.
¦ = linked connections (if R to S, may result from a 2-pole plug inserted in a 3-pole socket).
* inserting a 3-pole TRS plug in a 2-pole socket is unlikely to result in a reliable R to S link.
No cold conductor indicated = unbalanced screened cable used.

Balanced line
A line that operates in balanced mode. However, as balanced operation of a line is not only dependent upon the line itself, but also upon the source and destination equipment that it is connected to, the term 'balanced line' may be used to refer to a line that is intended to operate, or is capable of operating, in balanced mode − regardless of the equipment that it is connected to, if any.

Balanced mains
A mains powering arrangement in which a centre-tapped transformer is used to provide a full-voltage mains supply in which the two conductors of the current-carrying power circuit have a voltage to earth which is equal in magnitude but opposite in phase. For example, in a 230 volt balanced supply, the two conductors would provide anti-phase voltages of 115 volts to earth − so the total voltage between them would be 230 volts (the same as in a normal unbalanced supply). The centre-tap of the transformer secondary must be adequately connected to the installation safety earth. (This mains power arrangement is in contrast to a normal mains supply, in which the Neutral conductor is at essentially earth potential.)

Rarely used in mobile PA systems, but may be encountered in large fixed systems or in professional studios, where it substantially reduces mains earth currents and so helps to avoid problems from earth loops when a substantial number of unbalanced signal interconnections are in use between the items of equipment. In a PA context, earth loop problems are more usually avoided by use of balanced interconnections.

The reduction in voltage to earth in a balanced mains distribution arrangement gives an incidental safety benefit, but an RCD must be used at the output of the transformer, to protect against earth faults occurring from either of the current-carrying conductors (which are both effectively 'Live' conductors). See also Distro, Power conditioner, Star point earthing and Technical earth.

Balun
A transformer arranged to provide conversion between balanced and unbalanced connections, with or without also providing a change in impedance. The name is most usually applied in radio-frequency situations such as radio aerial systems and computer network connections, rather than to audio-frequency transformers. It is sometimes a centre-tapped transformer, in which case the centre tap of the balanced side would usually be connected to signal earth.

Banana plug
A single-pole 4 mm connector, the male of which consists of a single exposed pin. Sometimes used for speaker connections. May be mechanically linked in a pair, to form a 2-pole connector; in this case the centres are normally 0.75 inches (approximately 19 mm) apart. Unusually, in this kind of connector the spring(s) that provide a tight fit (and therefore a low contact resistance) are attached to the male connector and are therefore visible − their slightly curved shape gives the connector its name. See also Binding post and Speakon.

Band (1)
A range of values between a lower limit and an upper limit, especially a range of frequency values. For examples of use, see the definitions following Band (2). See also Graphic equaliser and Spectrum.

Band (2)
Strictly refers to a group of musicians who play together, but usually taken to include any associated vocalists who do not also play instruments.

Band track
A recording of musical instruments without vocals, typically used to provide musical accompaniment to a vocalist singing live, but without a live band.

Bandpass
Describes a filter that allows a specific range (or band) of frequencies to pass through relatively unaffected, while frequencies significantly lower or higher than this band are significantly attenuated. The boundaries between the range of frequencies allowed to pass and those stopped are usually defined as the frequencies at which the attenuation is 3 dB greater than that at the centre of the band of passed frequencies (the centre frequency). For an explanation of dB see the Decibels page. Compare Bandstop.

Band-limiting
Describes a filter essentially the same as a bandpass filter, but in which the emphasis is on excluding unwanted higher and lower frequencies rather than letting through specific wanted ones. Usually, the band of frequencies passed is quite wide. Often these filters are implemented using a high pass filter to stop the unwanted low frequencies, and a separate low pass filter to stop the unwanted high frequencies. An example of use of such a filter is in the restriction of the bandwidth of pink noise.

Bandstop
Describes a filter that provides a significant degree of attenuation to a specific range (or band) of frequencies, while frequencies significantly lower or higher than this band are allowed to pass through relatively unaffected. The boundaries between the range of frequencies stopped and those allowed to pass are usually defined as the frequencies at which the attenuation is 3 dB greater than that at the frequencies much lower or higher than the stopped band. (For an explanation of dB see the Decibels page.) The frequency midway between these two frequencies is called the centre frequency. If the band of stopped frequencies is quite narrow, and the transitions between the range of frequencies stopped and those allowed to pass are quite abrupt, then the filter is often referred to as a 'notch filter'. Compare Bandpass.

Bandwidth
In an analogue context, a measure of the difference in frequency between a lowest and a highest frequency − i.e. a range (or band) of frequencies. Typically this would be the range of frequencies that exist within a signal, or the range of frequencies that are satisfactorily handled by an item of equipment or by an interconnection between items of equipment, or which are passed by a filter. Full audio bandwidth is generally recognised to be 20 Hz to 20 kHz.

A bandwidth value is determined by simply subtracting the lowest frequency of the range from the highest, these two frequencies usually being the lower and upper cut-off frequencies. Like frequency, it is measured in Hz or kHz. When an analogue communication channel is used for the transfer of digital data, the maximum achievable bit-rate will depend upon the analogue bandwidth and upon the data modulation method employed by the modems used. See also Q.

In a digital context, a measure of the maximum bit-rate that is supported by an item of equipment or by an interconnection between items of equipment. Digital bandwidth is usually measured in kbit/s or Mbit/s.

A larger bandwidth means that the maximum possible information transfer rate is increased, allowing, for example:

Bank
A set, or an associated group, of identical or similar items of equipment. Or, a store of several (usually different) control-settings of digitally programmed equipment such as digital mixers, lighting desks and electronic keyboard instruments. See also Patch (2).

Bar (1)
A unit of pressure, equal to 100,000 (10⁵) Pascals (Pa). As this unit is so very large, figures relating to sound pressure levels (SPL) are more frequently expressed in microbars (µbar, 10⁻⁶ bar, equivalent to 0.1 Pa) or picobars (pbar, 10⁻¹² bar, equivalent to 0.1 µPa). Normal atmospheric pressure at ground level is variable around a value of approximately 1 bar (1000 mbar).

Bar (2)
The smallest section of a piece of music that contains a complete sequence of beats in the rhythm concerned (with rare exceptions). Used by composers, musicians and singers as a convenient means of dividing up the music into small pieces that (by and large) each occupy the same amount of time − provided that the tempo remains constant. In professional music scores, the bars are often sequentially numbered for ease of reference.

Bargraph meter
A level-indicator that operates by means of a row (or, more usually, a column) of individual segments. Each segment is generally marked with a value of relative or absolute level, in dB or dBu respectively. The segments are usually activated progressively with increasing level, so that at the maximum displayable level they are all activated. The segments may be constructed using any display technology, but often use either discrete LEDs or an integrated LCD device.

The main advantages of the bargraph indicator over mechanical meters are ruggedness, low cost, accurate calibration and rapid response to changes in level.

More sophisticated versions of these indicators are now in common use, which include a 'memory effect' on the highest active segment, in order to give a clearer indication of the peak level reached. See also PPM.

Barn doors
Hinged metal plates, painted black, that are located around the edges of a lantern and which may be adjusted in order to selectively block the passage of light and so control the area that is illuminated by the lantern. A lantern normally has four barn doors.

Barrier strip
An American term for a row of screw-terminals on an item of equipment, provided to enable the direct connection of cables (that is, without the use of connectors). Generally used only for hard-wired connections in permanent installations. The name arises from the insulating pieces which project from the main body of the strip between each of the terminals; these pieces act as physical barriers to reduce the possibility of an accidental short circuit (e.g. due to a stray strand of a cable conductor).

Baseband
Describes a signal which exists at its original range of frequencies, to contrast it with the case in which a signal has been processed to cause it to occupy a different (usually much higher) range of frequencies, as for example in the case of modulation onto a carrier.

Basic protection
In electrical safety, the collective name given to measures intended to provide protection against electric shock caused by direct contact, that is, electric shock occurring in the absence of any leakage (or short-circuit) of current to conductive parts that are not intended to be live (at a dangerous voltage) in normal use. This protection is provided by the presence of insulation or by the conductor being securely enclosed. Compare Fault protection.

Basket
The integral wire mesh covering which protects the diaphragm of a microphone, while allowing sound waves to pass through. It also usually supports the internal windshield. It is normally detached by unscrewing.

Bass (1)
A low audio frequency, typically below 250 Hz. A mixer will usually provide the facility to control the relative level of these frequencies separately for each channel, leaving higher frequency ranges essentially unaffected. This facility is usually provided by an equaliser having a shelving response and a cut-off frequency somewhere in the region of 60 to 250 Hz; such a control is often labelled 'LF'.

Some lowest fundamental frequencies are given below. Vocal ranges vary from person to person.

It is common practice for frequencies below around 100 Hz to be handled by separate amplifiers and speakers; these frequencies and the associated equipment are then described as 'sub-bass'. See also Bass bin, Treble, Mid-range and Crossover.

Bass (2)
One of several musical instruments designed to produce low frequency notes, including:

Baxandall
Describes a type of equaliser circuit, commonly used in simple 'tone control' arrangements, in which negative feedback is employed to provide for a variable cut (reduction) in level of the controlled bands of frequency, as well as a variable boost (increase). It often consists simply of bass and treble controls with shelving responses and fixed cut-off frequencies. Commonly used on 'Hi-Fi' equipment. Named after its inventor.

Be operating system
An operating system popular for live performance software applications. Developed by Be Incorporated in 1996. Abbreviation BeOS.

Beach
A slang term for a particular location within a venue, especially for the location of specific equipment; an alternative to 'land' or 'world'. For example, 'monitor beach', 'dimmer beach' (vs 'monitor land', etc).

Beaming
The characteristic tendency of a speaker to have a decreased dispersion angle at higher frequencies. This effect is particularly evident when a single type of driver is used to cover a wide frequency range, as is usually the case with guitar combos and cabs. The result is that the on-axis sound has an excessive treble content, whilst the off-axis sound is lacking in treble − this may only be noticeable at distances greater than 5 metres (15 feet) or so from the speaker.

To combat this effect some manufacturers incorporate an anti-beaming device in front of the driver(s); such devices, generally known as 'beam blockers' or 'beam diffusers' are also available separately for subsequent attachment. However, when the sound level from the guitar backline is low compared to the front-of-house sound, the issue of backline beaming may be insignificant.

BEIRG
An abbreviation for 'British Entertainment Industry Radio Group', a group representing the interests of radio spectrum users in the British entertainment industry. Their website is: http://www.beirg.org.uk. See also JFMG and PLASA.

Bel
A very rarely used term meaning ten decibels. For further information see the Decibels page.

Belden
A manufacturer of many different types of wire and cable. However, the term 'Belden cable' is often used to refer to a multicore, as Belden is a popular manufacturer of this type of cable. For multicore colour codes see Colour code.

Beltpack
A bodypack unit that is intended to attach to a performer's trouser-belt, or onto a body-belt worn specially for the purpose. Or, an alternative name for any bodypack.

Bessel
Describes a class of filter having the minimum possible amount of variation (or 'ripple') of phase over the whole width of its passband, a so-called 'maximally flat' phase response. Named after the person first to document this design. Sometimes used for crossovers. See also Butterworth and Chebyshev.

BGV
An abbreviation for 'background vocal(s)', an alternative term for 'backing vocal(s)' (BV). Compare LV.

Bi-amping
An arrangement in which the woofers and horns of one or more speakers are driven by separate amplifiers. This arrangement requires an electronic crossover to be connected at the input of the two amplifiers, so that the amplifier driving the woofers receives only low frequency signals, and the amplifier driving the horns receives only high frequency signals. The woofers and horns may be incorporated within the same enclosure (i.e. a 2-way speaker), equipped with separate external connections to those drivers, or may be contained within their own separate enclosures.

By extension, tri-amping is an arrangement of three separate amplifiers driving low frequency, mid-range and high frequency drivers, and multi-amping is a generic term for a similar arrangement of any number of separate amplifiers from 2 upwards.

It operates by using two consecutive logical states on the line to represent each data bit of the underlying signal. A state change always occurs at the start of each pair of logical states; a further state change occurs between the states of the pair if the represented data bit is a '1', but not if it is a '0'. So, a '1' is represented by the sequence '10' or '01' (depending on the preceding state), and likewise a '0' is represented by the sequence '11' or '00'. These coding rules have the effect that there are never more than two logical states before a state change, however the 'preamble' bits of AES3 and S/PDIF deliberately break the rules in order to be uniquely recognisable. Note that the use of two line states per data bit effectively doubles the bit-rate on the line.

Bias
In analogue tape recording, a signal at ultrasonic frequency (typically in the range 50 to 100 kHz) that is applied to the recording head along with the audio signal to be recorded. The bias signal serves to mitigate against the inherently non-linear characteristics of the magnetic recording process. The correct adjustment of its level is critical in achieving the optimum compromise between dynamic range and frequency response. See also Tape equalisation and HX.

Bidirectional microphone
A microphone that picks up sound equally from the front and the rear, having a decreasing sensitivity towards the sides. The angle of least pick-up is at 90º from the front (or rear) of the microphone, that is, directly at the sides. It is not often used in PA work. Also known as a 'figure-of-8' microphone, because its pattern of sensitivity, when plotted as a polar response graph, looks like the number '8'. See also Mid-side pair. For more information on microphones see the Microphones page.

Bifilar
Describes a coil wound back on itself in order to minimise its inductance. Used in wire-wound resistors, e.g. in crossovers. The term is from the French for 'two wires'.

Binary
Describes a number that is expressed in base 2, i.e. using just '0's and '1's. In such a number, each successive place, moving leftwards, has a significance twice as great. (Compare this with a normal decimal number, in which each place to the left is ten times as significant.) So, the right-most digit is the number of 1's, and successive places to the left are the number of 2's, 4's, 8's, 16's, 32's, 64's, etc.. For example, the number 375 would be written as 101110111 (= 256 + 0 + 64 + 32 + 16 + 0 + 4 + 2 + 1).

This scheme is useful because it enables numerical information, such as a digital audio signal, to be conveyed and/or stored using just two values or 'states'. This minimises the probability of misinterpretation (which may arise due to undesirable modification of the signal or interference with it) and allows storage of the information using two-state media such as RAM or disc. See also the next definition and Hexadecimal.

Binary coded decimal
A numbering scheme in which each digit of a decimal number is represented in binary form, four binary digits being used for each decimal digit. For example, the number 375 would be represented as 0011,0111,0101. Often abbreviated to BCD. (Note that this is different to a binary representation of the number; for example 375 would be represented in binary as 101110111.) See also Hexadecimal.

Binaural
Describes a stereo recording technique which makes use of a pair of microphones built into a model human head in such as way as to mimic the sound pick-up pattern of human ears. To achieve the desired effect, playback must be through headphones, so that the sound originally received by each microphone is reproduced directly into each ear of the listener.

Binding post
A connection terminal on a speaker or an amplifier, allowing the speaker cable to be connected directly − i.e. without the use of a connector. As it is a single-pole device, connection to a separate binding post is required for each conductor of the cable.

The insulation of the cable conductors must first be stripped back at the end of the cable to be connected − but only just enough to avoid the insulation being gripped by the connector. Having careful regard to ensure correct polarity (see below), each bare conductor is then either wrapped around the appropriate binding post or more commonly is fed through the small radial hole that is revealed when the terminal is partially unscrewed. The terminal is then screwed-up by hand to securely grip the conductor (but not so tight as to damage it).

When used properly this method can provide a lower resistance connection than using a connector, but is not convenient for rapid connection and disconnection and is prone to polarity errors and other problems such as loose terminals, conductor damage and short circuits between adjacent conductors or to chassis. Binding posts are normally colour-coded red for the 'hot' or '+' connection and black for the 'cold' or '−' connection.

Some types of binding post are equipped with an axial hole for a male 4 mm connector (banana plug), as an alternative means of connection to the terminal. See also Speakon.

Bit
Short for 'binary digit'. A single digit of a number that is expressed in binary form − i.e. a '0' or a '1'. Digital signals consist of a consecutive stream of bits − see Bit-stream.

Bit/s
A unit of bit-rate, equal to one bit per second. Sometimes written as 'bps' (always with a lower-case 'b', to avoid confusion with Bps). Large values of data rate are usually expressed in kbit/s or Mbit/s.

Bit error rate
A measure of the rate at which bits that are in error (i.e. a 1 instead of a 0 or vice versa) are occurring in a bit-stream, expressed as a proportion of the total number of bits passing in the stream in the same period. For example, if over a certain period a total of 10,000 bits pass, and 3 of them are in error, then the bit error rate is 0.0003, more usually written as 3 x 10⁻⁴ or 3 x 10E−4. Commonly abbreviated to 'BER'. See also Parity and Cyclic redundancy check.

Bit-rate
A measure of the rate at which bits are conveyed in a bit-stream, usually expressed in kbit/s or Mbit/s, sometimes written as kbps or Mbps (always with a lower-case 'b'). See also Bps and Bandwidth (digital).

Blackout
A point during a performance when all stage lighting is switched off, for dramatic effect. Back-stage lighting remains on (as must all emergency lighting and fire exit signs). Often abbreviated to 'BO'. Compare Dead blackout.

Blacks (1)
The black clothing worn by the stagecrew, to make them less visible to the audience.

Block
An identifiable sub-unit, forming a part of the whole. For example, an internal section of an item of equipment (see the next definition) or a specific group of frames in a bit-stream.

Block diagram
A type of diagram, frequently found in equipment user guides, which allows the path of the signals though the various internal sections (or 'blocks') of the equipment to be visualised without the detail of the actual electronic circuitry. Each block of the equipment is typically represented by a rectangle or square, which may contain standard symbols for common functions such as amplification or filtering. Compare Circuit diagram.

Blow
A slang term meaning to play a musical instrument. Originates from the playing of wind instruments, but now gets applied to any type of instrument.

Blu-ray
A video disc recording format that is able to store an increased amount of data (25 GB single-sided), as compared to DVD. This increased capacity makes it suitable for storage of high definition video material (see HDTV). The disc is written and read using a shorter optical wavelength than DVD, and so is not compatible with it. However, a 'dual-standard' machine may be able to play back (or even to record) in both formats.

Bluetooth
A standardised system for digital radio communication over very short distances, such as between nearby items of computer equipment. See also IEEE 802.11.

BNC
A bayonet-type connector often used for professional video and unbalanced digital audio connections. To connect it, place the slots of the male connector's locking ring over the lugs which protrude from the sides of the female connector, then push and twist the locking ring clockwise until the lugs lock into place. To disconnect, twist the ring fully anticlockwise to disengage the lugs, then pull the connector.

Note that there are two different versions, which look almost identical but should not be mated as they have different characteristic impedances: the 75 ohm version is always used in video and audio work, the 50 ohm version being used only in radio-frequency applications. (For radio microphone antenna connections, check which type is required by your equipment.)

This type of connector is also frequently used as a means of connecting and fixing gooseneck-supported illumination lamps over a mixing desk. BNC stands for 'Bayonet Neill-Concelman', Paul Neill and Carl Concelman being its inventors (though other explanations for the abbreviation are often quoted). See also MUSA and Littlite.

Board
Short for 'sound board', a rather outdated name for a mixer. May also be used to refer to a stage lighting board or to an electrical power distribution board.

Bodypack
A small unit that is intended to be worn, usually to provide a communication fuction. Their most common uses are as transmitters for lavalier radio mics or for wireless instrument applications, receivers for in-ear monitoring systems, and pre-amps for wired instrument applications. They are usually powered by one or more internal batteries. Alternatively known as a beltpack. See also Condom and Pick-up.

Bonding
A safety electrical connection between items of exposed metalwork, such as staging or gantry structures, or between such items and the main safety earth connection of the mains supply, in order to ensure that no harmful voltage can appear between them in the event of an electrical supply fault. Particularly important in the case of a TN−C−S supply. The term 'earth bonding' is sometimes used, which is strictly incorrect; the correct full term is 'protective equipotential bonding' (often abbreviated to 'protective bonding'), as its function is to keep the interconnected items at essentially equal electrical potentials (voltages). See also Indirect contact.

Boom
Short for 'boom arm' − the part of a microphone stand that allows the position of the microphone to be offset horizontally from a position directly above the base of the stand. Although this part is usually supplied together with the stand, it may be purchased separately and attached to the top of a 'straight' stand − that is, a stand without a boom. (The threads might be different, but thread adaptors are available.) Some types of boom arm have two (occasionally three) telescopic sections.

Or, in certain speech applications, a long hand-held pole (usually of multi-section telescopic construction) used to temporarily position a microphone close to (often just above) the person talking, while the operator remains some distance away, e.g. out of camera shot. Used in film sound, television interviews, live discussion forums (debates), etc.

Boost
A common term for gain; an increase in level, usually specified in decibels. The term may be refer to an increase across the whole audio spectrum, or, more often, refers to an increase at specific frequencies only − e.g. by use of an equaliser. Or, as a verb, to make such an increase in level. Sometimes the term 'lift' is used. Compare Cut.

Booth
Any enclosure, or partial enclosure, for use by one or more persons. The term may be used to refer specifically to the front-of-house mixing location, when an enclosure or partial enclosure is provided for it. (Such an enclosure may well also accommodate other equipment, e.g. lighting desk, video equipment, etc. and the associated personnel.)

Boundary microphone (Boundary layer microphone)
A special type of microphone which, when placed on or attached to a suitable surface, utilises the sound energy collected at that surface to provide an enhanced signal-to-noise ratio. Such microphones may be equally sensitive to sounds from all directions above the surface, or may exhibit directional characteristics. Typically used for speech, where a convenient surface such as a conference table, lectern or stage floor is available. Some types incorporate an integrated plate and so avoid the need for an external surface. Also called a 'pressure zone microphone' (PZM, a trade-marked term), a 'plate microphone' or a 'barrier microphone'. Patented by Ed Long and Ron Wickersham in 1982. For more information on microphones, see the Microphones page.

BPM
An abbreviation for 'beats per minute', a means of specifying the tempo of music. A term mostly used by musicians, but also used by DJs as a means of comparing the tempo of recordings that are to be played sequentially without a gap, when a dance tempo is to be maintained.

Bps
An abbreviation for 'bytes per second', a measure of the rate of digital information transfer. However, note that when this abbreviation is written with a lower-case 'b' the meaning is then 'bits per second'. See also Bit-rate.

Braid
Another name for a screen, strictly one that consists of many fine strands of wire which are woven together, rather than being laid parallel and spirally wrapped (lapped).

Brass (musical instruments)
Describes a particular class (or 'section') of orchestral musical instruments, including the trumpet, cornet, trombone, tuba, french horn and saxophone.

Break a leg
A theatrical term used in place of "Good luck!", for superstitious reasons.

Break switch
A button provided on some mixers, that typically mutes all inputs (apart from the 2-track inputs). Useful during performance intervals or other breaks. Not muting the 2-track inputs allows the playback of background music during such periods.

Breakthrough
Usually refers to the reception of an unwanted radio-frequency (RF) signal, that is being transmitted on a frequency other than that which a receiver (e.g. a radio microphone receiver) is intended to receive. This can occur when the unwanted signal is being transmitted at high RF power and/or from a nearby location, and is often due to the receiver being overloaded by the unwanted signal. Therefore, the unwanted audio may be distorted, and often starts and stops suddenly, as the changes in the unwanted RF signal level cause it to cross the overload threshold (in contrast to crosstalk, which is a linear phenomenon). See also RFI.

Breath blast
The sudden burst of air that is expelled from the mouth when speaking or singing certain plosive sounds, particularly the letter 'p' but also the letters 'b' and 't'. When this burst of air hits the diaphragm of a microphone, it can cause a 'popping' sound to be generated. This effect can usually be adequately reduced by use of a suitable external windshield, though most good quality microphones intended for vocal applications are equipped with an internal windshield.

Bridge (1)
The part of a stringed musical instrument on which the strings rest, at the sound board end of the instrument. The vibration of the strings is transmitted to the sound board largely through the bridge. In the case of classical instruments such as a violin, viola or double bass, specialised pick-ups are available for direct attachment to the bridge. In the case of an acoustic guitar, the placement of a contact pick-up in relation to the bridge has a substantial effect on the timbre of the sound that is picked-up.

Bridge (2)
An item of equipment that provides an interconnection between otherwise separate systems. Usually refers either to the interconnection of computers (or computer networks) or to the interconnection of audio equipment with a telephone network, enabling both sides of a live telephone conversation to be heard by the audience.

Bridging
A technique to improve the matching between the impedance of a speaker (or the overall impedance of several interconnected speakers) and the optimum load impedance of the available power amplifiers, so as to increase the maximum amount of power that the amplifiers can provide to that speaker(s). It is most useful when it is required to use more of the power-handling capacity of the speaker(s), or more of the power-providing capability of the amplifiers, than could be achieved with a simple amplifier-to-speaker connection.

For example, if an 8 ohm speaker with a peak power-handling capacity of 2 kW is driven from one side of a 2-channel amplifier that can supply a maximum of 1 kW per channel into 4 ohms, but only 500 W into 8 ohms, then the maximum obtainable power into that speaker is 500 W, and clearly both the speaker and the amplifier are under-utilised. If a second identical speaker is available and is connected in parallel with the first one, then a further 500 W could be drawn from the same amplifier channel − this fully utilises that amplifier channel, but each 2 kW speaker can still only receive a maximum of 500 W. The bridging technique brings into play the second channel of the amplifier (assumed to be 'spare'!) and thereby allows the original single speaker to be provided with a maximum power of 2 kW while at the same time fully utilising both channels of the amplifier.

Bridging is possible only when each channel of the amplifier is capable of driving an impedance of one-half of the overall impedance of the connected speaker(s) (without stressing the amplifiers or causing an unacceptable reduction in their maximum power output capability).

It should only be employed if approved by the amplifier manufacturer and if the speakers are suitably rated, otherwise serious damage to the amplifier or speakers may occur. WARNING: The output voltage of bridged high-power amplifiers can be high enough to cause electric shock. The speaker cable used must be suitable for the voltage and current. Properly termed a 'bridge-tied load' connection, frequently abbreviated to 'BTL'. For a more comprehensive explanation, see Bridging on the Amps and Speakers page. See also Series-parallel.

British EQ
Supposedly, an equaliser of a design popularised by British mixer manufacturers. In reality, a marketing term with little or no technical meaning.

BS 1363A connector
A normal fused UK 3-pole 13 amp mains plug. The suffix 'A' refers to the update of the specification to include insulated sleeving on the Live and Neutral pins, however as plugs without this sleeving are no longer available the suffix is often dropped.

Fuses used in this type of plug must be to BS 1362 and are available in ratings of 2A, 3A, 5A, 7A, 10A and 13A (of which the most popular are 3A, 5A and 13A). See also CEE-form connector and IEC 320.

BS 7671
The specification of requirements for fixed electrical installations in the UK, published by the IET. Informally known as the 'IEE Wiring Regulations'. See also BS 7909, PAT and National Electrical Code.

BS 7909
A code of practice for temporary electrical systems for entertainment and related purposes. See also BS 7671 and PAT.

BSI
An abbreviation for 'British Standards Institute', an organisation which sets technical standards for the UK.

BT431a
The modern telephone line connector used in the UK, originally specified by British Telecom.

Buffer
An item of equipment (usually an amplifier) intended to be connected in the signal path between two other items, which may not be connected together directly − usually due to an incompatibility in level or impedance, or when direct connection would cause the signal to be modified in an undesirable manner. See also Matching.

Build quality
A subjective measure of the reliability and ruggedness of equipment, regardless of how well it performs its required function when in an 'as new' condition.

Bum note
A (British) slang term for a musical note that is flat, sharp or defective in some other way. See also Fluff.

Burst error
In the transmission of digital information, bit errors (i.e. a '1' being received when it should have been a '0', or vice versa) do not always occur at a steady rate, but sometimes 'groups', or 'bursts', of errors occur over a short period of time. This is usually caused by some interfering signal of short duration, e.g. caused by operation of a power switch. See also Bit error rate.

Bus, Buss
A conductor, or a set of conductors, which interconnect several points in a system in order to carry information or power between those points. Power buses are also known as 'rails'.

In a mixer, the term bus (or buss) usually refers to the conductors on which the signals from each channel are combined, forming the mixes; these are therefore sometimes given the more descriptive name of 'mix buses'. In a typical mixer there will be buses for the Left, Right (and possibly Centre) main mixes (see LCR (1)), for the audio groups (usually arranged in pairs), for the auxiliary (Aux) mixes and for the PFL facility. Channel signals are routed onto the main mix and audio group buses by the channel routing switches, and onto the auxiliary mix buses by turning up the channel's Aux Send controls. See also Summing.

Butterworth
Describes a class of filter having the minimum possible amount of variation (or 'ripple') of attenuation over the whole width of its passband, a so-called 'maximally flat' amplitude response. Named after the person first to document this design. Often used for crossovers. See also Chebyshev and Bessel.

ButtKicker
A trademarked name for an ultra-bass transducer that, when mechanically coupled to a chair or floor, enables a performer to feel (rather than hear) extremely low frequencies. It is available in various versions, to suit applications ranging from home entertainment to concert stages. Typically used for drummers (coupled to a drum stool) or for bassists (coupled to the floor), the 'Concert' version has a quoted response down to 5 Hz. Note that its quoted impedance is only 2 ohms so the power amplifier used to drive it must be suitable to drive that impedance value. For further details see the manufacturer's website: www.thebuttkicker.com (this link opens in a new window).

BV
An abbreviation for 'backing vocals', an alternative term for 'background vocals' (BGV). Compare LV.

Bypass
A switch which enables an item of equipment (or a part of it, such as an equaliser) to be made ineffective.

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