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Glossary of PA Terms - L |
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The glossary pages provide definitions for over 2680 PA-related terms and abbreviations. 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.
In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.
L * L<number> * LAT * LAeq * Leq * Lambda * Lamp * Lampie, Lampy * LAN * Lantern * Lapel microphone * Lapped screen * Laser classes * LAT * Latency * Lav * Lavalier * Lavaliere * Law * Lay * Lay length * Lay-up * Layered * LC filter * LCD * LCR * LD * Lead * Lead sheet * Leading edge dimming * Leakage * Lectern * LED * LED display screen * Left * Leg * Legacy * LEQ * Leslie * Level * Level meter * Leveler * Leveller * Lex * LF * LFE * LFE channel * LFH * LFO * Lift * Light * Lighting bar * Lighting fixture * Lighting instrument * Lighting plot * Lighting truss * Lightning connector * Limiter * Limiting * Lin * Lin pot * Line * Line-up * Line array * Line check * Line code * Line conductor * Line driver * Line filter * Line input * Line level * Line microphone * Line output * Line sync * Linear * Linear PCM * Linear phase response * Linear potentiometer * Linear power supply * Linear taper * Linearity * Link * Linkwitz-Riley * Lip microphone * Lip sync * Listen wedge * Littlite * Live * Lively * LKFS * LM * LMID * Lo-Z * Load * Load impedance * Load-in * Load-out * Loaded * Lobar * Lobe * Log * Log pot * Log taper * Long throw * Longframe jack * Longitudinal choke * Longitudinal wave * Loom * Loom box * Loop * Loop amplifier * Loop driver * Loop system * Loss * Lossless * Lossy * Loud * Loudness * Loudness leveller * Loudspeaker * Low cut * Low end * Low frequency * Low impedance * Low pass * Low-Z * Lower mid-range * Lows * LPCM * LPDA * LPF * LR * LR-4 * LRA * LRC * LSF * LSHF * LSOF * LSOH * LSZH * LTE * LTP * LU * LUFS * Lumen * Lumens * Luminaire * Luminance * Lux * LV * lx * LX * LX tape * LXQ
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.
L
An abbreviation for 'Left
channel'.
See Stereo.
Compare R.
L<number> (1)
In sound
level measurement,
the level that is exceeded for <number> percent
of the time, over the period of the measurement,
usually expressed in
dB SPL.
For example, the L10 figure is the level that is
exceeded for 10% of the time during the
measurement interval.
See also Sound
level meter. Compare
Leq.
L<number> (2)
In equipment circuit
diagrams (schematics),
the component references
for inductors usually
take the form 'L followed by a number', e.g. 'L6'.
Such component references are sometimes also marked
on equipment circuit
boards (PCBs).
LAeq
See the next definition.
Leq
In sound
level measurement,
the continuous level that is equivalent to the average
of the actual levels over a stated period of measurement,
expressed as a value in
dB SPL. The measurement
is usually A-weighted,
in which case the correct term is LAeq,
though 'Leq = x dB(A)' is acceptable.
A measurement period of the order of 1 to 3 minutes
might typically be considered appropriate for
PA applications. This method
of measurement is more properly called LAT,
and is defined by standard IEC 61672-1.
Leq measurements are also often used to quantify sound exposure dosage for health and safety purposes, in which they specify the continuous level that is equivalent to the total sound energy exposure over the relevant time interval. For example, European regulations require the exposure for employees to be limited to an Leq SPL value of 85 dB (A) over an 8 hour period (per 24 hours), after the effects of any hearing protection measures are taken into account. For an Leq level of 88 dB (A) the maximum exposure period would be 4 hours and for 100 dB (A) it would be 15 minutes, as these exposures would give the same total sound energy dosage as 85 dB (A) for 8 hours. See also Sound level meter. Compare L<number> (1) and Time-weighted average.
Lambda
The Greek letter λ which is used as a symbol
for wavelength within
formulae (equations). Therefore, this term is occasionally
used in place of the term 'wavelength'. For
audio-related formulae
involving lambda, see
this entry on the
FAQ page.
Lamp
The component, within a
luminaire (such as a
a stage lantern), that converts
electrical energy into light.
Frequently called a 'bulb' by non-technical personnel
(a term deprecated by lighting engineers).
The term does not refer to the complete luminaire.
Lamps are of many different types, including
incandescent filament,
fluorescent, discharge and
LED.
Lampie, Lampy
A slang term for a lighting engineer.
See also LD,
Soundie,
Sparks and
LX.
LAN
An abbreviation for 'local area network',
an arrangement of data routing equipment,
cabling and connection points within a building
(or part of a building) that enables items of
connected data equipment to communicate with
each other. Connection points are usually
RJ45 types, and the cabling
is usually CAT 5e or
CAT 6
UTP.
See also AES50,
AES67 and
WLAN.
Lantern
The 'proper' name for a 'stage light' − an
enclosure
containing one or more light sources (and
possibly other devices such as lenses,
filters, shutters,
gobos etc.),
arranged to emit light
in a controlled way for applications such as stage lighting.
A luminaire specifically
designed for such applications.
Many different kinds of lanterns exist − see, for example, PAR can, Fresnel, Profile spot and Moving head.
With the advent of lighting and other equipment that is directly controllable using DMX, the term 'fixture' is now commonly used to refer to all such equipment; lanterns are therefore now often referred to as 'lighting fixtures'. The US term is 'lighting instrument'. See also Lamp and LED.
Lapel
microphone
See Lavalier.
Lapped screen
A screen that consists
of many fine strands of bare wire laid parallel with
each other in a helical (or 'spiral') wrapping around the
insulated inner
conductor(s), rather than
being woven into a braid.
Some screened cables have a lapped screen consisting of two
layers, one wrapped in a clockwise direction and the other
anticlockwise.
Laser classes
See Classes
of lasers.
Latency
The time delay introduced in a
signal by
digital signal
processing
or communications equipment. In the case of signal processing,
this delay is generally caused by the time taken to complete
the complex arithmetic operations which must be performed
on the signal in order to provide the required processing
function(s).
In the case of digital communications, latency
may result from use of a relatively low communications
bit-rate.
Such delays are more likely to be of significance when
some but not all signals are digitally processed
(or communicated) by equipment with a high latency,
because the difference in timing may then be more obvious
(for example, in AV
systems, poor lip sync).
Latency is usually most important in
real time processing,
for example live
programme applications.
As an example from digital communications, consider a MIDI 'note-on' message sent from a keyboard to a sound generator over a single MIDI link. This message consists of three bytes, each of 11 bits (including the start and stop bits). At the standard MIDI data rate of 31.25 kbit/s, the message will therefore take just over 1 ms to be sent. So even ignoring processing delays within the keyboard and sound generator, the latency between pressing a key and the sound beginning will be at least 1 ms. See also Time alignment.
Lav
A slang abbreviation for
lavalier; a short-hand term
for a lavalier microphone
(see the next definition).
Lavalier
(or, less commonly,
Lavaliere)
Originally, described a small
microphone that was worn
by being suspended from a cord around the wearer's neck,
to allow hands-free
use − e.g. by presenters or actors.
However, this method of attachment is now relatively
uncommon, and the term is now used to refer to any
chest-worn microphone − usually miniature types that
are attached to clothing by means of a clip.
As they are often clipped to a lapel or a tie,
they are alternatively known as lapel microphones,
clip microphones or tie-clip microphones.
They are usually
radio microphones,
and are used in conjunction with a
bodypack.
In some situations these microphones can be prone to problematic variations in output level, due to the changing distance from the wearer's mouth as their head is moved. To minimise this effect they should ideally be worn centrally, just below the point at which the wearer's chin would touch their chest. If they must be attached to one side, then this should be the side that the wearer predominantly faces (e.g. in an interview situation). Level variations can be further reduced by the use of omni-directional types, but these are more prone to feedback and leakage problems, especially when a high gain needs to be employed. A preferable approach is often a headset microphone.
The term 'lavalier' was taken from (by visual similarity) the name given to a type of single-stone pendant necklace. This name in turn originated from the French word 'lavalličre' for a necktie with a large knot, popularised by the Duchesse de La Valličre. See also Necklace microphone.
Law
A rule that expresses the relationship between two
or more quantities. For example, the law of a
potentiometer
(also referred to as its taper) defines how
its resistance
varies as its physical setting is changed, and
Ohm's law defines
the relationship between
voltage,
current and
resistance.
Lay
See the following two definitions.
Lay length
The distance taken up by each complete twist of the
insulated
signal
conductor
wires within a
balanced
cable.
Sometimes just referred to as the 'lay'.
See also
Twisted pair and
Star quad cable.
Lay-up
The particular physical arrangement of the
conductor
wires or
pairs within a
cable.
Sometimes just referred to as the 'lay'.
See also the previous definition.
Layered
Describes a sound, especially one from an electronic
keyboard system, in which several different source sounds
are produced simultaneously as the instrument is played.
The layers may be mixed by a small on-stage
mixer operated by the
keyboard player, or may be supplied to the main mixer
as separate signals which
are mixed there.
LC filter
An intentional filter
that is constructed using only
inductors and
capacitors, or an
unintentional one that exists as a result of the
inductance and
capacitance incidentally
present in a circuit.
Compare RC filter.
LCD
An abbreviation for 'liquid crystal display',
a device in which liquid crystals are placed under
the influence of an electric field in order to control
their polarisation and hence their ability to pass
light
(when used with appropriate polar filters).
By division of the display area into segments or
pixels, each under
separate electrical control, information such
as numbers, characters or full images can be formed.
When each pixel has its own adjacent controlling
transistor(s) fabricated
into the LCD, the display
is described as 'TFT', an abbreviation of 'thin
film transistor'.
The liquid crystals themselves do not emit light; the display must either incorporate a source of light (e.g. a 'back-lit' display) or rely on the reflection of ambient light. See also IPS (2) and LED.
LCR (1)
An abbreviation for 'left, centre and right'; describes
a routing
arrangement that in addition to the facility for
providing Left and
Right
mixes
(for eventual delivery to the Left and Right
speakers of a
stereo
PA system), also
provides the facility for a
Centre mix (for eventual
delivery to centrally located speakers).
Alternative abbreviation: LRC.
Some mixers that provide such routing arrangements have pan controls that are configured to pan from a Left-only feed (when fully anti-clockwise), to a Centre-only feed (when at the 12 o'clock position) to a Right-only feed (when fully clockwise). The lack of any feed to the Left and Right mix buses when the control is at 12 o'clock is in contrast to the usual stereo routing arrangement, in which an equal feed is supplied to those buses when the control is in that position. Some mixers providing LCR routing have the facility to select whether the pan controls operate in LCR mode or in stereo mode. See also Cluster.
LCR (2)
A combination of the
component designations
for inductors (L),
capacitors (C) and
resistors (R).
Usually used to describe a
filter which makes use
of all three of these component types.
LD
An abbreviation for 'lighting designer'. Or, in
television production, an abbreviation for
'lighting director'.
See also Lampie and
LX.
Lead (1)
A common alternative name for a
cable, usually one
that is either fitted with
connectors at both
ends or (less commonly) that is permanently attached
to an item of equipment. Typically preceded by a word
to describe the type of cable referred to, or to describe
its intended purpose, e.g. 'guitar lead',
'mic lead',
'instrument lead',
'speaker lead',
'mains lead', etc.
Lead (2)
Describes an instrument or vocal part whose sound
is intended to be dominant in the overall sound
of the band, as perceived by the audience, or to
be dominant in at least some parts of some songs.
For example, lead vocals (LV) sings the main
melody
line; lead guitar plays (mostly) single notes
(rather than chords) and often has a solo part
in at least one song.
See also Instrumental
break and
Rhythm guitar.
Compare Backing.
Lead sheet
A document that specifies the foundational elements
of a song, such as the lyrics, the
melody line,
chords for basic
accompaniment, and the song's structure (i.e.
sequence of verses, choruses, instrumentals etc.).
The information provided on a lead sheet can be of use to
sound engineers,
e.g. to indicate when a guitar solo will be played.
Leading
edge dimming
See Phase-angle
control.
Leakage
In general, something occurring where ideally it would not.
Most often refers to pick-up by a
microphone
of sounds other than those
intended to be picked-up by that
microphone, particularly pick-up of musical instruments
situated close to the target instrument. Usually undesirable in
PA work (but see
Creative leakage).
In the sense that leakage constitutes a part of the unwanted
signal present, it may be
considered to be part of the
noise. Alternatively known
as 'spill' or 'bleed'. See also
Separation,
Noise gate and
Earth leakage.
Lectern
A stand which supports books, documents, a laptop
computer, etc., used by a person delivering a
presentation. It may also support a
microphone, often a
gooseneck type.
A lectern is often located on a
dais.
LED
An abbreviation for 'light-emitting diode',
a semiconductor
device that emits light when a
current is passed
through it. High brightness LEDs are now increasingly
used as light sources in lanterns,
where they can provide multiple colours and much greater
efficiency
and longer life than
incandescent lamps.
Although the brightness of an LED varies with the current passed through it, such a method is generally unsuitable for accurate dimming applications (especially below around 20% power) because of variabilities between the LEDs in a group and because of undesirable changes in the colour of the emitted light. The dimming of LEDs is therefore usually accomplished using pulse width modulation or pulse density modulation techniques, which in some cases can result in problematic levels of radio-frequency interference (RFI). See also LCD. Compare Incandescent lamp and Discharge lamp.
LED display screen
Usually refers to a display screen in which the image
is formed by pixels that are
each made up of a red, a green and a blue
LED.
The light level of the individual LEDs is controlled
in order to give the brightness and colour required for
each pixel. Very large such screens, suitable
for use in large auditoria,
may be created by inter-linking smaller modules. In such
screens, the pitch
of the pixels is an important factor
in determining the clarity of the images produced −
especially for the viewers located closest to the screen.
As a rough guide, the minimum viewing distance in metres
can usually be considered to be slightly less than the pitch
in millimetres, e.g. 6 m for a 7 mm pitch screen.
It should be noted that the video
signal processing
associated with these screens can sometimes introduce
a substantial delay in the displayed image − see
Latency. The brightness
of display screens is usually specified in
nits.
Left
In a stereo system,
the channel that is
destined for the speakers
located at the left as viewed by
the audience (i.e. those located at
stage right).
Leg
One of the signal-carrying
conductors of a
balanced
pair. The two legs of
the pair are commonly referred to as the
'hot'
and 'cold' legs, or as the
'+' and '−' legs. Note that in a
star quad cable
two wires are used to make
up each leg of the pair (making four wires in total).
See also One-legged.
Legacy
Describes equipment of an earlier design or style
than what is now being manufactured. This may mean
that the equipment is many years old, but in the case
of equipment using rapidly-changing technology
(such as effects units
and computer-based equipment) the term might be used
of equipment even less than a year old. The term is
most frequently used in a comparative sense, to refer
to the earlier equipment that remains installed,
or in use, after some refurbishment or extension of the
system (often by new owners or new operators of a venue).
Such a description does not necessarily imply that the
equipment referred to is no longer fit for purpose.
Leslie
Describes a speaker
that incorporates an internal motor-driven rotating device
which interferes with the passage of the
sound emerging from the
driver. A means is usually
provided to switch the motor on and off, and to control
its speed of rotation (e.g. start/stop and
fast/slow foot switches). Effective
miking of these
speakers can be difficult, and is sometimes achieved
by use of two microphones,
e.g. at the top and side apertures of the
cabinet.
Such speakers were originally used with early electric organs to produce a rapid 'vibrato-like' sound effect typical of classic 'theatre organs'. The effect is now produced internally in keyboards by digital means. Named after its inventor.
Level
The amount of something; a measurement or indication
of its magnitude. In particular, the magnitude of
an audible sound
(usually expressed in
dB SPL or in
µPa)
or of a signal (usually
expressed in volts,
dBV,
dBu
or dBm).
By convention, such levels are nearly always expressed as
RMS values.
An alternative term for level is 'amplitude'.
Audio signal levels are important in a relative sense because they impact upon the perceived loudness of the sound finally produced and upon the relative contribution of each sound source in a mix. They are also important in an absolute sense: a) at the interconnections between items of equipment in order to ensure compatibility and b) within equipment to ensure that signals are neither so small that contributed noise becomes significant (see Signal-to-noise ratio) nor so large that significant distortion occurs as a result of overload.
The levels of analogue audio signals at the interconnections between items of equipment in a PA system usually fall into one of the following categories:
Digital audio signal levels are usually expressed as a value relative to the maximum level that can be handled by the digital signal format concerned, i.e. as a negative dB FS value.
Current levels are measured in amps (or mA, etc).
Power levels are measured in watts (or mW, etc) − but see also VA.
Sound pressure levels are measured in dB SPL (or sometimes in µPa).
Lighting levels are usually measured in lux.
See also Metering, PPM, VU, Dynamic range, Amplifier, Attenuator, Gain, Clipping, Volume, Input and Output.
Level meter
A device that indicates the
level of an
audio
signal − see
Metering.
Or, an abbreviation for
sound
level meter.
Leveller or Leveler
A device that is intended to remove or reduce variations in
average signal
level, for example so as to
remove or reduce variations
in average programme
loudness when switching
between different audio
signal sources. A simple version of such a device would
basically be a compressor
with an appropriately low
threshold
setting and appropriate settings of
compression ratio
and attack and
release times. Further,
specific side chain
filtering may be employed in order to provide a suitable
assessment of loudness. However, more sophisticated loudness
levellers operate by measuring programme loudness according
to the standardised LUFS method,
and adjusting levels in order to achieve a target LUFS value.
'Leveler' is the US spelling. See also
Automatic gain
control.
LF
An abbreviation for 'low
frequency'.
See Bass (1).
LFE
An abbreviation for
'low-frequency effects',
typically referring to the low-frequency effects added
to film sound-tracks.
See Bass (1).
LFE channel
A surround sound
recording channel
used solely for
sub-bass
signals,
typically for the low-frequency effects (LFE) added
to film sound-tracks. Use of a separate channel for this
purpose avoids a need to record the
higher frequency
content of the programme
at a lower average
level in order to
accommodate high-level
sub-bass signal peaks within the
available dynamic range
of the main recording channels. An improved
signal-to-noise
ratio can therefore be obtained at those higher frequencies,
where noise is more prevalent
and more noticeable. For proper reproduction of the LFE
channel a sub-woofer
is required.
See also 5.1 and
7.2.
LFH
An abbreviation for 'low fire hazard', typically
relating to the hazards that
may be created by some types of
cables when subjected to
fire conditions.
See also CPR (2) and
LSF etc.
LFO
An abbreviation for 'low frequency oscillator'.
It is most commonly encountered in relation to
modulated
time-domain effects such as
chorus,
flanging,
phasing,
tremolo and
vibrato,
where it refers to the
oscillator that
runs at a very low frequency
(typically between 0.1 Hz and
20 Hz) in order to produce the repetitive variations
in sound that are characteristic of these effects.
Lift (1)
In the context of
equalisers,
an alternative term for
boost.
Lift (2)
An abbreviation of
earth lift.
Light
Visible electromagnetic radiation, emitted by a
source such as lantern.
Not the lantern itself (though non-technical
personnel frequently use the term in that way,
as in 'a stage light'). See also
Lumen and
Lux.
Lighting bar
A solid or hollow metal bar of rectangular cross-section,
drilled at intervals to enable
lanterns or other
lighting fixtures to be bolted directly to it.
Or, a metal tube of circular cross-section
to which clamps may be attached at any point,
for the mounting of lanterns or other lighting fixtures.
May be referred to as just a 'bar'. See also
T-bar,
Truss and
SWL.
Lighting fixture
See Lantern.
Lighting instrument
An American term for a lantern.
Lighting plot
A diagram (or set of diagrams), produced during the
planning of an event, to indicate the stage area that is
to be illuminated by each
lantern during each scene,
and to specify the lighting colours, textures, special
effects, etc. that
are to be used. Often abbreviated by lighting engineers
to just 'plot'.
See also Cue and
Sound plot.
Lighting truss
See Truss.
Lightning connector
A reversible very thin flat micro-format
connector
developed by Apple® for
digital
signal and
charging interconnections to thin mobile devices.
The connector has 16 contacts in total,
but only the 8 poles
on one side are used at any one time,
depending on which way up the connector is inserted.
Digital audio and
video information may
be conveyed.
Adaptors to USB, and to other
interfaces, are available.
Compare DisplayPort.
View Lightning connector image
Limiter
A signal processing
unit that prevents the peak
level of a
signal from significantly
exceeding a pre-determined value (the limiting
'threshold'),
either as an effect or to
restrict its
dynamic range in order
to avoid the overload of
amplifiers,
speakers, recording
equipment or broadcast equipment.
If the limiting threshold is crossed by the
input signal, the limiting
action that takes place will reduce the difference between
the signal's average level and its peak level, so the
required headroom is reduced.
Effectively, a limiter is a compressor operating with a very high compression ratio above the threshold. This is usually referred to as an 'infinite' compression ratio (i.e. 'infinity to 1'), but in practice a ratio of 10:1 or more provides a limiting effect. See also Dynamics processor, Speaker management equipment and PPM.
Limiting
The effect on
dynamic range that is
intentionally produced by a
limiter, or that is
caused by some other means (intentional or otherwise)
that has a similar effect.
Lin
An abbreviation for 'linear'.
Lin pot
An abbreviation for 'linear
potentiometer'
− see Potentiometer.
Line (1)
A physical
signal-interconnection
between two points (usually a significant distance apart),
made by means of a cable.
In the case of a multicore
cable, the same cable incorporates several lines.
Note that a line may carry any type or
level of signal for which
the cable is suitable (not just
line-level
analogue signals).
The signal path
over a line may be referred to as a
circuit or a
channel. To assist
in the reduction of interference picked-up on the
line, lines are often screened
and/or balanced.
See also Line check,
Link (1),
Feed,
Pair,
Leg,
100 volt line,
Returns,
Line driver,
Termination (1) and
Termination (2).
Line (2)
The mains power for an
item of equipment, or a cable
carrying such power. See also
Line conductor and
Line filter.
Line-up (1)
The particular set of musical instruments used by a
band during a performance, or the particular set of
performers involved.
Line-up (2)
The process of adjusting the relative
signal
levels throughout a
system, so as to achieve the required
gain structure.
See also Tone (2),
SOL and
Programme level.
Line array
A speaker system
consisting of a vertical column of
drivers, either all contained
within a single enclosure or
(more usually) each driver having its own enclosure.
The array is usually flown, and may hang straight downwards or be 'articulated' into a curved shape. A common shape is the so-called 'J-curve', in which the units are angled increasingly downwards towards the lower end of the array, forming the shape of the letter 'J' when viewed sideways from stage right. (The amount of curvature necessary can be reduced by use of downfill or front-fill speakers.)
The resulting sound field has very limited dispersion in the vertical plane, but provides a broad horizontal coverage. It can be pictured as a segment of a horizontal slice of a vertical cylinder (i.e. like a wedge-shaped piece of a circular cake). The result is that, for a particular range of frequencies and up to a particular distance (the 'critical distance'), the on-axis sound level decreases by 3 dB for each doubling in distance, rather than by the 6 dB that is applicable to a point source (see Inverse square law).
The critical distance is dependent upon the length (i.e. height) of the array and upon frequency, but opinions differ as to how best to define this distance and calculate its value. Mark Ureda (of JBL) proposed in a 2001 AES convention paper (preprint 5304 − click to open in a new window) that a suitable approximate calculation would be (array length2 x frequency) / 700, where the array length and the resulting distance are in metres, and the frequency is in Hz. However, in a 2002 paper (AES ref 5649 − click to open in a new window) he proposed a revised approach giving a significantly increased critical distance, calculated (as an approximation) using 0.006 x array length2 x frequency. These calculations show that quite long (i.e. tall) arrays are needed to obtain any useful effect at low frequencies. The high-frequency limit of the effect is determined by how close together the individual drivers of the array can be placed. See also Column speaker. Compare Cluster.
Line check
A specific part of the
sound-check procedure,
during which the allocation and correct operation of the
lines from the stage is
checked. At the conclusion of the line check, it will be
proven that each
microphone and
instrument source is connected to the correct
input of the
mixer (or to the correct
inputs of the
front-of-house and
monitor mixer, where
these are separate) and that there are no problems with
the connections.
Line code
A set of rules that are used by equipment to encode a
digital data stream prior
to making it available at an
output of the equipment.
(For the meaning of 'line' in this context, see
Line (1).)
Using a line code does not add any additional information to
the data being output; rather its purpose is to facilitate
transmission through the interconnection and to facilitate
recovery of the original data stream by the destination
equipment. Line codes are often particularly important for
clock recovery and
frame
synchronisation.
An example of a line code used for
digital audio
interconnections is the
bi-phase mark code.
Line conductor
In mains supplies, a
conductor that is
intended to be at
supply voltage (230 V
in the UK and much of Europe) with
respect to safety earth.
A single-phase supply
has just one line conductor, whereas a
3-phase supply has three
electrically-independent line conductors.
Alternatively known as a phase conductor or, less
formally, as a live
conductor or a hot
conductor.
Line driver
A device whose function is to provide
signal
output characteristics
suitable for connection to a line.
In terms of audio lines,
this usually means providing a
balanced signal at
line level, with sufficient
drive
current capability to
cater for the relatively high
capacitance
values of lengthy cables.
In PA work, line drivers are typically integrated circuit components internal to equipment that has electronically balanced (rather than transformer balanced) outputs. There are several different types of such outputs, which behave differently to each other in some circumstances (see Balanced). For example, quasi-floating line drivers may introduce relatively high levels of noise if used to drive an unbalanced input or an unbalanced cable without linking the cold leg of the output to signal earth.
Line filter
A filter that is intended to
be used in a mains power
feed, with the intention of
reducing mains-borne
radio-frequency interference
(RFI). Such filters must be
rated adequately for the
current demands of the
equipment supplied through them. See also
Power conditioner,
Distro and
VA.
Line input
An abbreviated form of
'line-level
input'; an input that is
intended to accept only a line-level
signal. The
connector is usually
a quarter-inch (6.35 mm)
jack
or a female
3-pole
XLR. Line inputs may be
intended to accept
unbalanced signals only,
or may be designed for
balanced signals
(at line-level).
Line inputs that are intended for balanced signals
can usually also accept unbalanced ones, provided that
they are not supplying
phantom power
(rare for line inputs) and that the
connecting plug is
appropriately wired − check the
equipment manufacturer's instructions. Compare
Microphone input.
Line level
Usually refers to a signal
whose nominal
level is in the region of
0.3 V to 2 V
(−8 to +8 dBu),
or refers to an input or
output
connection intended to carry such a signal.
Typically, line-level signals are used for interconnections
between mixers,
power amplifiers and
outboard equipment such as
graphic equalisers,
effects units
and active
crossovers.
Nominal levels for these signals are standardised at either −10 dBV or +4 dBu.
- −10 dBV (equivalent to approximately −8 dBu or 0.316 V) is used by most semi-professional equipment, home studios, etc. These connections are typically of the unbalanced type.
- +4 dBu (equivalent to approximately +2 dBV or 1.23 V) is used by most professional equipment. These connections are typically of the balanced type, in which case +4 dBu refers to the level between the hot and cold conductors (not from each of these conductors to signal earth).
The name derives from the level of signals typically used on analogue lines between distant stations (such as broadcast studios and transmitter sites). See also 100 volt line. Compare Microphone-level and Instrument-level.
Line
microphone
An alternative name for a
rifle microphone.
Line output
An abbreviated form of
'line-level
output'; an output that
provides a line-level
signal.
Line sync
In an analogue
video
signal, the
synchronisation
pulses that indicate the start of each horizontal
line of the picture.
In a 625-line picture, the pulses occur at
a frequency of 15.625 kHz,
an interval of 64 µs.
The short black-level interval preceding the line sync pulse
is called the 'front porch'.
The slightly longer black-level interval following the line
sync pulse is called the 'back porch' and in a
composite video
signal carries the
chrominance
reference burst; it is also used for black-level
clamping.
Both line and field sync
pulses occupy the negative-most
0.3 volts of a
standard 1 volt peak-to-peak video waveform.
Linear
Describes a system, or a component of a system, that
has a truly proportional response. That is to say,
the output of the system is always in direct
proportion to its input (under normal operating
conditions and within a practical and appropriate
accuracy of measurement). So, a doubling of input
produces a doubling of output, etc. Where the
input is a signal, such
a component
or system has the important property that no new
frequencies related
to the original ones are introduced into the signal by it.
The term literally means 'relating to a (straight) line', which refers to the fact that if a graph is plotted of output values versus input values, under normal operating conditions, an essentially straight-line graph is obtained over the range of interest. For examples of the use of this term, see the following definitions.
A system or component that does not behave linearly in some respect may be described as 'non-linear' in that respect, and this behaviour may result in intermodulation. See also Interpolation.
Linear PCM
PCM that utilises
linear
quantisation, which
means that equal changes in
analogue
sample values
result in equal changes in the numeric value assigned
to each sample. This scheme is used in standard
CD recordings and in S/PDIF
and AES3
digital audio
interconnections. It is often abbreviated to LPCM.
For further information see
Pulse code
modulation and
Analogue
to digital conversion.
Linear phase
response
If the phase response
of a system whose
group delay
is constant over a particular
frequency range is plotted
against frequency, the result will be a straight line over
that frequency range; this is described as a 'linear phase
response'. So this term is just another way of saying
'constant group delay', which is a good thing in high-quality
audio systems.
A system having a linear phase response over a particular
frequency range is sometimes described as being
'phase linear' over that range. A system having a linear
phase response is not necessarily a
minimum phase system.
Linear
potentiometer
A potentiometer (pot)
whose track has a
constant resistance
profile along its entire length, known as a
'linear taper'. In an appropriate
circuit it therefore
provides an output voltage
that (for a fixed input voltage) is directly proportional
to the angle that the control is turned (rotary pots)
or the distance it is moved (slider pots).
For further information see
Potentiometer.
Linear
power supply
A power supply
whose transformer
operates at mains
frequency −
i.e. a power supply that is not of the
switched-mode
type. A linear power supply is sometimes referred
to as a 'conventional' one. For further information
see Power supply.
Linear taper
See Taper.
Linearity
The degree to which a system or component behaves in a
linear manner. See also
Intermodulation.
Link (1)
An interconnection path
between equipment or between
locations, a term often used when there is a
considerable distance
involved. Although sometimes used as short for 'radio link'
(i.e. a wireless interconnection), it may also
be used to refer to a physical interconnection −
see Line.
Linkwitz-Riley
Describes a crossover which
uses 4th-order
Butterworth
filters. So-called because the
engineers S. Linkwitz and R. Riley determined that this
arrangement gives an improved result over 3rd-order designs
− not only by virtue of a steeper
slope, but also in that the
low-pass and
high-pass outputs are well
co-ordinated in amplitude and
phase through the region of
the crossover
frequency. This is important because in this region
both drivers
are operating simultaneously, and so problems in
co-ordination of their drive signals can lead to
undesirable variations in the overall output
level and in the
directivity of the
speaker. Abbreviation LR, or
LR-4.
Lip microphone
A microphone which is
intended to be held extremely close to the mouth. It usually
has a shield protruding from its top surface; this is intended
to touch the user's face between the upper lip and the nose −
hence the name. It is generally used only for
sports commentary applications, and is usually of the
noise-cancelling
type.
Lip sync (1)
In audio-visual applications (such as TV or film), the
degree to which the sound and vision tracks are in
synchronism, characterised by the degree to which the
sound of a person's voice corresponds with the movement
of their lips. Lip sync may be adversely affected by the
latency of
signal processing
equipment such as digital
vision mixers, or by a large distance between the
speakers
and the audience (see Speed of sound).
Lip sync (2)
To move one's lips in synchronism with a previously made
recording of the required vocal sounds, e.g.
to mime to a pre-recorded vocal
sound-track. Often a
microphone will be
used for purely visual effect − it will generally
be muted so as to avoid
any accidentally-made vocalisations appearing in the
mix, unless the performer
wishes to add further vocals or speech at one or more
points.
Listen wedge
The monitor
speaker
that is provided for use by the
monitor engineer.
It enables him/her to listen to a particular performer's
monitor mix − this is especially
useful when making adjustments to the mix. Typically the
listen wedge is fed from the 'control room' output of the
mixer, and the mix to be
monitored is selected by activating the
AFL of the appropriate
aux send. The monitor
should be of the same type as those used by the performers,
and any outboard
processing (most commonly graphic equalisers)
that is used for the monitor mixes should be
patched into the aux send
inserts of the mixer, so
that their effect is apparent in the listen wedge.
See also Wedge.
Littlite
A trademarked name for a particular brand of miniature
illumination devices for equipment and for musicians'
music stands. The gooseneck
variety is frequently used for illumination of
mixer control surfaces, where
it is often powered and supported via a
BNC
connector. The most popular
incandescent lamp
rating
is 12 volts
5 watts.
LED versions are also available.
Live (1) (electrical)
Describes an item of equipment that is currently
in use, or that is connected to a source of electrical power
− especially mains power.
(More formally, live mains supply distribution equipment or
cabling is described as
'energised'.)
Or, describes a conductor
that is at (or is intended to be at) a significant
voltage with respect to the
general mass of earth; that is, a
non-earthy
conductor, sometimes referred to as a
'hot'
conductor, or marked '+'.
Warning: In energised mains supplies, all the
current-carrying conductors
(including the Neutral) should be
treated as being 'live', i.e. at a dangerously high
voltage with respect to earth, and must never be exposed
to the touch. See also
Line conductor.
Live (2)
(microphones)
Describes a microphone
that is currently 'open' − i.e. the sound that it
picks up is currently being used in some way (amplified,
recorded, broadcast, etc.).
See also NOM.
Live (3) (programme)
Describes a programme
originating from a performance, or other event, that is
happening at essentially the present time; not one
previously recorded.
Live programme material must be handled in
real time, and
latency may therefore be
important.
Lively
Describes acoustics
having a long
reverberation
time.
Compare Dead (1).
LKFS
An abbreviation for
'loudness, K-weighted, relative to full-scale',
the term defined by the ITU
(ITU-R BS.1770-4) for a standard measurement of integrated
programme
loudness employed
primarily in the broadcast industry. It is now simply
an alternative name for the identical standard
LUFS measurement
defined by the EBU [though
prior to 2011 it was different as it was then an ungated
measurement]. For more detailed information see
LUFS.
LM
An abbreviation for
'lower
mid-range'.
LMID
An abbreviation for
'lower
mid-range'.
Lo-Z
See Low
impedance.
Load (1) (electrical)
[Noun:] The item of equipment, or an
input
of it, that is the destination of a
signal or of a
source of electrical power.
The impedance of the
load is often significant (see the
next definition), particularly in regard to
speakers.
Or, the amount of current
drawn by, or amount of power
required by, one or more items of equipment. Where the
equipment has (or potentially has) a poor
power factor,
the load may be expressed in terms of
VA rather than
watts.
Or, [Verb:] The effect that connection of one or more loads has on the output of the source equipment − particularly as regards the resulting reduction in output voltage level. Specifically, to cause a current to be drawn from a source of voltage (e.g. an output of an item of equipment), or to cause power to be drawn from a source of electrical energy (such as a mains supply point).
See also Overload (1), Overload (2), Voltage-matched and Impedance-matched.
Load (2) (mechanical)
The tensile or compressive force exerted on an item of
mechanical equipment such as a chain, rope,
truss, hoist, staging, etc.,
typically specified as a weight value in kg, pounds or tons.
Such loading forces must under all circumstances be less than
the Safe Working Load
(SWL)
of the equipment sustaining them − this may require
the SWL to be considerably higher than the actual weight
of the item(s) to be supported.
Or, a term for the equipment (or other influence, such as
the wind) exerting such a force.
Load impedance
Usually, the combined
input impedance
value of all the loads
that are connected to a particular
output
channel of an item of
equipment, whether via a single output
connector of the
equipment, via multiple internally
paralleled connectors,
daisy-chained load
equipment or other
signal splitting arrangements.
Or, the term may refer to the input impedance value
of a particular load.
Where two or more identical loads are connected to an output channel in any arrangement that effectively connects them in parallel, their combined load impedance is the impedance of one of them divided by the number of them. In order to avoid a reduction in the performance of equipment, and/or possible damage to it, it is necessary to ensure that the combined load impedance connected is not less than the minimum specified for the output channel concerned. This is of particular importance in the interconnection of speakers and power amplifiers − see the Impedance section on the Amplifiers & Speakers page. See also Headphones, Input impedance, Output impedance, Parallel, Series, Voltage-matched and Impedance-matched.
Load-in
The process of moving a performer's equipment,
or a rig, from the transport
vehicle(s) into position in
the venue before an event. Also referred to as the 'get-in'.
Compare Load-out.
Load-out
The process of moving a performer's equipment,
or a rig, from its location
in the venue into the transport
vehicle(s), after an
event. Also referred to as the 'get-out'.
See also De-rig.
Compare Load-in.
Loaded (1)
Describes a signal
output
or an electrical supply source
that has a load
connected to it. See also
Output impedance,
Termination (2),
Overload (1) and
Overload (2).
Compare
Open-circuit (1)
and Unloaded (1).
Loaded (2)
Describes a speaker
enclosure or
cab that has
drivers fitted.
Compare Unloaded (2).
Lobar
Describes a
microphone whose
polar response
contains multiple lobes.
An alternative term for a
rifle microphone,
because this type has such a polar response.
Lobe
An imaginary 3-dimensional region in space,
emanating from a speaker
or a microphone,
the boundary of which area defines a given degree of
sensitivity of that
particular device at a particular
frequency.
For example, the rear lobe of a microphone is the
region of pick-up on the microphone's rear
axis, which
occurs in the case of
super-cardioid and
hyper-cardioid
polar responses.
Log
An abbreviation for 'logarithm'. The mathematical power to
which some number (called the 'base') must be raised in order
to give a particular value. Logarithms to the base 10 are
generally assumed, so the log of a number is the power
to which 10 must be raised to give that number.
For example the log of 1000 is 3, because
103 = 1000. Similarly, the log of 0.01 is
−2, and the log of 1 is 0.
Or, an abbreviation for 'logarithmic', which describes a component, scale, or response in which the 'output' varies in accordance with the logarithm of the 'input' (or approximately so, or within defined limits). An example of this usage is 'log pot' (see Potentiometer).
The main relevance to sound engineering is in relation to decibels, which is a logarithmic scale − see the Decibels page. See also Taper.
Log pot
See Potentiometer.
Long throw
Describes a speaker
that has narrow dispersion
angles, enabling it to provide sound effectively to an
audience at some distance from it (say, greater than 12
metres). See also
Directivity.
Compare Short throw.
Longframe jack
See Jack.
Longitudinal
choke
An inductor whose
purpose is to introduce an opposition to
signals carried by
all the conductors
of a cable, rather than to the
signals appearing between the conductors.
Most often used to reduce
common-mode
radio-frequency (RF)
electromagnetic interference (EMI) picked up by,
or radiated by, computer equipment or other
digital signal
processing
equipment. In such cases the choke is usually implemented
as a ferrite cylindrical or
toroidal
core through which the whole
cable is passed either once or twice; this is seen as a
cylindrical 'bulge' close to one (or both) ends of the cable.
Sometimes called a 'common mode choke'.
('Choke' is just another term for an inductor.)
Longitudinal
wave
A physical wave in which the displacement of the
medium occurs in the same
direction as the travel of the wave. Or, more formally,
occurs along the same axis as a line drawn between
successive crests.
For example, a sound
wave in air is a longitudinal wave consisting of alternate
regions of compression
and rarefaction.
See also Propagation
of sound and
Speed of sound.
Loom
Short for wiring loom or cable loom.
A group of wires or
cables secured together,
either permanently or temporarily. For example, the fixed
cabling that inconnects items of equipment permanently
installed in an equipment rack or behind interconnected
patch bays. Or, a set
of cables, temporarily secured together using
cable ties or
PVC tape, that run as a
group between two points.
Or, the grouped-together wiring internal to an item of
equipment, which interconnects its circuit boards,
panel connectors,
controls, etc.
Alternatively known as a harness (wiring harness,
cable harness).
See also the next definition.
Ribbon cable.
Loom box
A compartment of an equipment
flight case,
intended to contain the cables connecting to the equipment
so that they may remain connected while the equipment is in
transit. It is usually
located at the rear of mixer
flight cases, and usually has a hinged or removable lid.
The slang term 'dog box' is sometimes used.
Loop (1)
Short for
Induction loop.
Loop (2)
Short for Effects loop.
Loop (3)
A recording that, when played, is arranged to restart each
time that it reaches the end of the recorded material.
Loop amplifier / driver
See Induction loop.
Loop system
See Induction loop.
Loss
The amount of
attenuation given to a
signal between two specific
points in a system or between two points within an item
of equipment. Or, the amount of reduction in response (or
sensitivity) of an
item of equipment at a particular
frequency, as compared to its
response at another frequency (often a standard reference
frequency such as 1 kHz)
− see
Frequency response.
Loss is usually measured in
decibels; if it is understood
that the value is a loss then strictly it should be quoted as
a positive value, though very often a negative value is given.
Compare Gain.
Lossless (1)
See
Compression (2).
Lossless (2)
Describes equipment or an interconnection having
no attenuation.
Lossy (1)
See
Compression (2).
Lossy (2)
Describes equipment or an interconnection having
a significant
attenuation.
Loud
A subjective term, describing
sound judged to be of
high loudness relative
to an implied, specific or arbitary reference.
See also
Sound pressure
level. Compare Quiet (2),
but take note of Quiet (1).
Loudness (1)
The apparent magnitude or 'volume' of an audible
sound, as perceived by an
average human listener (therefore also called 'perceived
level', or 'perceived volume').
On average, loudness is subjectively judged to double for
each 10 decibels
increase in
sound pressure
level.
Usually measured in phons
(occasionally in sones).
Contrast with sound pressure level (SPL), which (in
unweighted form) is the
actual magnitude of the sound waves, regardless
of how an average listener might perceive their level.
(Note, however, that SPLs may also be quoted as
weighted values, which
do provide an approximation to the perceived sound level.)
See also VU,
Weighting,
LUFS and
LRA.
Loudness (2)
A control, usually found on a domestic
Hi-Fi
amplifier,
which may be operated to activate
equalisation
that partially compensates for the apparent
reduction in sound quality which occurs when
listening at low
sound pressure
levels.
It works by boosting
the bass
frequencies
to compensate for the ear's reduced sensitivity to
those frequencies at low
levels, and also
boosting the treble
frequencies for the same reason and to compensate
for the masking effect
of low-level
ambient noise.
See also Fletcher-Munson
curves.
Loudness leveller
See Leveller.
Loudspeaker
See Speaker.
Low cut
The process of reducing the
bass content of an
audio
signal.
Or, the name of a switch or
control(s) that provides
this function (e.g. on a
mixer, where it may be
considered to be a part of the
channel
EQ section).
Some types of microphone
are equipped with switches providing a low cut
(or bass roll-off)
function, sometimes giving control over the
cut-off frequency
and/or slope; this facility
is generally provided only on
condenser types.
May be abbreviated to 'LC', or may alternatively be
referred to as 'high pass' (HP).
On a mixer, a variety of forms are found, depending
on the mixer sophistication − for details see
Equalisation
on the Mixing Facilities page.
For cross-references to more filtering-related
terms see Filter (1).
Compare High cut.
Low end
The bass end of the
audio-frequency
spectrum, also called the 'bottom end'.
Compare High end.
Low frequency
See Bass (1).
Low impedance
Usually describes equipment or interconnections in which the
source impedance
is, or is intended to be, no greater than
600 ohms.
Sometimes written 'Low-Z' or 'Lo-Z', because Z
is the symbol for impedance.
Note that inputs intended
for the connection of low-impedance
microphones usually
have an
input impedance
significantly greater than 600 ohms (typically
2 kilohms),
but are nevertheless referred to as 'low-impedance inputs'.
For further information on the impedance of microphones,
see the Microphones page.
A speaker that is not
a 100 V line
type is sometimes referred to as a 'low-impedance speaker'.
See also Matching.
Compare High
impedance.
Low pass
Describes a filter that
attenuates
('cuts')
frequencies higher
than a particular value, but allows frequencies lower
than that value to pass through relatively unaffected.
The boundary between the range of frequencies allowed to pass
and those cut is called the
cut-off frequency,
however in practice the transition between 'cutting'
and 'passing' is not abrupt, but takes place over a range
of frequencies.
The cut-off frequency of a low pass filter is usually
considered to be the frequency at which the
attenuation
is 3 dB greater
than the average attenuation at frequencies that
are low enough to be clear of the transition region.
A low pass filter is sometimes referred to as a 'high cut'
filter. For cross-references to more filtering-related
terms see Filter.
Compare High pass.
Low-Z
See Low
impedance.
Lower mid-range
Describes an
audio frequency
at the lower end of the mid-range
frequencies, typically between 250 and
800 Hz.
Often abbreviated to 'LMID' or 'LM'.
Compare
Higher mid-range.
Lows
Another name for bass
frequencies.
Compare Highs.
LPCM
An abbreviation for
'linear PCM'.
LPDA
An abbreviation for 'log-periodic dipole antenna'.
A type of antenna
that is sometimes used with receivers for
radio microphones
or with transmitters for
in-ear monitoring
(IEM) systems. Its directional behaviour provides
some gain in the
forward direction and (when receiving) provides some
rejection of interference in the opposite direction.
Transmit antenna gain can be particularly useful for
improved signal integrity in IEM systems, because IEM
receivers are usually
non-diversity types.
This type of antenna is sometimes constructed in the shape of a paddle-shaped or fin-shaped plate, and these types are therefore sometimes referred to as 'paddles' or 'shark fins'. Note, however, that some paddle-shaped antennae are omni-directional types, not LPDA ones. See also Antenna distribution unit. Compare Helical antenna.
LPF
An abbreviation for
'low pass
filter'.
LR, LR-4
See Linkwitz-Riley.
LRA
An abbreviation for 'loudness range', the
dynamic range
of a standardised LUFS (or
LKFS) measurement of
programme
loudness, quoted
in LUs. In order to avoid
extreme events such as single gunshots or periods
of silence affecting the overall result, the top 5%
and the lowest 10% of the total loudness range is
excluded from the LRA measurement. For further
information see LUFS.
LSF, LSOF, LSHF, LSOH, LSZH
An abbreviation for 'low smoke and fume'. Describes a
cable that emits little
smoke or hazardous fumes when exposed to extreme heat
or to fire. Such cables are sometimes specified for
permanent PA
installations in certain types of building.
Some types of cable may be referred to as LSOH, LSZH or OHLS
(low smoke, zero halogen)
or LSHF (low smoke, halogen-free), or as
'plenum cable'. However, as considerable differences
exist between the smoke and fume emissions from
different types, always be sure to install an
appropriate type of cable for the situation concerned.
See also CPR (2) and
LFH.
LTE
An abbreviation for 'long term evolution',
a standard for public mobile communications (including data)
that uses fixed radio base-stations connected to a
telecommunications network. It has versions known as
'3G',
'4G' etc.
Care must be taken to avoid possible
interference between LTE equipment and equipment used in
PA systems, particularly
wireless equipment such as
radio microphones
and in-ear monitoring
(IEM) systems.
LTP
In stage lighting, an abbreviation for 'latest takes
precedence'. This refers to the method used to determine
the DMX control values sent by
lighting desks
to fixtures such as
moving-head
lanterns, as the more usual
'highest takes precedence'
(HTP) method would not be applicable
to control parameters such as pan, tilt and
colour-wheel.
In the LTP method, the most recent value determined by the
desk is always output, regardless of whether it is higher
or lower than any other values that might be concurrently
relevant for the same DMX channel.
Compare HTP.
LU
An abbreviation for 'loudness units'. One LU is
a difference of 1 dB
in a LUFS (or
LKFS) measurement of
programme
loudness, or in a
LRA measurement of
loudness range.
LUFS
An abbreviation for
'loudness units, relative to full-scale',
the term defined by the EBU
for a standard measurement of integrated
programme
loudness employed
primarily in the broadcast industry.
In this method, the 'average loudness' of a broadcast
piece is determined by averaging the
K-weighted
true-peak
level of the program
over the duration of the piece, excluding any
periods during which the true-peak level is more than
10 LU
below a version of the measurement that made no exclusions.
The purpose of this exclusion, or 'gating', technique
is to prevent extended periods of very low level or
silence from skewing the measured loudness away from
what would actually be perceived for the piece as a whole,
which is governed mostly by the loudest passages.
An alternative name for the same measurement is
LKFS,
defined by the ITU
(ITU-R BS.1770-4)
[though prior to 2011 LKFS was defined differently
as it was then an ungated measurement].
A loudness leveller
will adjust the level of a programme
signal to provide
adherence to a standard 'target loudness' figure. Such
target values are set
according to the destination of the material, e.g.
for UK television typically −12 to −16 LUFS
is used. See also dB FS
on the Decibels page,
LRA
and Weighting.
Lumen, Lumens
The unit of measurement of total perceived
light output from a source,
or present within a defined beam. More strictly, it is
called the 'luminous flux', and is abbreviated to 'lm'.
Scientifically speaking, a value in lumens is determined
by weighting the overall spectrum of the electromagnetic
radiation emitted by the source in accordance with the
spectral response of an average human eye.
This unit is commonly used to specify the light output of video projectors. As there are several possible ways to make the measurement, the method laid down by ANSI is often used, and values measured this way are usually quoted in 'ANSI lumens'.
The amount of illumination (strictly, 'illuminance') provided onto a surface from a given number of incident lumens, uniformly distributed, will depend upon the area of the surface and is measured in lux. How bright the surface appears to be as a result, when viewed from a particular direction, depends on the light returned from the screen in that direction per solid angle, and is measured in nits. This value will depend on the screen's optical properties, particularly the amount of reflection and the extent of scattering that it provides. The contrast ratio achievable from a particular type of (front-projection) screen will depend not only on the screen gain but also on the amount of light returned from the screen's surface (in a given viewing direction) originating from other sources of incident light, as compared with the amount it returns in that direction as a result of the projected light.
Luminaire
Any complete apparatus that is designed to emit
light for the purpose of
illumination. A specific class of luminaire is the stage
lantern, of which there
are in turn many types. Compare
Lamp.
Luminance (1)
The part of a
composite video
signal that carries the
monochrome (i.e. black and white) information,
that is, the brightness information.
Abbreviation 'Y', thought to be because of its
appearance being indicative
of converging beams of the three additive primary
colours (red, green and blue), whose total illuminating
strength equals the value of luminance.
See also S-video,
Grey scale and
IRE.
Compare Chrominance.
Luminance (2)
The apparent brightness of an illuminated or light-emitting
surface, when viewed from a given direction. In other words,
the amount of light returned or emitted from one square metre
of the surface in that direction, per solid angle. In the case
of an illuminated surface, its luminance depends on the amount
of light falling on the surface per unit area (measured in
lux)
and upon the optical characteristics of the surface. For more
information see Nits, the
(unofficial) unit of luminance.
Lux
The unit of measurement of illumination (strictly,
'illuminance') provided onto a surface, or returned from
a surface. The illuminance, in lux, provided by a given
total amount of incident light
on a particular surface
(measured in lumens), uniformly
distributed, is given by the number of lumens divided by
the illuminated area of the surface (in square metres).
For example, 2000 lumens uniformly illuminating
an area of 4 m2 will provide an
illuminance of 500 lux. Abbreviated to 'lx'.
LV
An abbreviation for lead
vocal(s).
Compare BGV
and BV.
lx
An abbreviation for lux.
For the upper case form, see the next definition.
LX
Originally an abbreviation for 'electrics' ('LectriX).
However it now refers to all aspects of stage and
auditorium
lighting, due to the extensive
electrical supply cabling
and equipment that are involved with lighting.
May be pronounced 'lex'. See also
Power distribution,
Lampie and
Sparks.
For the lower case form, 'lx', see the previous
definition.
LX tape
Electrical insulating
tape, usually made from PVC. The name 'LX tape' derives
from its use by lighting engineers and electricians (see
the previous definition). For further details
see PVC tape.
LXQ
An abbreviation for 'lighting
cue'.
See also LX and
Q (5).
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This page last updated 12-Apr-2018.