| Glossary of PA Terms - L
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The glossary pages provide definitions for over 2680 PA-related
terms and abbreviations.
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In the list below, the most commonly looked-up terms
are in bold, lighting-specific terms are in
and video-specific terms are in
Lampie, Lampy *
Lapel microphone *
Lapped screen *
Laser classes *
Lay length *
LC filter *
Lead sheet *
Leading edge dimming *
LED display screen *
Level meter *
LFE channel *
Lighting bar *
Lighting fixture *
Lighting instrument *
Lighting plot *
Lighting truss *
Lightning connector *
Lin pot *
Line array *
Line check *
Line code *
Line conductor *
Line driver *
Line filter *
Line input *
Line level *
Line microphone *
Line output *
Line sync *
Linear PCM *
Linear phase response *
Linear potentiometer *
Linear power supply *
Linear taper *
Lip microphone *
Lip sync *
Listen wedge *
Load impedance *
Log pot *
Log taper *
Long throw *
Longframe jack *
Longitudinal choke *
Longitudinal wave *
Loom box *
Loop amplifier *
Loop driver *
Loop system *
Loudness leveller *
Low cut *
Low end *
Low frequency *
Low impedance *
Low pass *
Lower mid-range *
LX tape *
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.
An abbreviation for 'Left
the level that is exceeded for <number> percent
of the time, over the period of the measurement,
usually expressed in
For example, the L10 figure is the level that is
exceeded for 10% of the time during the
See also Sound
level meter. Compare
In equipment circuit
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
See the next definition.
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.
Sound level meter.
Compare L<number> (1)
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
involving lambda, see
this entry on the
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
fluorescent, discharge and
A slang term for a lighting engineer.
See also LD,
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
See also AES50,
The 'proper' name for a 'stage light' − an
containing one or more light sources (and
possibly other devices such as lenses,
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,
Profile spot and
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
A screen that consists
of many fine strands of bare wire laid parallel with
each other in a helical (or 'spiral') wrapping around the
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
The time delay introduced in a
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
In the case of digital communications, latency
may result from use of a relatively low communications
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
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
(including the start and stop bits).
At the standard MIDI data rate of
31.25 kbit/s, the
message will therefore take just over
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
A slang abbreviation for
lavalier; a short-hand term
for a lavalier microphone
(see the next definition).
(or, less commonly,
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
and are used in conjunction with a
In some situations these microphones can be prone to
problematic variations in
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
types, but these are more prone to
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
A rule that expresses the relationship between two
or more quantities. For example, the law of a
(also referred to as its taper) defines how
varies as its physical setting is changed, and
Ohm's law defines
the relationship between
See the following two definitions.
The distance taken up by each complete twist of the
wires within a
Sometimes just referred to as the 'lay'.
Twisted pair and
Star quad cable.
The particular physical arrangement of the
pairs within a
Sometimes just referred to as the 'lay'.
See also the previous definition.
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.
An intentional filter
that is constructed using only
capacitors, or an
unintentional one that exists as a result of the
present in a circuit.
Compare RC filter.
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
(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
into the LCD, the display
is described as 'TFT', an abbreviation of 'thin
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
An abbreviation for 'left, centre and right'; describes
arrangement that in addition to the facility for
providing Left and
(for eventual delivery to the Left and Right
speakers of a
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
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.
A combination of the
for inductors (L),
capacitors (C) and
Usually used to describe a
filter which makes use
of all three of these component types.
An abbreviation for 'lighting designer'. Or, in
television production, an abbreviation for
See also Lampie and
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',
'mains lead', etc.
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
line; lead guitar plays (mostly) single notes
(rather than chords) and often has a solo part
in at least one song.
See also Instrumental
A document that specifies the foundational elements
of a song, such as the lyrics, the
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
e.g. to indicate when a guitar solo will be played.
In general, something occurring where ideally it would not.
Most often refers to pick-up by a
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
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
Noise gate and
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
A lectern is often located on a
An abbreviation for 'light-emitting diode',
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
and longer life than
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
techniques, which in some cases can result in
problematic levels of
(RFI). See also LCD.
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
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
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
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
One of the signal-carrying
conductors of a
pair. The two legs of
the pair are commonly referred to as the
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.
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.
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
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.
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
or of a signal (usually
expressed in volts,
By convention, such levels are nearly always expressed as
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
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:
signal levels are usually expressed as a value relative to the
maximum level that can be handled by the digital signal
i.e. as a negative
dB FS value.
Current levels are measured in
Power levels are measured in
mW, etc) − but
see also VA.
levels are measured in
dB SPL (or sometimes in
Lighting levels are usually measured in
A device that indicates the
level of an
signal − see
Or, an abbreviation for
Leveller or Leveler
A device that is intended to remove or reduce variations in
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
setting and appropriate settings of
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
An abbreviation for 'low
See Bass (1).
An abbreviation for
typically referring to the low-frequency effects added
to film sound-tracks.
See Bass (1).
A surround sound
used solely for
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
content of the programme
at a lower average
level in order to
sub-bass signal peaks within the
available dynamic range
of the main recording channels. An improved
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
See also 5.1 and
An abbreviation for 'low fire hazard', typically
relating to the hazards that
may be created by some types of
cables when subjected to
See also CPR (2) and
An abbreviation for 'low frequency oscillator'.
It is most commonly encountered in relation to
time-domain effects such as
where it refers to the
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.
In the context of
an alternative term for
An abbreviation of
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
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
An American term for a lantern.
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
A reversible very thin flat micro-format
developed by Apple® for
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
Adaptors to USB, and to other
interfaces, are available.
Lightning connector image
A signal processing
unit that prevents the peak
level of a
signal from significantly
exceeding a pre-determined value (the limiting
either as an effect or to
dynamic range in order
to avoid the overload of
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
with a very high
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
management equipment and
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.
An abbreviation for 'linear'.
An abbreviation for 'linear
− see Potentiometer.
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
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
See also Line check,
100 volt line,
Termination (1) and
The mains power for an
item of equipment, or a cable
carrying such power. See also
Line conductor and
The particular set of musical instruments used by a
band during a performance, or the particular set of
The process of adjusting the relative
levels throughout a
system, so as to achieve the required
See also Tone (2),
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
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
for each doubling in distance, rather than by the 6 dB
that is applicable to a point source (see Inverse square
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 −
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.
A specific part of the
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
instrument source is connected to the correct
input of the
mixer (or to the correct
inputs of the
monitor mixer, where
these are separate) and that there are no problems with
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
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
An example of a line code used for
interconnections is the
bi-phase mark code.
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
A device whose function is to provide
suitable for connection to a line.
In terms of audio lines,
this usually means providing a
balanced signal at
line level, with sufficient
current capability to
cater for the relatively high
values of lengthy cables.
In PA work, line drivers are typically
components internal to
equipment that has
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,
line drivers may introduce relatively high
noise if used to drive an
input or an unbalanced
cable without linking the cold
leg of the output to
A filter that is intended to
be used in a mains power
feed, with the intention of
(RFI). Such filters must be
rated adequately for the
current demands of the
equipment supplied through them. See also
An abbreviated form of
input'; an input that is
intended to accept only a line-level
connector is usually
a quarter-inch (6.35 mm)
or a female
XLR. Line inputs may be
intended to accept
unbalanced signals only,
or may be designed for
Line inputs that are intended for balanced signals
can usually also accept unbalanced ones, provided that
they are not supplying
(rare for line inputs) and that the
connecting plug is
appropriately wired − check the
equipment manufacturer's instructions. Compare
Usually refers to a signal
level is in the region of
0.3 V to 2 V
(−8 to +8 dBu),
or refers to an input or
connection intended to carry such a signal.
Typically, line-level signals are used for interconnections
power amplifiers and
outboard equipment such as
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
+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
The name derives from the level of signals typically
used on analogue
between distant stations (such as broadcast studios
and transmitter sites). See also
100 volt line.
An alternative name for a
An abbreviated form of
output'; an output that
provides a line-level
In an analogue
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
signal carries the
reference burst; it is also used for black-level
Both line and field sync
pulses occupy the negative-most
0.3 volts of a
standard 1 volt peak-to-peak video waveform.
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
or system has the important property that no new
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
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
PCM that utilises
means that equal changes in
result in equal changes in the numeric value assigned
to each sample. This scheme is used in standard
CD recordings and in S/PDIF
interconnections. It is often abbreviated to LPCM.
For further information see
to digital conversion.
If the phase response
of a system whose
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
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.
A potentiometer (pot)
whose track has a
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
A power supply
operates at mains
i.e. a power supply that is not of the
type. A linear power supply is sometimes referred
to as a 'conventional' one. For further information
see Power supply.
The degree to which a system or component behaves in a
linear manner. See also
An interconnection path
between equipment or between
locations, a term often used when there is a
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 −
Describes a crossover which
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
high-pass outputs are well
co-ordinated in amplitude and
phase through the region of
frequency. This is important because in this region
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
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
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
equipment such as digital
vision mixers, or by a large distance between the
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
that is provided for use by the
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.
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
connector. The most popular
is 12 volts
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
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
conductor, sometimes referred to as a
conductor, or marked '+'.
Warning: In energised mains supplies, all the
(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
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
Live programme material must be handled in
real time, and
latency may therefore be
having a long
Compare Dead (1).
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
primarily in the broadcast industry. It is now simply
an alternative name for the identical standard
defined by the EBU [though
prior to 2011 it was different as it was then an ungated
measurement]. For more detailed information see
An abbreviation for
An abbreviation for
Load (1) (electrical)
[Noun:] The item of equipment, or an
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
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
the load may be expressed in terms of
VA rather than
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
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).
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
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.
Usually, the combined
value of all the loads
that are connected to a particular
channel of an item of
equipment, whether via a single output
connector of the
equipment, via multiple internally
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
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
section on the
Amplifiers & Speakers page.
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'.
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.
Describes a signal
or an electrical supply source
that has a load
connected to it. See also
Overload (1) and
and Unloaded (1).
Describes a speaker
cab that has
Compare Unloaded (2).
contains multiple lobes.
An alternative term for a
because this type has such a polar response.
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
For example, the rear lobe of a microphone is the
region of pick-up on the microphone's rear
occurs in the case of
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
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
The main relevance to sound engineering is in relation to
decibels, which is a logarithmic scale − see the
See also Taper.
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
Compare Short throw.
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
electromagnetic interference (EMI) picked up by,
or radiated by, computer equipment or other
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.)
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
For example, a sound
wave in air is a longitudinal wave consisting of alternate
regions of compression
See also Propagation
of sound and
Speed of sound.
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,
Alternatively known as a harness (wiring harness,
See also the next definition.
A compartment of an equipment
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.
Short for Effects loop.
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.
See Induction loop.
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)
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.
Describes equipment or an interconnection having
Describes equipment or an interconnection having
A subjective term, describing
sound judged to be of
high loudness relative
to an implied, specific or arbitary reference.
level. Compare Quiet (2),
but take note of Quiet (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
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,
A control, usually found on a domestic
which may be operated to activate
that partially compensates for the apparent
reduction in sound quality which occurs when
listening at low
It works by boosting
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
See also Fletcher-Munson
The process of reducing the
bass content of an
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
Some types of microphone
are equipped with switches providing a low cut
(or bass roll-off)
function, sometimes giving control over the
and/or slope; this facility
is generally provided only on
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
on the Mixing Facilities page.
For cross-references to more filtering-related
terms see Filter (1).
Compare High cut.
The bass end of the
spectrum, also called the 'bottom end'.
Compare High end.
See Bass (1).
Usually describes equipment or interconnections in which the
is, or is intended to be, no greater than
Sometimes written 'Low-Z' or 'Lo-Z', because Z
is the symbol for impedance.
Note that inputs intended
for the connection of low-impedance
significantly greater than 600 ohms (typically
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.
Describes a filter that
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
however in practice the transition between 'cutting'
and 'passing' is not abrupt, but takes place over a range
The cut-off frequency of a low pass filter is usually
considered to be the frequency at which the
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.
at the lower end of the mid-range
frequencies, typically between 250 and
Often abbreviated to 'LMID' or 'LM'.
Another name for bass
An abbreviation for
An abbreviation for 'log-periodic dipole antenna'.
A type of antenna
that is sometimes used with receivers for
or with transmitters for
(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
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
types, not LPDA ones. See also
distribution unit. Compare
An abbreviation for
An abbreviation for 'loudness range', the
of a standardised LUFS (or
LKFS) measurement of
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.
See LCR (1).
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
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
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
Care must be taken to avoid possible
interference between LTE equipment and equipment used in
PA systems, particularly
wireless equipment such as
and in-ear monitoring
In stage lighting, an abbreviation for 'latest takes
precedence'. This refers to the method used to determine
the DMX control values sent by
to fixtures such as
lanterns, as the more usual
'highest takes precedence'
(HTP) method would not be applicable
to control parameters such as pan, tilt and
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.
An abbreviation for 'loudness units'. One LU is
a difference of 1 dB
in a LUFS (or
LKFS) measurement of
loudness, or in a
LRA measurement of
An abbreviation for
'loudness units, relative to full-scale',
the term defined by the EBU
for a standard measurement of integrated
primarily in the broadcast industry.
In this method, the 'average loudness' of a broadcast
piece is determined by averaging the
level of the program
over the duration of the piece, excluding any
periods during which the true-peak level is more than
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
defined by the ITU
[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,
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
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.
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
The part of a
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
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
and upon the optical characteristics of the surface. For more
information see Nits, the
(unofficial) unit of luminance.
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'.
An abbreviation for lead
An abbreviation for lux.
For the upper case form, see the next definition.
Originally an abbreviation for 'electrics' ('LectriX).
However it now refers to all aspects of stage and
lighting, due to the extensive
electrical supply cabling
and equipment that are involved with lighting.
May be pronounced 'lex'. See also
For the lower case form, 'lx', see the previous
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.
An abbreviation for 'lighting
See also LX and
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This page last updated 20-Mar-2018.