| Glossary of PA Terms - U
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The glossary pages provide definitions for over 2680 PA-related
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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
Uniform directivity *
Uninterruptible power supply *
Unity gain *
Unity power factor *
Unpowered mixer *
Unpowered speaker *
Unregulated frequency *
Unregulated power supply *
Unscreened cable *
Unshielded cable *
Upper mid-range *
Upright piano *
Upward masking *
Upward scaling *
US terminology *
Utility output *
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 'units', meaning a unit of vertical
measurement on a 19
inch rack system. One U is equal to 1.75 inches
(approximately 44.5 mm). Note, however, that the mounting
holes in the rack system are at a smaller spacing than this.
These units may be referred to as 'rack units', but this term
should not be abbreviated to 'RU' as the correct
abbreviated form for these units is 'U'.
An abbreviation for
'unity gain', sometimes
used as a panel marking to indicate such a setting of a
control (e.g. on some Mackie mixers).
A symbol for voltage,
sometimes used in formulae
in place of 'V'. Note that 'U' should never be used as
an abbreviation for 'volts', the unit of voltage.
A microbar, a unit of pressure equal to one millionth
of a bar, or to
one million pbar.
The Ábar is a unit sometimes used for quoting
levels (SPL), as the bar is very much too large for
that purpose. One Ábar is equivalent to
See also Pascal and
on the Decibels page.
An abbreviation for 'universal disk format',
specification for data file storage on optical
An abbreviation for 'microfarad', one millionth of a
A common unit for the measurement of
See also nF and
An abbreviation for 'ultra high definition'.
Refers to video displays
or projection that provides images of very high
resolution, such as
See also 3G (2),
For information on the most common
video resolutions see VGA.
An abbreviation for 'ultra high frequency'.
Refers to radio frequencies
in the range 300 MHz to
1000 MHz, used by many types of
radio instrument systems and
systems. For more details, see the
radio mic information
on the Microphones page.
An abbreviation for 'uuser interface' −
the complete set of controls, indicators and displays
on an item of equipment.
Describes a sound
whose frequency is
above the generally accepted
range but is below
That is, sounds in the frequency range
20 kHz to around
100 kHz. Compare
signal interconnection arrangement
having only one un-earthed
conductor. The 'return'
current path is via
a signal earth
conductor, often the
screen of the interconnecting
cable. The screen therefore
forms the 'cold'
conductor of the circuit.
Unbalanced interconnections can be vulnerable to a number
of problems, particularly when the interconnection is used
to carry low-level signals.
Such problems include:
For these reasons, this type of interconnection is
suitable only for use with moderate lengths of
cable (up to around 5-30 metres, depending upon the
interconnection's impedance and signal level).
For low-level signals (such as in
interconnections) or for long lengths of cable,
are much preferred.
An unbalanced interconnection is sometimes
referred to as 'one-legged'
or 'single-ended', particularly when operated over a cable
that is suitable for balanced interconnections.
See also Coaxial cable,
(A table comparing the most common types of balanced
interconnections and balanced-to-unbalanced interconnections
is provided under the 'Balanced' entry.)
A person who learns the part of a principal performer,
so as to perform that part in the event of the principal
performer being unwell or otherwise unavailable. Or, any
person in training for a particular role or duty, such
as Sound Engineer.
The process of erasing the table of contents (TOC)
on a re-writable audio compact disc
in order to allow additional tracks to be recorded. This
operation cannot be performed on a recordable compact disc
Describes a microphone
that is more sensitive
to sound at the front than at the sides or the back.
The most common types of uni-directional microphone are
See the Microphones page for more
Uninterruptible power supply
An item of equipment that is able to provide a
continued supply of operating power, up to a given
load, to items of
during failure of the mains supply of a
given maximum duration. Usually abbreviated to 'UPS'.
Under normal conditions, the protected load equipment
takes its mains power via
the UPS equipment. Most types
of UPS operate using internal rechargeable batteries,
which must be in good condition in order for the UPS
to function as specified. Frequently a UPS will
systems that are used for safety-critical announcements,
a UPS would not usually be employed to provide
uninterruptible power to a complete
PA system. However, it may
be advantageous to employ one in conjunction with
certain types of associated equipment, such as
computers. Some types of UPS are intended
to provide power only for sufficient time to
allow computer systems to be shut down gracefully;
these types often provide a data connection
to the computer, so that shut down may be performed
automatically within the required time.
The situation in which two or more instruments
are playing notes of the same
pitch, or in which two or
more voices are singing at the same pitch. The term is
generally only used when this applies to a reasonably
lengthy sequence of consecutive notes.
The situation in which, between two specified
points in a system, there is overall neither
That is, the gain between these two points is
0 dB −
a multiplying factor of one (a value otherwise known as 'unity').
On some mixers (e.g. Mackie), the position of a control
which corresponds to a unity gain condition is marked 'U'.
Unity power factor
Describes the situation in which, in an
current and the
in phase with each other.
In this situation the
power factor has a
value of one (a value otherwise known as 'unity').
The term is generally used only in reference to the supply
and utilisation of mains
Describes a signal
or an electrical supply source
that has no load
connected to it. See also
Open-circuit (1) and
Compare Loaded (1).
Describes a speaker
cab that has no
Compare Loaded (2).
Apart from the obvious meaning, describes a musical
performance or recording in which the sound
mix is relatively uncluttered;
in particular being largely devoid of added
effects and in which
heavy drums, sustained or
and electronic keyboard fills are usually absent.
The term is most often applied in the context of a solo
artist (often with an acoustic
guitar) or vocal group, and derives from the intention of
producing an overall sound that appears similar to
what might be expected of the artist(s) concerned if giving
an entirely unamplified live
performance (of vocals and acoustic instruments)
in a suitably small venue.
A mixer that does not incorporate
facilities in the same unit. Compare
Powered mixer and
See Passive speaker.
A power supply
that is designed to provide output(s)
may vary significantly, even under normal operating conditions.
A cable that does not incorporate
a screen. For example,
most speaker cables are
unscreened. Also called 'unshielded cable'. See also
Describes a cable,
or a circuit
(or line) within a cable,
that has no physical
that is neither wired to a
connector nor directly to
Describes an output
or a line
that has no
at its final destination. In the case of
interconnection, this is an unsatisfactory situation.
In the case of
interconnection, however, this is a standard condition
for specificiation of a signal
output level (commonly quoted
in dBu or in
− also known as an
Describes a level
measurement to which no
weighting has been
Further away from the audience − towards the 'back' of
the stage. So-called because of the slight upwards incline
('rake') in this direction on a theatrical stage.
(This is the origin of the term 'to up-stage someone'.)
A micropascal, a unit of pressure equal to one millionth of a
The ÁPa is a unit sometimes used for quoting
levels (SPL), as the Pascal is rather large for
that purpose. One ÁPa is equivalent to
An SPL of 20 ÁPa is equivalent
to 0 dB SPL. For further details see
Pascal. See also
on the Decibels page.
An acoustic piano
that has its strings and
sound board fixed
An abbreviation for
The process of converting the
format of a
signal (or of a set of similar
or related signals) to a format that is capable of
carrying higher quality versions of that type of signal.
This does not increase the quality of the signal being
converted, but merely makes it compatible with other
signals in the higher quality format. Such compatibility
may be required for example, to allow several signals
in that format to be handled or processed together.
(It should be noted that the higher quality format
may in some cases be less resilient than the original
one, e.g. to being carried over long
distances, or to the presence of a particular type of
interference.) Upscaling is usually performed by a
scaler. It is sometimes
referred to by the fuller term 'upward scaling'.
A specific form of the masking
phenomenon, in which a sound
masks another sound which is at a significantly higher
An abbreviation for 'universal
a standardised digital
interface for the
interconnection of computers and peripherals.
The original '1.0' specification is now superceded by
the '2.0' version. USB connections are made using
made for the purpose, equipped with either 'Type A' or
'Type B' connectors.
See also IEEE 1394 and
View USB A
View USB B
An abbreviation for 'United States Institute of
Theatre Technology'. See also ABTT
A 'general purpose'
output of a
mixer, provided to
supply auxiliary items of equipment such as an
Utility outputs are often stereo,
but may be mono.
An abbreviation for 'unshielded twisted pair', a type of
cable containing one or
more twisted pairs
and no integral screen
(also called a shield). It is mostly used to
interconnect computers (and some other
computer-related equipment) in computer networks
using an Ethernet
It typically contains four twisted pairs and is
This type of cable is now commonly used between 'senders'
and 'receivers' that convey
signals over moderate
distances. For example, it is commonly used in
VGA extender systems. Another
common application is in
It is important to use the correct 'category' of UTP
cable, to suit the speed of the computer network or
whatever other equipment it is used with.
For example CAT 3 cable is suitable for use at a
bit-rate of only up
to 10 Mbit/s
whereas CAT 5 supports up to 100 Mbit/s
(e.g. 100Base-T Ethernet).
A popular category is CAT 5e; this is an enhanced
version of CAT 5 having a
and is capable of carrying
100Base-T over longer distances
or 1000Base-T over short
distances. See also CCA.
An abbreviation for 'ultra extended graphics adaptor'.
A standard interface
for the connection of display
equipment (such as monitors and projectors) to
computers. The standard gives a resolution of
1600 pixels horizontally
and 1200 pixels vertically. The
See also VGA,
The most commonly encountered standards, their resolutions
and aspect ratios are tabled under the entry for
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This page last updated 22-Jan-2018.