| Glossary of PA Terms - I
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IEC 268-5 *
IEC 320 *
IEC 60268-10 *
IEC 60268-17 *
IEC 60268-18 *
IEC 60309 *
IEC 60320 *
IEC 60958-3 *
IEC 60958-4 *
IEC 61938 *
IEC 651 *
IEC connector *
IEC noise *
IEC noise weighting *
IEEE 1394 *
IEEE 802.11 *
Image frequency *
Image rejection *
Impedance-balanced output *
Impulse impedance *
In-ear monitoring *
Incandescent lamp *
Indirect contact *
Indirect sound *
Induced interference *
Induction loop *
Inductive coupling *
Infinite baffle *
Infinite impulse response *
Ingress protection *
Input impedance *
Input stage *
Inrush current *
Insertion loss *
Installation cable *
Installation speaker *
Instantaneous value *
Instrument cable/lead/cord *
Instrumental break *
Insulating tape *
Insulation displacement *
Integrated circuit *
Interference tube *
Intermediate frequency *
Intraaural or Intraural *
Inverse square law *
IRT norm *
Isolated ground *
Isolating transformer *
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.
See IEEE 1394.
An abbreviation for
"input / output". A designation commonly
used for an interface,
etc. that may be used either as an input or an
output, whether simultaneously or alternately.
For example, see SCART.
See also Duplex.
An abbreviation for 'input'.
An abbreviation for 'International Alliance of Theatrical
Stage Employes' (yes that's how they spell 'employees'), a
stage-workers' union. Their website is
An abbreviation for
An abbreviation for 'insulation displacement
connector'; any type of
connector that uses
An abbreviation for 'International Electrotechnical
Commission', an organisation which defines standards for
use in the electronics industry. In PA
work, the term 'IEC' is most commonly used (as an
abbreviation for 'IEC 320') to refer to a particular
common type of 3-pole
connector. For further
information see IEC 320.
Note, however, that the IEC produce very many other
specifications, including ones for other types of mains
(such as the CEE-form).
Some of the specifications most relevant to PA work are
listed in the definitions following this one.
See also DIN,
See IEC noise.
The original designation of the
IEC standard covering a
common type of 3-pole
frequently used for the connection of mains supply
cables to equipment.
It is commonly referred to as an 'IEC connector'.
The IEC 320 standard is now redesignated
IEC 60320 (or EN 60320), and details several
incompatible types of mains connector (2-pole as well
as 3-pole types). The specific 3-pole connector
most commonly referred to by the terms 'IEC' or
'IEC 320' has two sub-designations: C13 for the
female (outlet connector,
usually cable-mounted) and C14 for the
male (e.g. for
equipment power inlets).
However, some items of equipment that become hot in use,
such as some types of lantern,
are fitted with a heat-resistant type of IEC 320 connector,
properly termed a 'hot condition' type. Such items frequently
have a special type of IEC 320 inlet connector
(C16 or C16A), which is able to accept only hot condition
outlet-cable connectors (C15 or C15A respectively)
that are identified by the presence
of a keyway in their widest side. The 'A' versions are
additionally keyed in their opposite side.
IEC 320 C13/C14 connectors are
at either 6 or 10 amps.
As there is no obvious physical difference between the
6 amp and 10 amp versions, and there is no
built-in means to prevent use of the wrong type,
it is important to ensure that equipment rated at
more than 6 amps is always used with a 10 amp
rated connector and cable.
Items of equipment that draw
currents higher than
10 amps, such as some
use a higher-current version of IEC 320 connector,
designated C19 for the female (outlet, usually cable-mounted)
and C20 for the male (inlet, usually equipment-mounted).
These are usually rated at either 16 or 20 amps.
Warning: All the above types of connectors are used
on both 230 V
and 110 V equipment (and on equipment that is adjustable
for operation on either of these voltages). Before
connecting, always be sure that the mains supply voltage
is suitable for the equipment and/or
that the equipment is set for the mains voltage to be used.
In the UK, a mains cable
fitted with an IEC connector at
one end is frequently fitted with a
('13 amp') plug at the other end. If the IEC connector
is a 6 amp type
then the BS 1363A plug should be fitted with
a 5 amp fuse; if it is a 10 amp type then the plug
may be fitted with a 13 amp fuse (provided that the
cable's current rating is adequate for these fuse values).
IEC 320 connectors may, in non-professional circles,
sometimes be referred to as a
and a mains cable fitted with it referred to as a
'kettle lead'; both are deprecated slang terms.
[Kettles (and other such domestic equipment that becomes hot)
are generally fitted with a 'hot condition' inlet connector
− often the C16 type is used, requiring use of a keyed
C15 female cable connector.]
See also PowerCon and
IEC (IEC320 mains) image
See IEC 320.
connector complying with
an IEC standard. However,
in the context of PA work
the term is most often used to refer to an
IEC 320 connector.
A type of noise
intentionally generated for the purpose of testing
and especially for specifying the
speakers. It is
to pink noise,
but is specially
filtered in order to
provide a better
match to the power/frequency distribution of actual
material. The IEC
standard for this noise is
IEC 268-5, which is very
similar to the DIN 45573 noise specification.
Speakers must withstand the quoted
IEC 268-5 power
level for 100 hours
continuously. See also
White noise and
Ratings on the
Amplifiers and Speakers
An abbreviation for the 'Institution of Electrical
Engineers', a UK-based organisation which set standards
for electrical installations. The IEE no longer exists as
an institution, having become part of the
IET when it was formed in 2006.
However, 'IEE' is still sometimes incorrectly used to
refer to the IET Wiring Regulations
(see BS 7671).
See also PAT.
An abbreviation for the 'Institute of Electrical
and Electronics Engineers', an American organisation
which sets standards for electronic equipment
(e.g. see the following definitions).
interface for high speed
communications, especially with computer equipment.
Also known as FireWire (a trade mark of Apple Computers)
and I.Link (a trade mark of Sony). It can operate using
two types of connector,
a flat 6-pole type which
carries power, and a smaller 4-pole type which does not.
An updated version of IEEE 1394 is known as IEEE 1394b,
or FireWire800; so the original version is now sometimes
referred to as IEEE 1394a, or FireWire400. (400 and 800
relate to the approximate maximum possible
in Mbit/s.) The updated
version uses a 9-pole connector.
The bit-rate in use over a particular interconnection
is identified by 'S100', 'S200', 'S400', etc,
where 100, 200 and 400 relate to the approximate bit-rate
in Mbit/s. (The corresponding actual bit-rates
98.304, 196.608 and 393.216 Mbit/s.)
See also USB and
Firewire (4 pole) image
Firewire (6 pole) image
Firewire (9 pole) image
communications, especially with computer equipment.
Also known as Wi-Fi and AirPort.
See also Bluetooth.
An abbreviation for
An abbreviation for the 'Institution of Engineering
and Technology', a UK-based organisation which
(amongst other activities) sets standards for electrical
installations. Their website is www.theiet.org
(opens in a new window or tab).
became part of the IET when it was formed in 2006. See also
An abbreviation for
An abbreviation for 'interruptible fold back'.
A term used mostly in a broadcast context to refer to a
facility enabling live presenters to hear
cues and other
directions from the producer and possibly to hear other
remote audio (such as an incoming telephone caller,
an outside broadcast or another studio), without that
audio being picked-up by the presenter's studio microphone.
In radio broadcast the presenter will typically wear
headphones for this purpose, while in television in-ear
devices are worn − these are often wired earpieces
but may be wireless types.
The term arises from the need to provide the person's
own voice (or a full station mix) to them most of the
time (to assist in natural speech), but to interrupt that
foldback in order for the producer's voice to be clearly
heard. However, this term is sometimes used to refer
to a general-purpose wireless
system, as used by many on-stage performers.
An abbreviation for 'infinite impulse response'.
Describes a time-domain digital
filter which incorporates
feed-back paths. This enables complex filtering
functions to be readily implemented, however care must be
taken to ensure stability and to avoid the effect of
compounded rounding errors in the computations; the
response can also be problematic. These difficulties are
avoided with FIR types.
The name arises because the filter's response to an
impulse input is not
An abbreviation for
An abbreviation for 'intermodulation' or for
See Stereo image.
A frequency at which
a radio receiver exhibits some unwanted sensitivity as a
result of the conversion from received
radio frequency (RF)
frequency (IF) that
takes place within the receiver. The image frequency will be
either twice the IF higher than, or twice the IF lower than,
the intended RF reception frequency. The extent of the
receiver's sensitivity at the image frequency is specified
in terms of its 'image rejection', which is a function of
the quality of RF filtering employed within the receiver.
An abbreviation for 'intermodulation distortion'.
The value of the ratio of
to current of a
signal at some
point in a system, at a particular
Or, a measure of the opposition to the flow of current,
due to the combined effects of
at an input or
output of an item of equipment,
or through an electrical
at a particular frequency.
As the impedance value generally varies across the frequency
range of interest, a nominal
value is usually quoted, being an 'average' or 'typical'
value over the relevant range of frequencies.
For a DC
circuit, it is the
same as the resistance of the circuit.
Like resistance and reactance, it is
measured in ohms.
It is given the symbol 'Z'.
Impedance is important in various situations, for a number
of different reasons:
For illustrative purposes, some typical impedance values
are summarised as follows:
Load impedance governs
how much current is drawn by a
load, for a given applied
voltage, and therefore affects how much
power will be
the load, for a given applied voltage (because power =
voltage times current, provided that they are
in phase). For example,
the load impedance of a
is usually 4, 8 or 15 ohms (except for
100 V line
types, which have a much higher load impedance). For further
information on speaker impedance, see the
section on the
Amplifiers & Speakers page.
In a voltage-matched
of a signal source and the
input impedance of
the equipment that it connects to are co-ordinated so that
the source voltage is not significantly reduced as a
result of making the interconnection.
For example, a
microphone having an
output impedance of, say, 150 ohms would typically
be connected to a mixer or
having an input impedance of around
2 kilohms, but a
source such as an electric guitar would need to be
connected to equipment (such as a
DI box) having a much
higher input impedance − typically at least
interconnection, the output impedance of a signal source
and the input impedance of the equipment that it connects
to must be the same, in order to maximise the transfer of
power between source and load.
For impedance-matched interconnections that use
signals, or which are very lengthy, the
impedance of the interconnecting
connectors must also be the
same as the output impedance and input impedance values,
in order to maximise the
Speakers (not 100 V line types): 8 or 4 ohms
(sometimes 15 ohms).
Headphones and earphones:
Early consumer types:
Modern consumer types: Usually in the range 24 to
40 ohms; most commonly 32 ohms.
Professional types: Usually 250 or 400 ohms.
Low-impedance: Usually 150 to 300 ohms
(otherwise 50 to 600 ohms).
High-impedance: Usually 5 to 15 kilohms.
Low-impedance (e.g. microphone inputs on
mixers): Usually around 2 kilohms.
line inputs on mixers
and inputs of
10 to 50 kilohms.
Inputs of instrument amplifiers (e.g. guitar amps):
200 kilohms to
Inputs of passive DI boxes:
10 to 50 kilohms.
Inputs of active DI boxes:
200 kilohms to 1 megohm.
Passive guitars and
20 to 150 kilohms (usually varies heavily
according to the setting of the instrument's controls;
also often frequency-dependent).
Active instruments, including
guitars and basses (unbalanced
outputs): 100 ohms to 20 kilohms.
50 to 600 ohms.
Speaker outputs (not 100 V line): 0.005 to 0.1 ohm.
Impedance-matched interconnections: See
The term 'impedance' is frequently used as short-hand for
'load impedance', 'output impedance', 'input impedance' and
'characteristic impedance'; in most cases the intended
meaning is indicated by the context of use.
A term that is usually used as an alternative name for a
Describes an interconnection in which the
load impedance is
equal to the source
impedance. This arrangement is employed where
long lengths of cable are involved,
such as in analogue
digital audio and
DMX lighting control
interconnections. In such cases, it is vital that cable
and connectors of the correct
impedance are used in order to maximise signal transfer
and minimise troublesome signal reflections.
It is also important to avoid
The impedance values used for some common types of
impedance-matched interconnections are as follows:
Note that, in general,
interconnections are not impedance
rather the load impedance is significantly higher than
the source impedance; this is sometimes referred to as a
interconnection. Exceptions to this rule
circuits and the
For most practical purposes, an alternative term for
A system which provides a performer with
sound through one or two
Often abbreviated to 'IEM', or else the slang term 'ears'
is used. Sometimes referred to as 'EWM' (ear-worn monitoring).
Most usually the earphones are connected to a
battery-powered portable radio receiver
in order to provide wire-free operation, but sometimes
they are instead wired to a
monitor output(s) of the
In simple wired set-ups a headphone
is used, providing the same monitor
mix to several performers,
while in more complex set-ups each performer may be
provided with a separate mix via individual
headphone amplifiers or via a
In the case of wireless systems, the receiver receives
from a transmitter which is connected to a
monitor output of the mixer. If required, several receivers
can be used with each transmitter.
The use of IEM is generally superior to the use of
monitor speakers −
usually floor monitors
because it avoids the monitor sound being heard by
the audience or by other performers, and avoids
leakage from monitor speakers
into the microphones
(often a major source of potential
Additionally, in the case of wireless systems, it
enables the performer to move freely around the stage without
loss of monitor sound.
Many wireless types offer two-channel
which allows the performer's receiver to be supplied with
both an overall monitor mix
and just the performer's own sound;
the performer can then adjust the balance between these to
suit his or her own preference, using a control on the
receiver. However, the two channels are instead often used to
provide the performer with a stereo
mix, as this helps to give a sense
of space and so helps to reduce the disorientation that can
sometimes occur when using in-ear monitoring. It can also be
helpful in reducing this latter effect if an
is included in the performer's mix.
These systems must only be operated on the frequencies that
are permitted for use in the area concerned. This means either
frequencies or else licensing the equipment to use a
The frequencies used must also be compatible
with the frequencies used by any
in use at the same time. (See the
radio mic information
on the Microphones page for information
Wireless IEM systems can be particularly prone to signal integrity
problems such as drop-out,
partly because their receivers are usually
A directional transmit antenna
such as an LPDA or
helical type is
therefore sometimes used to increase the effective signal
power transmitted in the direction of the
Describes the situation in which a
voltage (or current)
changes at essentially the same time and in the same
direction (i.e. positively or negatively) as
that of some reference signal at the same
frequency or carrying
the same information, i.e. there is no
phase difference between
them. (They may, however, be of different
Or, the situation in which the changes in a
instantaneous pressure occur at essentially the same time
and in the same direction as those of a reference
sound wave carrying the same information, at specified
location(s). If two in-phase
sound waves combine then they will reinforce one another,
giving an increase in
level; this is termed
interference. In order to avoid the opposite effect
interference), it is important to ensure that
when two or more speakers
cover the same area they are operating in-phase −
unless deliberate timing differences are being used to
provide a specific directional coverage pattern.
There is therefore a need to ensure correct
polarity of all connections.
In an AC
circuit, when the
current are in phase then
the average power
can be calculated by multiplying the
by the RMS current, a situation referred to as
'unity power factor'.
Out of phase and
A lamp in which the
light is produced as a result
of raising the temperature of a metal filament to a
suitable point by passing an appropriate amount
of electric current through
it. Such lamps are therefore also known as filament lamps.
This method of producing light is very
most of the electrical power
consumed gets converted to heat rather than to light.
Therefore, other types of lamp such as
or discharge types
are often used where appropriate.
Mains-powered incandescent lamps
are commonly used in lanterns
for stage lighting, and such lanterns are generally suitable
for dimming by
However, the increased efficiency and convenience of
LED-based lanterns (which often incorporate their own
brightness and colour control) means that they are
increasingly used instead. Incandescent lamps are
sometimes used in speakers
to provide driver protection
− for further information see
safety, the potentially lethal situation where a person
comes into contact with an accessible
conductive part that is
not intended to be live at a dangerous
voltage (e.g. at
mains voltage), but that has
become live at a dangerous voltage because of a fault
condition. Such a conductor would typically be the metal
case of an item of mains-powered equipment, or something
connected to it (such as the strings of a guitar), or some
other item of metalwork (such as a steel gantry).
This situation is protected against by the connection of
accessible conductive parts to a
('earthing' or 'bonding'),
or by the provision of additional
insulation between such
conductors and parts that are intended to be live
Further protection against indirect contact may be
provided by a suitable RCD,
but this should not be the sole means of protection
against indirect contact. These measures
are collectively referred to as 'fault protection'
against electric shock. See also
Sound that has
travelled from its source to the listener, or to a
microphone, by a path(s)
other than a single essentially straight line;
or sound that has undergone one more
Compare Direct sound.
See also Free field,
Diffuse field and
The property of a conductor
which causes it to oppose the flow of a changing current
to a greater extent than it opposes the flow of a steady
current (DC); this property is
measured in henrys. More importantly,
the higher the frequency
of the current, the greater the opposition caused.
Therefore, this property can be used to construct
filters, selectively passing
some frequencies and blocking others.
The inductance of a conductor depends not only on its
dimensions but also on its shape. This is because the
inductive effect has a magnetic origin: coiling a conductor
considerably increases its inductance because it concentrates
the magnetic field, and when such a
component is deliberately
formed it is called an
The inductance which exists, undesirably, in the
conductors of a cable
(or other wiring) may be a cause of high-frequency
attenuation in the cable.
The inductance of a straight piece of normal
wire is around 2 µH per metre
of length (0.6 µH per foot).
Note, however, that the round-trip inductance of a 2-core
cable is less
than its end-to-end inductance, because the
magnetic fields of each conductor are in opposition
and partially cancel one another.
See also Back-emf,
listening facility which provides an
field suitable for pick-up by hearing aids equipped with
a T setting.
Such systems are frequently used to satisfy the requirements
of the DDA (UK legislation).
Code of Practice BS 7594
applies to the installation of these systems in the UK.
The system consists of a single loop of
cable installed at a constant
height and arranged so as to surround the area to be served.
There is an 'ideal' height for installation of the cable
(which depends upon the shape of the loop), but because of
practical considerations it is usually installed either at
ground level or at around 2.5 metres height.
The two ends of the loop cable are
connected to a purpose-designed loop-driver
amplifier, which is
from an output of the PA system.
The magnetic field created by the loop is not contained
entirely within the area enclosed by the loop cable,
but also extends some distance outside of
(and above and below) that area. This
fact can sometimes enable the area of the loop to be
reduced, but also means that loop systems can cause
interference with PA (and other) equipment even when
located outside the loop (more details below), and that
multiple loop systems in close proximity cannot be
Loop systems must meet the requirements of standard
(EN 60118-4 in Europe).
This requires the complete system to have a
from 100 Hz to
5 kHz, and to include
compression in order to
adapt the signal to the
limited dynamic range
of hearing aids. In practice, loop amplifiers generally
have a frequency response of around 80 Hz to 10 kHz.
However, the frequency response actually achieved will be
influenced by the inductance
of the loop, which depends on its size and shape and on
the presence of nearby ferrous materials such as structural
steelwork or reinforcing meshes embedded in concrete floors
and ceilings. Some induction loop amplifiers have the
facility to provide adjustable compensation for the
presence of such materials, usually referred to as a
'metal compensation' control.
The average magnetic field strength inside the loop,
at listener height, is required to be
per metre (A/m)
RMS. Although compression is
applied, its time constant must be sufficiently
long to preserve the essential dynamics of speech,
so as not to impair
Allowing 12 dB of
headroom for such
dynamics implies that it must be possible to achieve a
peak field strength of
0.4 A/m RMS. The amount of
current around the loop that is
needed to create this peak level of magnetic field depends upon
a number of factors, particularly the physical dimensions and
layout of the loop; typically it will be in the region of
6 to 12 amps RMS.
The required value of peak current is the main factor in the
selection of an appropriate loop amplifier, although it must
also be able to produce sufficient
voltage to force this amount
of current through the highly
impedance of the loop, at the
highest frequency at which
such high peaks of current may occur (typically 1.5 to 2 kHz).
The loop cable gauge must be
selected to be suitable for carrying the required current,
and also in order to ensure an appropriate value of loop
impedance. Consult the loop amplifier handbook to determine
the required gauge for your loop dimensions and total cable
length. (As a guide, it is common for modern loop amplifiers
to require a loop resistance
of between 0.5 and 1 ohm for correct operation.)
Loop amplifiers must also avoid the radiation of
electromagnetic interference from the loop, usually achieved
by the application of appropriate output
filtering and by incorporating
limiting to avoid the
possibility of clipping.
A possible problem for PA systems is
from the loop into
(especially if not of the
humbucker type) and
occasionally into other system components such as
poor magnetic screening. Coupling into a guitar pickup
will cause any source (including vocals) that is being
fed to the loop to be heard in that
guitar's backline (after processing
by any guitar pedals
etc.). Those sources will also be fed
from the backline back into the PA system via the guitar's
backline microphone (and by any other
mic(s) that are picking up the
guitar's sound) and/or via
the guitar's DI connection.
If any PA channels
that are receiving the guitar signal/sound
(whether intentionally or not) are fed into the induction loop,
then a circular signal path is created that may result in the
occurrence of feedback.
(This feedback sometimes happens only at low input levels
into the induction loop amplifier, when that
amplifier is not compressing the signal,
and/or only happens when the guitar is held in certain
orientations relative to the loop cable).
To assist in the avoidance of these problems, it is
highly preferable for the loop area to be kept well clear
of the stage area occupied by the band. (It is not
sufficient to avoid the loop area encompassing the stage,
due to the fact that the magnetic field from the loop
extends some distance outside the loop area.)
The phenomenon whereby a
current that is flowing on
one conductor impresses
itself to some degree on a nearby conductor because of the
that exists between the two conductors. Such coupling
is usually undesirable (except internally within a
transformer, and from
an induction loop
to a hearing aid),
and is normally avoided by use of
In such interconnections, the two signal conductors are usually
twisted around each other along the length of the cable,
in order that each conductor experiences a near-identical
inductive pick-up from interfering sources.
The remaining induced interference is then mostly
Undesirable inductive coupling can also be caused by the
close proximity of items of equipment which use transformers
− for example if a
DI box were placed on top of
a combo then it may pick up an
unwanted hum. (Note, however, that
a similar effect can also be caused by
earth loops, and under
certain circumstances such multiple interfering sources
of hum may partially cancel one another when mixed.)
See also Crosstalk.
An electrical component
whose purpose is to introduce
inductance into a
In PA work,
they are most usually encountered in passive
Sometimes called a 'choke'. Inductors most commonly use
enamelled copper wire in their
inductors are most usually
components use a laminated core.
Inductors inevitably also introduce a certain amount of
resistance into the circuit,
but this is usually arranged to be small compared
with their inductive
reactance at the
frequencies of interest;
the amount of resistance introduced is often specified by
the Q of the inductor.
See also Resistor and
See Sealed box.
Describes a sound
whose frequency is
below the generally accepted
range, i.e. is below
20 Hz. (The term
sometimes incorrectly used for this meaning.) See also
A connection point, e.g. on an item
of equipment, intended to accept a
signal from elsewhere,
typically from an output
connection of some
other item of equipment (or, rarely, from an output
of the same item of equipment). Or, a similar connection
point of a stage, module,
or component internal to an item of equipment.
Inputs of modules
processing systems may be virtual
connection points, physically unidentifiable in the hardware.
Or, the signal that is applied to such a connection point.
The term is sometimes used quantitatively of a signal in
reference to its level,
e.g. "How much input does that equipment need?"
input may be classified in many ways, e.g. as
line level or
low impedance or
Often abbreviated to 'I/P'.
See also Connector,
I/O and the next definition.
The impedance that a
input connection on an item
of equipment presents to an applied signal. This is a measure
of the current that will be
drawn by the input, for a given
and therefore indicates the degree to which this input will
output to which it is connected.
For examples of typical input impedance values see
The degree to which the level
of an output's signal will drop, as a result of it
being loaded, is dependent upon
the output impedance
of the signal source and upon the combined input impedance of
all the loads connected to that same output (the overall
As with all impedances, the value of an input impedance
may well change significantly with
frequency − this is
particularly evident in the case of
speakers. See also
High impedance and
The circuitry, within
an item of equipment, which initially processes the
signals that are applied
to the input
of the equipment. See also
current that initially flows
for a very short period (typically less than 1 or 2 seconds)
when a supply voltage
is first applied to an item of equipment. The term is
most commonly used in reference to switch-on of
that is particularly prone to this effect, such as
and some types of
In the case of power amplifiers, the inrush current is
caused largely by the initial charging of the internal
power supply reservoir
capacitors and (in the
case of linear power supplies)
the initial energisation of the
transformer. In the
case of filament lamps it is
caused by the resistance
of a cold filament being very much less than that of one
at operating temperature.
The inrush currents of some equipment can be as much
as 50 times the normal operating supply current, so
for this equipment need to be arranged so as to not
spuriously operate during a normal inrush current, but
only when there is a genuine excess-current situation
of a duration
likely to cause danger or
damage. For example, the mains input fuse of a power
amplifer is typically a
or a thermal
When inrush current problems manifest themselves in spurious
operation of protective devices, it is often the case that
such operation occurs on apparently random instances of
switch-on of the equipment concerned.
This is generally due to random variations in the point,
within the cycle of the
AC supply voltage,
at which the power switch is operated.
On occasions when the switch is closed while the
supply voltage cycle is near a maximum of its
value, the instantanous current will
commence at a higher value. On such occasions a higher
inrush current will result than is the case on occasions
in which the switch is operated near a minimum of
instantaneous supply voltage. This makes the likelihood
of spurious operation of protective devices variable
between different occasions of switch-on.
Equipment such as high-power
power amplifiers, whose inrush current would (by default)
be so high as to make spurious operation of fuses etc.
a likely issue, typically incorporate a design feature
specifically intended to reduce their inrush current to
an acceptable value and so avoid such problems.
Such a feature is commonly referred
to as 'soft start'.
A connection, on a mixer
or amplifier, which
allows the connection of a
serial effects unit
into the internal signal path of
the equipment, by interrupting that path when a plug is
put into the connector.
It is usually a ¼″
(6.35 mm) TRS ('stereo'
The tip is generally used for the
send connection and the
for the return connection
(pre-1990 Soundcraft mixers used the opposite
convention). Inserts may be provided for individual
the main mix,
The insert points are usually
but are sometimes
(check the block diagram
in the handbook!) − this means that effects units
connected this way are handling the un-equalised signal.
This may be an important issue in some cases, such as
processors (but using their
side chain EQ can help).
A channel insert connection may usually also be used as
a direct output
connection, either by partially inserting the jack plug
(not good practice), or by using a special lead which has
the tip and ring of the jack connected together.
See also Normalling.
A measure of the reduction in
level that is caused in a
signal chain by the
presence of a particular item
of equipment (such as a filter
or an earth isolator.)
Numerically, it is the reduction in level (usually expressed
in decibels) that occurs when
the item in question is connected ('inserted') into the
signal chain, as compared to the level obtained when the
item is absent − continuity of the chain then being
made by a direct connection between the previous and
following items of equipment.
A cable that is designed
to be fixed in place within a building,
rather than for short-term use. Such cables do not have the
requirement to withstand frequent flexing or mechanical
abrasions and stresses, and so are usually of lower cost
and are less thick and of lighter weight. Such cables are
usually installed within trunking or are secured with cable
clips. See also LSF.
A speaker that is designed
for use as a long-term fixture in a building,
rather than for portable or mobile use.
Such speakers are styled to be visually discrete, and
are usually provided with a means of fixing to a wall
or ceiling, rather than being floor-standing or
pole-mounted. (Note, however, that any
required may have to purchased as a separate item.)
Some models have various
degrees of weather resistance, for outdoor use.
Types intended for use with
systems are frequently of the
100 volt line
See also A & E
The value of a signal's
power at a single moment
in time. Or, the value of sound
wave pressure at a single moment in time. Note that this
value continually varies, even for a signal or sound of
Compare RMS and
In lighting, the American name for a
A cable intended for
connection to a musical instrument, generally referring
to a cable intended to carry
signals from the instrument
to equipment such as a combo,
DI box or instrument
Such cables are unbalanced
most usually fitted with 2-pole
¼″ (6.35 mm) jack
connectors at both ends.
They are also often used for other unbalanced instrument-level
audio interconnections, such as connections between the items
As interconnections between instruments and the type of
equipment listed above usually operate at
most instrument cables are specifically designed to
minimise the undesirable effects sometimes encountered
with high impedance interconnections. Such effects include
a loss of treble due to
the cable's capacitance,
and can be particularly troublesome when the
very high − as is usually the case with
guitars and basses.
Enhanced mechanical flexibility and resistance to kinking
are also often important design features of such cables.
Note that unbalanced cables fitted with 2-pole jack
plugs are frequently referred to as
however such a cable is not necessarily of a type
specifically designed for high impedance instrument
A rather imprecise term referring to the
level that can typically be
expected from musical instruments − especially electric
guitars and acoustic guitar
pick-ups. The term is imprecise
because different types and makes of instrument are likely
to produce substantially different typical output levels,
and because the level obtained from a particular instrument
at any point in time will usually depend on the settings
of its controls and on the way it is being played.
However, the term is generally used to refer to a
nominal level of around
(−18 dBu or
−20 dBV). Such a
signal is likely to peak at
around 250 mV (−10 dBu or −12 dBV),
but again this will depend on the instrument and on how it
is played. Compare
A section of a song, during which no words are sung.
There is often a solo part by one or more
during this section.
A flexible tape that is specifically designed to have
very good insulating
properties. The most common type of insulating tape
is PVC tape, though
this may not be suitable for use in some situations.
That part of a cable,
connector, or equipment,
which is intended to present a barrier to an electric
current and so prevent
an unwanted flow of it from a
conductor. As such a flow
of current could cause a fatal electric shock or a fire,
it is essential for safety
that insulation remains intact and in good condition.
Insulation is frequently
colour-coded to enable
identification of the associated conductor.
See also Insulator,
PVC tape. Compare
Describes a connector
or a cable
having one or more metal blades that slice through the
insulation of the
individual wires of the
cable, so making contact with the
conductors of the
wires. Such a connection may
be referred to as a 'punchdown connection'. A special tool,
designed to suit the particular terminal type in question,
is required to insert the wires correctly.
This method of connecting cables is
popular with certain types of cables which have to be
terminated in large quantities (e.g. at
patch bays or in computer
networking), as, when the wire size is compatible with the
connector, it can give a rapidly-made connection of
high quality without
the use of solder or screws. However, some types of
insulation displacement connector cannot be re-used,
once terminated. Insulation displacement connections
are most frequently used
(rather than stranded) cables,
such as are often used in fixed installations.
See also Krone.
Any material which presents a very high
resistance to the
flow of current.
Or, a component part whose purpose is to prevent electrical
contact between conductors.
See also Insulation and
which is fabricated on a single piece of silicon, and
packaged as a single component.
Commonly abbreviated to 'IC'. Also referred to as a
'silicon chip', or just a 'chip' (a slang term).
There are very many different types of ICs, each for a specific
purpose. Their internal circuitry ranges from fairly basic
(typically a few tens of transistors)
to extremely complex (many millions of transistors).
Many types fall into general categories such as
Like most components, many types of ICs are available in both
The design of an item of equipment may be referred to
as 'integrated' if it makes use of ICs, even though other
kinds of components will usually also be present. See also
Solid state and
The clarity of a
sound. In particular, the
degree to which spoken words are correctly recognised
or (in a negative sense) the degree to which deficiencies
in a PA system or in a room's
unfavourably on such recognition. Good intelligibility
of speech is heavily dependent upon the degree to which
consonants can be clearly distinguished from one another.
Several methods are available to assess intelligibility,
RASTI. In general,
an assessment of intelligibility is meaningful only
when it takes into account the characteristics of
both the specific room and the PA system in use,
given a specific set of system adjustments.
Note, however, that such standardised assessments do
not take into account such factors as the
clarity or accent of the talker, or the quality
of the listener's hearing. See also
A point on an item of equipment or system, at which
connection is made with other
item(s) of equipment or
system(s). Typically an
or an output, though many
types of computer equipment interfaces (such as
are bi-directional. Or, an item of equipment that
allows the interconnection of two otherwise incompatible
items of equipment.
See also Connector,
In a video display system, a technique for the
reduction of flicker at low
It operates by arranging for alternate lines of
the raster to be scanned
on successive fields;
two such fields being required in order to complete a
whole frame. So, the odd-numbered lines are first
scanned (forming an odd-numbered field), followed by
the even-numbered ones (forming an even-numbered field), then
the odd ones again, etc. This arrangement suffers
from the disadvantage that adjacent lines are scanned
at substantially different times, giving a blurring effect
to objects in the picture that have a large horizontal
velocity across the screen. See also
Field sync. Compare
A fixed radio-frequency
to which the radio
signal picked up by a receiver
is converted prior to
Amplifying and demodulating the signal at an intermediate
frequency (rather than at the received frequency) has the
advantages that these processes may be performed at a fixed
frequency regardless of any changes to the received
frequency, and at one which is (usually) lower than the
received frequency. Commonly abbreviated to 'IF'.
An American term for an
interval, i.e. a
period of time between the major sections of a performance.
The process, usually undesirable, whereby two
signals, when passing through a
system component that is not perfectly
interact to produce new signal(s)
with frequencies that are
either the difference or the sum of the frequency
of the original two signals or of their
harmonics. For example,
signals at frequencies 'A' and 'B' may interact to produce
signals at frequencies A+B,
When this effect occurs between two
resulting in the generation of unwanted new audio frequencies,
the result is termed 'intermodulation
When this effect occurs between two
resulting in the generation of unwanted new radio frequencies,
the result is termed 'intermodulation interference'.
This may be experienced when two or more
systems are operated together, on frequencies that are
not members of a compatible set for that model of system.
A period of time (typically around 20 minutes)
between the major sections of a performance,
during which no acts are on stage and the audience move about
to take refreshments etc. See also
A musical term for the difference in
pitch between two notes,
corresponding to the ratio of
See, for example,
Note that, in terms of frequency, intervals multiply
rather than add. So since an octave corresponds to a
frequency ratio of 2, three octaves correspond to a
ratio of 8 (not 6).
The process of calculating an intermediate value between
two given values, usually on the assumption of a straight
line joining all three points (strictly,
Although not a true
system, this process may be used as a crude method for
improving the (apparent) quality of
signals, by inserting an
interpolated value when an error detection system
(such as parity) indicates
that the received value is invalid. However, it can only
work effectively for occasional
See also Bit error rate.
that are worn inside the ear canals. See also
An abbreviation for 'introduction'.
The very first section of a song; the section that
begins it. Compare Outro.
The rule that describes the way in which
levels to change with
changing distance between the source and the listener
It states − assuming a point source, free
dispersion (i.e. a free field) and no
absorption by the
medium − that the
sound intensity decreases proportionally with the
square of the factor by
which the distance has increased.
For example, if the distance increases by a factor of 3,
then the sound intensity level (in
watts per square metre)
reduces by a factor of 9,
because the area through which the same amount of radiated
power is flowing
will have increased by a factor of 9.
In terms of decibel (dB)
measurements of sound intensity, the change in
level is 10 times the
log of the
factor by which the intensity has changed. Therefore,
due to the necessary squaring of the distance factor,
the change in level is
−20 (rather than −10) times the log of the
factor by which the distance has increased.
Put more simply, we can say that the level
decreases by (very nearly) 6 dB for
every doubling in distance.
For our example of a tripling in distance, the
change in level is approximately −10 dB
(which will sound about 'half as loud').
[All references to logs here are to the base 10.]
However, for several important reasons, in
PA work sound levels are
measured and expressed in terms of
pressure level (SPL) rather than sound intensity level.
Contrary to popular belief, sound pressure does not
follow the inverse square law − it decreases
proportionally with the factor by which the distance has
increased (not with the square of that factor).
For example, if, in the same free field, the distance
increases by a factor of 3, the sound pressure level (in
also reduces by a factor of 3.
Now when we express this change in sound pressure
as a value in dB, in accordance with dB conventions we
have to make the calculation with regard to the effective
change in sound power (or intensity, which is power
per unit area), which requires us to square the factor by
which the pressure has changed. This means that the dB
value changes by 20 (not 10) times the log of the
factor by which the pressure has changed, which in turn
means that the SPL has changed by −20 times
the log of the factor by which the distance has increased.
Put more simply, we can say that the SPL
decreases by (very nearly) 6 dB for
every doubling in distance.
For our example of a tripling in distance, the
change in level is again approx −10 dB
(which will sound about 'half as loud'). N.B. This is the
same change in dB as we calculated for changes with
distance in sound intensity with distance.
SPL 'itself' doesn't follow the inverse square law,
when SPL is expressed in dB the figures give the
appearance that it does. We could, perhaps misleadingly,
consider that SPL values in dB do follow 'an' inverse
square law, but not 'the' inverse square law.
The inverse square law applies only within the
free field of the source,
and indicates the reduction in level due to the effect of
dispersion only. The presence of natural
and, in outdoor situations, wind speed and
direction will usually modify this basic law
Note that a line array
is not a point source and therefore does not follow this
law (within the distance and
frequency ranges over which
it effectively operates as a line source − see
Below is indicated the change in direct sound level
(to the nearest dB) according to the inverse square law,
with reference to the level at a
distance of 1 metre from a point source
speaker. (1 metre is
the distance at which speaker
usually quoted.) But, as stated above, this will rarely
be the actual reduction in overall sound level that occurs
in practice, because of the effects of natural reverberation,
- 1 m ........ 0 dB
- 2 m ........ −6 dB
- 5 m ........ −14 dB
- 10 m ...... −20 dB
- 15 m ...... −24 dB
- 20 m ...... −26 dB
- 25 m ...... −28 dB
- 30 m ...... −30 dB
- 35 m ...... −31 dB
- 40 m ...... −32 dB
- 50 m ...... −34 dB
- 75 m ...... −38 dB
- 100 m .... −40 dB
For guidance on how to use this law to calculate
the input power
required to a speaker, see
Sensitivity on the
See also Needed
An item of equipment which produces
voltage from a low-voltage
DC supply (usually a 12 V or
24 V battery). These devices are ideal for powering small
mobile systems (especially when vehicle-mounted), but are suitable
only for relatively low-power
applications (up to about 1.5 kW mains power requirement).
It is very important to ensure that neither the continuous nor
the short-term power
ratings of the inverter are exceeded.
For high-power systems, or when long-term use is required and
connection to the battery of a vehicle with a running engine
is not practicable, a better approach is to use a
IP (1) (followed by 2 digits)
An abbreviation for 'ingress protection', a classification
which is used to indicate the degree to which the
enclosure of an item of
equipment is resistant to the infiltration of solid particles
The first of the two digits following 'IP' indicates the
resistance to ingress of solid particles and objects, as follows:
- 0 (or X) − no protection
- 1 − 50 mm diameter and greater
- 2 − 12.5 mm diameter and greater
- 3 − 2.5 mm diameter and greater
- 4 − 1 mm diameter and greater
- 5 − Dust-protected
- 6 − Dust-tight
The second of the two digits following 'IP' indicates the
resistance to ingress of moisture, as follows:
- 0 (or X) − no protection
- 1 − Water droplets falling vertically
- 2 − Water droplets falling vertically, when the
enclosure is tilted at up to 15º from the vertical
- 3 − Water spraying at an angle of up to 60º from
- 4 − Water splashing from any direction
- 5 − Water jets from any direction
- 6 − Powerful water jets from any direction
- 7 − Temporary immersion in water
- 8 − Continuous immersion in water
An optional additional or supplementary letter may also
be added. If in any doubt about the precise meaning of
any of the above classification codes for a particular
product, check with the manufacturer.
An abbreviation for 'Internet protocol', the networking
protocol that is used
by the Internet and also by most local data networks and
some point-to-point links. See also
An abbreviation for 'iso-propyl alcohol', a cleaning
agent commonly used for electronic equipment as it is
harmless to metals and to most plastics, and dries rapidly
leaving no residue.
An abbreviation for 'inches per second', a measure of the
speed of recording tape during recording and playback.
The quality of the reproduction improves with increasing
speed. The standard speeds are:
- 17⁄8 (1.875) IPS for cassette tapes,
- 33⁄4 (3.75) IPS for consumer
reel-to-reel (now rarely
- 71⁄2 (7.5) IPS for semi-professional reel-to-reel
- 15 IPS for professional reel-to-reel.
An abbreviation for 'in-plane switching', a type
of LCD display panel offering
a wider viewing angle than earlier types.
See also TFT.
An abbreviation for 'Institut für Rundfunktechnik', the
German Institute for Radio Technology.
An abbreviation for 'Institute of Radio Engineers' (of
America). This no longer exists, as it merged with the
IEEE. However, the term
remains in use to refer to a scale of relative
luminance, such as is
represented by a video
signal. This scale, formalised
by the IRE, specifies relative luminance values as a
percentage of peak white level, so 'IRE 0' refers to
black and 'IRE 100' refers to peak white. See also
Describes a product or an equipment
interface that is
compliant with the relevant IRT
An abbreviation for 'Institute of Sound and
Communications Engineers'. Their website is:
See also PLASA and
An abbreviation for 'industrial, scientific and medical',
a description applied to several
bands of the
spectrum in order to
indicate the use to which they have been allocated.
One of the ISM bands is the so-called
"863-865 MHz" band (also known as
allocated for licence-free use
of radio microphone
and in-ear monitoring
frequency and ETS.
An abbreviation for the 'International Standards Organisation',
a body which defines standards for (amongst other things)
A specifically US term, describing the
particular type of 'domestic style' fixed
outlet (or 'receptacle')
having a safety earth
pole that is
isolated) from the metallic frame of the
outlet. This allows the outlet to provide a
safety earth that is connected via a dedicated
insulated earth wire
(along with the other dedicated wiring supplying that
outlet) to the
independent of the safety earth carried by the
metallic conduit or trunking to the outlet. In some cases,
such a separate dedicated safety earth connection may provide
impedance coupling of interference into the equipment
supplied from the outlet, avoiding other potentially
'dirty' earth currents
that may be flowing in the conduit system.
However, such problematic earth currents may
nevertheless couple into the dedicated safety earth
conductor by means of
This distinction between types of 'domestic style'
(13 amp, BS 1363 outlets)
does not apply in the UK in the same way, because:
In non-industrial premises it is now relatively
unusual for these outlets to be fixed to a metallic back-box
that is earthed by means of metallic conduit or trunking.
The majority of metallic back-boxes are earthed via a
protective conductor that is either insulated or
is contained within sheathed
cable, and is dedicated to
the circuit concerned.
These outlets are not normally individually cabled
from the distribution board, but rather several are
supplied from the same circuit (a ring or radial circuit).
Therefore, even with a dedicated earth connection for
that circuit, interfering earth currents from
equipment connected to other outlets may still be a
problem. (In contrast, the
typically used in entertainment situations are
usually individually supplied from the distribution
board and independent earth connections to them are
readily provided, as standard types have no inherent
link between their earth pole and any metallic
back-box or fixing point.)
Nevertheless, BS 1363 outlets
with a so-called 'clean earth' capability are
available in the UK for specialised applications where
a safety earth must be provided that is independent of
that provided by the metallic back-box or fixing point,
such as in high-risk medical environments.
Not to be confused with the term
which relates to
isolation. See also
A transformer used
with the main purpose of providing electrical separation,
rather than for other purposes such as
or balancing/unbalancing (for the latter see
The term is used for two entirely different kinds
of transformer, used respectively for:
- Power applications. Here the purpose is usually to
provide a local mains
supply that is not referenced to
either to reduce problems from
currents or for
safety purposes. Some types are fitted with an
screen that should be connected to
Warning: An isolating transformer does
not provide isolation in the sense of
Such transformers usually provide an
output voltage that is
nominally the same as
its input voltage,
i.e. at a mains
voltage constituting a potential shock
hazard. Supplies taken
from the transformer output must be considered to be
as hazardous as those from the original supply,
and all appropriate safety measures must be applied
to both supplies. (Unless, exceptionally, the output
voltage of the transformer is at a safe level −
e.g. less than 50 volts).
Here the purpose is usually to provide
so as to reduce problems from earth loop currents,
while still passing the required signal without
significant degradation. Some types are fitted with an
inter-winding screen that should be connected to
Separation measures taken to substantially reduce unwanted
transmission or coupling. Such measures are typically provided
at a suitable location on the route(s)
of coupling between a source of
interference and another point or location where that
interference might otherwise cause a problem.
The complete disconnection of equipment or of a power
distribution system (or part of such a system) from
its source(s) of electrical supply
(e.g. the mains supply),
so as to enable that equipment or system to be
safely worked on
for maintenance, alteration or repair, or in order to make
it safe in an emergency situation such as electric shock
or fire. Literally, 'separation'.
An abbreviation for 'International Standard Recording Code',
an alpha-numeric identifier code for a particular recording
of a particular song or music video. The code is made up of:
A two-character country identifier.
A three-character registrant code.
A two-digit year identifier.
A five-digit designation code, assigned by the registrant.
Codes are registered via the agency applicable to the
country concerned − for details refer to the ISRC
section of the
(opens in a new window or tab). ISRC codes are managed
in the UK by PPL.
An abbreviation for 'intake switch unit',
the item of equipment that controls the
mains power at its point of
entry into the distribution system for a temporary
installation, from the fixed electrical installation
or other source of power.
See also Distro and
An abbreviation for 'information technology equipment'.
Some equipment, such as
power supply units,
is marked with this abbreviation in order to signify
that they comply with the relevant standards for
information technology equipment, for example
as regards their emission of
An abbreviation for 'International Telecommunications Union',
an organisation which sets technical standards in
telecommunications and related fields.
See also EBU and
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This page last updated 11-Jul-2017.