| Glossary of PA Terms - R
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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
Rack tom *
Rack unit *
Radio-frequency interference *
Radio microphone *
Radius of reverberation *
Ratio control *
RC filter *
Real time *
Real time analyser *
Rear lobe *
Record deck *
Red Book *
Reference level *
Reflection factor *
Reflection filter *
Reflex enclosure *
Regulated frequency *
Regulated power supply *
Relative humidity *
Releasable cable tie *
Repetitive waveform *
Residual current circuit breaker *
Residual current device *
Resonant frequency *
Return leg *
Return loss *
Reverberant field *
Reverberation time *
Reverse phase *
Reverse phase-angle control *
Reverse polarity *
RF capacitor microphone *
RF condenser microphone *
RG cable *
Rhythm guitar *
Ribbon cable *
Ribbon microphone *
Rig check *
Rigging motor *
Ring circuit *
Ring main *
Ringing out *
Ripple voltage *
Risk assessment *
Room acoustics *
Roving microphone *
Rude solo *
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 'Right
Part of a drum kit; medium and small-sized
drums, several different sizes of which are
attached to the framework of the kit
(or, in small frameless kits, to the top of the
Sometimes referred to as 'mounted toms'.
Compare Floor tom.
See U (1).
At right-angles to the direction of the
In the case of an electronic
or other part, this usually
means "across the width" of the component.
Describes a signal at
a frequency of
100 kHz or higher, or
something that uses, processes or carries such frequencies.
Or, as a noun (without the hyphen), any such frequency
itself. So-called because originally such frequencies
were used only as carriers
for radio transmissions. The name has remained, even
though now some types of
baseband signal are at
these frequencies; the term is now used regardless of
the purpose to which such frequencies are put −
there need be no connection with radio.
Often abbreviated to 'RF'. See also
A microphone that
is not connected to the PA
system by a cable, but which instead transmits a
which is picked up by a receiver some distance away.
The receiver converts the radio-frequency signal into
signal, which is supplied to the
mixer just as if (− in
theory, anyway) the microphone were connected directly.
Also called a 'wireless microphone'.
For more details, see the
radio mic information
on the Microphones page. See also
Channel 38 and
An imaginary surface at the boundary between
regions in 3-dimensional space, made up of all points that
are at the critical
distance from a particular sound source. Or, another term for
that distance. At any point on this surface, sound waves
arriving direct from that sound source are at the same
level as the
resulting from that source alone.
See also Distance
inside an item of equipment (especially an
or (more usually) the term refers to the
voltages that are
typically present on those connections.
See also Hitting
the rails, Clipping,
Overload (1) and
A slope, especially the downward slope of a
stage towards the
auditorium or the
downward slope (or terracing) of an auditorium floor
towards the stage. Auditorium seating that is arranged
so that rows progressively further from the stage are
at an increasingly higher level may be described as
'raked seating'. Raked stages are much less common
in modern performance venues. See also
An abbreviation for 'random access memory'. A means of
storage for digital
information, which enables extremely rapid access
to any element of the storage and extremely rapid writing
to it and reading from it, regardless of the sequence in
which the elements are accessed. However, retention of the
information is dependent upon a constant supply of power
(unless the RAM is described as
Used extensively in computers
and other equipment which requires the rapid storage and
retrieval of data, such as
A lowering of pressure, in relation to the
propagation of sound
through a gaseous medium
such as air; the opposite of
Sound waves in air consist of alternate regions of
compression and rarefaction along the length of the wave
in its direction of travel, and are therefore described as
In a video display system,
the complete set of horizontal
lines which make up the picture. In a
CRT display, the lines are
formed by a dot of light which is rapidly scanned across the
display, varying in brightness and colour as it goes, to
construct the required image. The dot is formed by a
sharply focused electron beam hitting a phosphor coating
on the inside of the screen surface. The scanning movement
is not visible because of the speed of the dot relative
to the persistence of vision and because of the persistence
of the screen phosphor. The rate at which lines are scanned
is called the line frequency.
displays there is no moving dot; the image is formed by
means of individual pixels.)
A complete set of scanned lines is called a
frame, and the rate at
which frames are scanned is the frame rate (or frame
If the raster lines are scanned alternately
scanning) then each frame consists of two
fields: an odd-numbered
field (consisting of the odd-numbered lines) and
an even-numbered field (consisting of the even-numbered
lines). So in this case the field frequency is twice the
frame rate. If however the raster lines are scanned
consecutively (called progressive
scanning) then the field frequency is the
same as the frame rate. In the UK 625-line
standard-definition broadcast video format, the
line frequency is
15.625 kHz and the
field frequency is 50 Hz
(giving a frame rate of 25 Hz, since this is an
In general terms, for direct-display CRT systems
the scanning is from left to right on each line, starting
with the line at the top of the screen and finishing with
the line at the bottom. For back-projection or
mirror-reflection systems the scanning direction may need
to be reversed to give a correctly-oriented image.
In order for images to be correctly displayed, the
start of each line scan and of each field scan must be
timed to correctly match the
programme content of the
video signal. This
is achieved by incorporating
line sync and
field sync pulses
into the video signal.
After each individual line has been scanned, the spot
rapidly 'fly back' to the other side of the screen to
be ready to start scanning the next line. During this
horizontal flyback time, the electron beam is shut off
by a process known as 'horizontal blanking'. Similarly,
when the last line of the field has been scanned, the
spot must rapidly return to the top of the screen ready
for the next field; vertical blanking is applied to
the beam during this vertical flyback time. See also
An abbreviation for 'room acoustics speech transmission index'.
For further information see STI.
The name of a control on an
effects unit, whose setting
determines how frequently the effect is repeated.
(The name 'frequency'
is not used in order to avoid confusion with
A numeric value (with the relevant units stated)
typically specifying the
capability of an item of equipment, or specifying
its operating voltage.
See also VA,
De-rating and the
section on the
Amplifiers and Speakers page.
A method of comparing two values, in which one value
is divided by the other. For example, if a
transformer has an input
200 ohms and
an output impedance of
50 kilohms, then it
would be said to have an impedance ratio of 1:250
(pronounced "1 to 250"), because 50,000 divided
by 200 is 250. A ratio of 1:1 means that the two values
Ratios of sound
signal levels are usually
as a value in decibels −
see, for example,
ratio. Note that ratios expressing an amount of
dynamic range are
usually comparing decibel values − see
ratio (1) and Expansion
ratio (1) and Expansion
An implementation of the
A intentional filter
that is constructed using only
capacitors, or an
unintentional one that exists as a result of the
present in a circuit.
An example of an unintentional RC filter is the
low pass filter
created by the
of an instrument output
(assumed to be purely resistive)
in combination with the capcitance of the
used to connect it to a backline
Some low cut filtering
may result from the series
coupling capacitors used at the
and/or output of amplifiers and between their internal
Calculators for simple RC low cut filters are available
Compare LC filter.
Another name for a phono
connector, so-named after the company which originally
designed it (Radio Corporation of America).
An abbreviation for 'residual current breaker with
overload protection'. A device which incorporates the
functions of an RCD and an
MCB in a single unit, usually
with a single operating switch. Generally found only in
boards of fixed electrical supply installations.
Another name for an RCD.
An abbreviation for 'residual current device'.
A safety device, connected in the
mains supply in order to provide
a degree of protection against
hazards that may occur in the
event of a fault which causes a potentially
of mains current to flow
to a safety earth.
RCDs are most commonly installed in order to provide
supplementary protection agains electric shock
Just 40 mA can be enough
to kill a healthy adult, but supply
require several hundred times that value to trip
(or to 'blow'),
so MCBs (or fuses) alone do not provide
protection against fatal electric shock
(except when they very rapidly disconnect the supply
due to a short circuit
between a Live
conductor and an adequately earthed
conductor, or conductive
object, that might be touched).
In a single-phase
supply, an RCD operates by rapidly cutting
off the supply when the current flowing in the
Live conductor differs from that flowing in the
Neutral conductor by
more than a specific amount for a sufficient length of
time. This difference in currents is known as the residual
current − i.e. the residue value that
remains after subtracting the Neutral current from
the Live current − and is equal to the current
flowing to earth (assuming that the earth current is
in-phase with the
supply current). RCDs are also available for use on
In order to provide adequate protection against fatal shock,
an RCD must have a suitably low trip current and a
suitably rapid response time. In the UK, modern-type (BS-EN)
RCDs intended for shock protection must have a
nominal trip current of
no more than 30 mA. They must also trip within
300 milliseconds (ms) at the nominal trip current
and must trip within 40 ms when tested at
a residual current equal to 5 times the nominal
Even when a 30 mA RCD is in use, a very serious shock
may still be obtained from a current that is too small to
operate the RCD, passing through the body to earth.
However, such low currents are unlikely to cause the death
of a healthy adult by electrocution. Any RCD used for safety
protection purposes must be manufactured to the relevant standards
and be regularly tested to ensure correct operation.
Note that an RCD does not provide any protection
against overload, against
or against shocks caused by simultaneous
contact with the Live and Neutral conductors.
An RCD should never be relied upon as the sole means of
shock protection, for example in a situation in which the
appropriate primary means of protection against
direct contact or
are absent or defective.
Specialist RCDs are available that incorporate delayed
operation, and/or which operate at higher residual currents
than RCDs used for shock protection. Some types provide
the facility for adjusting the delay time and/or the nominal
operating current. Some specialist types
incorporate increased immunity to impaired operation
caused by superimposed DC
currents or by pulsating AC.
Such specialist types
are sometimes used in the portable power distribution
units employed in the entertainment industry.
Note that the nominal trip current of an RCD (e.g.
30 mA) is the value of residual current at which the
device must operate − this is in contrast to the
way in which the rated current
is specified for MCBs and fuses. In addition to their
nominal trip current, RCDs also have a maximum rated
(e.g. 63 A), which must not be exceeded in use.
Also known as an RCCB (residual current circuit-breaker)
or, in the USA, as an EFI (earth fault interrupter),
a GFI (ground fault interrupter) or a GFCI (ground fault
circuit interrupter). A mains outlet that incorporates
an RCD is called an SRCD ('socket RCD').
See also Earth fault,
and Power Breaker.
An abbreviation for 'remote device management', an
allows compatible devices such as dimmers and moving lights
to communicate with each other bi-directionally over a standard
DMX link. For further information see
(external link, opens in a new window).
The process of replacing some or all of the
capacitors in an item
of equipment, in an effort to restore (or improve upon)
some aspects of its original as-new performance.
In the case of equipment of a substantial age there
is technical justification for this,
because some types of capacitor, particularly
electrolytic ones, deteriorate with age and with duration
of operation − especially when subject to elevated
temperatures. For this reason often just the electrolytic
capacitors are replaced. (A notable exception to this
is in the case of vintage equipment,
where the substantial age of other capacitor types of
early design may also have a detrimental effect on the
The current-opposing effect
of the inductance and/or
capacitance in a
circuit or in an
at a particular
It is measured in ohms.
reactance is given by 2 π f L, where
f is the frequency in hertz
and L is the inductance in henrys.
reactance is given by
1 / (2 π f C), where
C is the capacitance in farads.
Inductive and capacitive reactances can be considered
as being of opposite sign, and are therefore capable of
partially or completely cancelling each other out.
When the current-opposing effect of any
resistance present in
the circuit (or component) is taken into account along
with the reactance, the resultant total effect is termed
Describes equipment that handles information (e.g.
programme material) at
the rate that it is being produced, and is therefore
suitable for processing the information 'as it happens'.
A common requirement for such equipment is to have low
See also Live (3).
A connector (of either
gender) that is attached
to equipment or to a fixed surface rather than to a
cable. Also referred to
as a socket. In the US, a term
for a fixed mains
The process of refurbishing a coned
driver by replacing the
cone and its attached
This is usually a highly skilled process that needs
to be performed by the manufacturer (or their agents)
using a suitable jig. However, the cost can be
substantially less than that of replacing the driver
in its entirity.
An item of equipment for playing
vinyl records. Often abbreviated
to just 'deck', though strictly
that term has a more general meaning.
Also called a 'turntable'. See also
A circuit whose function
is to convert AC to
DC, usually either as part
of a power supply
or in order to
obtain a DC voltage that
represents the level of
a signal (e.g. in order
to drive a
or as part of a
Rectifier circuits usually make use of one or more
An abbreviation for the European Union's
Radio Equipment Directive, a standard that
must be complied with by
nearly all new commercial and domestic equipment,
sold in the EU, that has a wireless transmission or
reception capability. It replaces (from June 2016) the
Radio and Telecommunications Terminal Equipment Directive
(R&TTE). See also ETSI.
See CD standards.
The provision of more equipment than is necessary for
a system to operate satisfactorily,
in order to allow for the possibility of an unexpected
failure of part of the system. For example, a vital part
within the system may be provided in duplicate.
There are several ways that this may be done, depending
on what is most appropriate in the circumstances for
each part of the system requiring redundancy:
The additional item(s) may be incorporated as part of
the working system, so that all the items of that type
normally share the relevant task between them. In
the event of a failure, the system is designed such
that the remaining item(s) are adequate to take over
the whole of that task − usually without any
apparent break in the operation of the system.
Some manual adjustment may then be necessary,
e.g. in order to rebalance the loading
of the system.
The additional item(s) may be on 'cold standby',
i.e. normally switched off and/or physically
disconnected. Manual intervention is required to bring
them into use to restore normal operation of the system
− a short break in operation can therefore be
The additional item(s) may be on 'hot standby'
i.e. switched on and connected ready for use,
but not normally contributing to the operation of the
system. In the event of a failure, minor manual
intervention may be needed to bring them into operation,
or this may be arranged to occur automatically in order
to shorten the break in operation or to provide for
such a change-over in an unattended part of the system.
In the case of 'hot standby' and 'cold standby'
arrangements, items of equipment that are normally
(or currently) fulfilling the relevant task may be
referred to as 'workers'. See also
Describes a tape recording system which uses unenclosed reels
of tape (in contrast to cassette tapes), now generally used
only for the reproduction of old recordings.
Also known as an 'open reel' system.
For stereo applications
the standard tape width is quarter-inch
(6.35 mm). There are various standard tape speeds, depending
upon the recording quality required and the length of time for
which a spool of tape is required to last (for details see
On loading, the tape
must be manually threaded through an aperture to contact the
heads. There are two formats for stereo operation:
The format for consumer applications is called quarter-track
stereo, as each channel, or
occupies one quarter of the total tape width. This
format allows the reels to be turned over and used again on the
'other side', in the opposite direction of tape travel
(the same track layout is used in cassette tapes).
The format for professional applications (such as mastering
in recording studios, before the advent of
digital recording techniques)
is called half-track stereo, as each channel occupies
one half of the total tape width. This gives a better
ratio but allows the tape to be used on 'one side' only.
For optimum performance, it is essential that the tape heads
are regularly cleaned and demagnetised, and are maintained
in correct mechanical adjustment − particularly with correct
azimuth. It is also essential
that the machine is correctly set up with a recording
and bias to suit the particular
type of tape being used (usually done using an alignment tape),
and that the appropriate standard of
is selected. See also Zenith
level against which other
levels (at the same point, or equivalent points, in the
system) are compared for measurement purposes, usually
on a decibel scale.
For examples see
0 dB (1).
See also Zero level,
Tone (2) and
The phenomenon whereby sound
waves bounce off objects they encounter on their path. This is a
because it only occurs to any significant degree when the
wavelength of the sound
is smaller than the physical size of the object.
The result is that treble
sounds are readily reflected by relatively small obstacles in
their direct path, while bass
sounds are not.
See also Absorption.
In video projection,
a more formal name for
In microphone usage
(most particularly in studio recording of vocals), an absorbent
device, usually curved in shape, that is positioned behind a
microphone in order to reduce the reflected sound energy from the
room reaching the rear and sides of the microphone and to reduce
the unwanted direct sound from the performer passing beyond
the microphone and into the room, from where it could be reflected
back and picked up by the microphone. Abbreviated to RF (but
take care to avoid confusion with
Reflection filters are frequently used in conjunction
with additional sound-absorbent material behind the
performer, whose purpose is to reduce the reflected sound energy
from the room reaching the front of the microphone, which is
in most cases much more sensitive than its rear and sides.
The phenomenon whereby
sound waves undergo a
change in their direction of movement as they pass
through a change in density of the air, caused by
a difference in temperature. In practice this effect
is usually insignificant for the majority of
PA work, because variations
in venue air temperature are usually fairly small.
See also Diffraction.
A specific section of some songs, that occurs at
least twice (typically several times) within the song
in substantially the same form. It is commonly referred
to as a 'chorus', though in strict musical terminology
that term has a different meaning − see
Chorus (2). See also
Bridge (6) and
An item of equipment which improves the quality
of a signal, or of
mains power, by producing
a fresh (supposedly 'near-perfect') copy of the
signal 'from scratch' − as compared to
simple filtering of the
input signal to remove undesirable components.
In the case of signal regeneration, the
level will also be
corrected. Signal regenerators are often referred
to as repeaters
(though historically that term was used of devices
and filtering − not true regeneration).
They are sometimes referred to as
though that term is much abused.
Mains power regenerators are a particular class of the
devices generally known as
conditioners. Sometimes abbreviated to 'regen'.
(e.g. for use by
or IEM equipment)
that is subject to licensing.
For further details see
'Wired or Radio'
on the Microphones page.
A power supply
that is designed to provide output(s)
are maintained within close
normal operating conditions. The tolerances are often
quoted as percentage values of 'line regulation' (output
voltage variation versus variation in input voltage) and
'load regulation' (output voltage variation versus
variation in applied load).
A reduction in response or in
usually in reference to this occurring under
particular circumstances. In most cases, the term
is used to refer to a wanted behaviour −
either one that is intentionally designed-in
to a piece of equipment or one that is
intentionally set-up by its user. For example, a
microphone provides rejection of
sound pick-up from
particular angles, the controls of an
equaliser may be
adjusted to provide rejection of a particular
rejection of interference on condition that
it arrives at an equal
level on the
Note, in such technical senses
of the term, that although the degree of rejection
may be very substantial it is rarely total.
See also Notch out
See Cable tie and
A control which adjusts the rate at which a
compression, after the
expiry of the hold time.
Sometimes this control is labelled 'decay'.
Compare Attack (2).
One of the parameters
commonly used to define the
envelope of a musical note
− for details see ADSR.
A device which extends the maximum length of a
digital interconnection by
the digital signal
with its original amplitude
and timing. (Note that this is not just an
Such a device will generally be specific to a particular
type of digital signal. They are sometimes referred to as
though that term is much abused.
A waveform whose shape
follows an exactly repeating pattern and maintains an
level. For example, a
sine wave, a
square wave, etc.
Examples of non-repetitive waveforms are
current circuit breaker
The property of any electrical
which causes it to oppose the flow of a
current to some extent.
This property is
measured in ohms.
The thinner or longer a conductor of a
cable, the greater its
resistance. In practice,
the resistance of a signal
cable is usually only important in very low
circuits (less than 50 ohms), such as in the case of the
cables connecting speakers to
The resistance of a conductor is directly proportional to
its length, and inversely proportional to its cross-sectional
area (that is, to the square of its diameter).
It is also dependent on the material of which the conductor
The passage of a current through a resistance will result
in a voltage drop across the
resistance, which can be calculated using
As a result of this, power
will be dissipated in
the resistance (calculated by multiplying the current
flowing by the voltage dropped).
For other electrical calculations, see the
How do I calculate ...?
question on the FAQ page.
Whilst resistance is generally considered to be a nuisance
in the case of cable conductors, in other situations (such as
it is sometimes required to be deliberately introduced.
This is done using a
component called a
However, note that audio signals are
not DC but AC, and in addition to the
resistance in their path these currents are subject to an
additional opposition (called reactance) due to the
of the path; the higher the
frequency of the
alternating current, the greater this additional opposition is.
The combined effect of resistance and reactance
is called impedance.
See also Capacitance
Describes an electrical device or circuit
which possesses only
having a negligible amount of
See also Impedance.
An electrical component
whose purpose is to introduce
resistance into a
In PA work, they are most often
encountered in passive
but are used in every type of electronic equipment.
The nominal resistance
value and the tolerance
of fixed resistors of low-power
(typically up to 3 watts) is usually indicated
by a colour code.
The exact power rating of these types may be difficult
to accurately assess, but the physical size of the
resistor gives a good indication. (The physical size gives
no indication of the resistance value.)
Higher-power types usually have their resistance value,
tolerance and power rating printed on them.
Often the letter 'R' is used to indicate the position
of the decimal point in the resistance value, or for higher
values the appropriate
multiplier letter (K or M) is used. On these types, the
tolerance is often indicated by the appropriate code letter
For example '6K8J' indicates a
6.8 kilohm resistor with
a tolerance of 5% while '3R3G' indicates a
3.3 ohm resistor with
a tolerance of 2%.
When replacing burnt-out or faulty resistors, at least
the following factors must be considered (in addition to
the usual safety considerations):
Why did the original resistor fail? This may be due to prior
failure of one or more other components (often
Use the same resistance value as the original.
Ensure that the tolerance of the replacement is no higher
than the original.
Ensure that the power rating of the replacement is at
least as high as the original.
In certain performance-critical applications such as
stability and noise
level may be important.
In certain safety-critical applications such as
factors such as flammability may be important.
When two or more resistors are connected in
series, their combined
value of resistance is the sum of the individual
When two or more resistors are connected in
parallel, to find their
combined value of resistance it is necessary to sum the
reciprocals of the individual resistor values and
then take the reciprocal of that sum. See
'How do I calculate ...'
on the FAQ page.
A resistor whose resistance can be mechanically varied is usually
referred to as a potentiometer
(although strictly a potentiometer is a 3-terminal device).
See also Inductor and
A measure of the detail with which something is
digital audio work
the term generally refers to the number of
bits used to
represent each sample
to digital conversion), however
the term is more commonly used in reference to
and displays, in which it refers to number of
up the image. For information on the most common
video resolutions see VGA.
The tendency of something to vibrate more strongly
when subjected to a stimulus (typically
sound waves or an electrical
signal) at one or more particular
than when it is subjected to a stimulus at other frequencies.
Resonances in acoustic
musical instruments can be a good thing, to the extent that
they contribute to the required
timbre of the instrument.
However in a PA context, resonances
of the room or of objects within it, or resonances of
speakers, are generally
undesirable. For example, the natural
acoustics of a
room will typically accentuate
several frequencies due to resonances of the air space
within it, and these frequencies may need to be reduced
in the output of the PA system (e.g.
by using a graphic
Resonance is also known as "ringing", especially
when applied to a room in which the PA system
is operating close to the point of acoustic
See also the next definition,
Q (3) and
The natural frequency
of vibration of an enclosed space, an object, or a mechanical
or electrical system; the frequency at which
Note that although there will typically be many
resonant frequencies, the term usually refers to the
dominant one (often the lowest, or
An input; a connection
point for a signal
entering an item of equipment, e.g. from
an effects unit.
See also Returns.
A measure of the degree to which equality of the
load impedance and
impedances is achieved in a particular
interconnection, usually expressed in
decibels. A higher
figure indicates a greater loss
on the return path − i.e. less reflected
signal as a result of
more-nearly equal impedances. (As the figure is a value
of loss, it is a positive
quantity. Unfortunately, however, it is sometimes
erroneously indicated as a negative value.)
that carry the mixed
signals from the
mixer to the
typically through a multicore.
This may be the same multicore as carries the source
signals from the stage to the mixer, but in
large systems a separate cable is often used.
Returns are needed for each
main mix signal
(typically Left, Right and maybe also Centre)
and for each
The multicore normally provides
balanced connections for the
returns using XLR
A stagebox is
typically provided with male
XLRs for balanced returns, and these may be connected to
inputs of the power amplifiers
using standard balanced
In small systems (where either the mixer does not provide
balanced outputs or the
amplifiers do not have balanced inputs)
the returns are frequently operated in
mode, which can give rise to
earth loop problems.
Stageboxes are sometimes
equipped with jack returns
connectors to facilitate unbalanced connections to
See also Powered
multicore and Line (1).
See Diffuse field.
The phenomenon whereby sound
waves continue to exist
within an enclosed space (such as a room) after the
source of the sound has ceased, due to multiple
of the sound continuing to occur between the surfaces
enclosing and within the space. Or, an
effect unit which simulates
this phenomenon. As rooms differ in the manner and degree
to which they behave in this way, such effect units usually
provide some control over the type and extent of the
reverberation effect they produce, in order to provide the
facility to simulate various different room
These units are useful in reducing the
of a sound. See also
distance (1) and the next definition.
The time taken for the
sound pressure level
to decrease by 60 dB when the
source of a continuous sound ceases abruptly.
This is a frequency-dependent
value, because the time for which sound of a particular
frequency persists in a space depends on the degree of
absorption provided at
that frequency by the surfaces surrounding and within that
space (and, to a lesser degree, by the air within the space).
Also called the RT-60.
See also ALCONS,
See Phase reversal.
See Polarity reversal.
An abbreviation for
An abbreviation for
condenser microphone (RF capacitor microphone)
A special type of
microphone in which, instead of the
being given a DC
the capacitor controls the frequency of a
within the microphone.
So, as the sound waves impinge upon the
and thereby affect the capacitance of
the capacitor, the frequency of the oscillator
is modified in sympathy. The
is then obtained by means of a 'discriminator'
circuit (also within the microphone) which produces
an output voltage
that varies according to the RF oscillator's frequency.
The Sennheiser MKH series of microphones employ this
principle of operation.
An abbreviation for
interference'. Any interference (to a
signal) that has
its origins in a radio-frequency transmission −
regardless of whether such transmission be intended,
accidental, or of natural causes.
See also EMI,
An abbreviation for 'request for proposal'.
A document setting out the broad requirements of an
intended commercial project (such as the installation
of a PA or lighting system),
in order to provide information for the companies who
are being invited to submit detailed proposals as to
how they would meet those requirements. Such companies
may also be requested to supply quotations for the
work involved. An RFP may be produced by the customer
themselves, or may be produced for them by a company
hired specifically for that purpose.
See Coaxial cable.
An abbreviation for 'red, green and blue', the three
additive primary colours that are combined in the
necessary proportions to create any required colour.
Usually refers to a video
interconnection in which the
levels of these three colours
are conveyed by separate
signals. Although the
signals are physically separate, their respective
usually bundled into a single
cable. The term applies
equally to live/recorded video and to connections
between computers and display equipment.
See also VGA.
video and S-video.
An abbreviation for 'relative humidity'; a measure of
the amount of water vapour present in the air, expressed
as a percentage relative to full saturation. Along with
ambient temperature, the RH is an important factor
in providing a suitable storage and operating
environment for equipment, and also because it affects
the transmission (or 'propagation') of
sound waves − see
RH should preferably be between 40% and 70%.
discharges are more likely at RH values
significantly below 40%.
An abbreviation for 'rehearsal'.
A guitar on which chords are played
so as provide a rhythmic instrumental
backing to the overall
sound. Compare Lead (2).
An abbreviation for 'Recording Industry Association
of America'. In PA work,
usually refers to the specific fixed
equalisation which is
needed for the proper reproduction of sound from
vinyl records, the standard
for which was published by that association.
This equalisation is incorporated into
designed for the purpose, and exactly compensates for
the equalisation that was applied when the record was
produced. The main purpose of this equalisation is to
reduce the width of the groove, and therefore
increase the length of time for which a record
(of a given diameter) will play. It also provides some
See also Phono (2).
A cable consisting of
three or more
insulation of each one
being bonded to, or contiguous with, that of its neighbours
so as to form a flat cable whose conductors run side-by-side.
The insulation may be bonded in a similar fashion to
zip cable or may be
a continuous plastic sheet in which the conductors are
embedded or on which they are printed as a metallic film.
As this type of cable is intended for making the internal
interconnections within an item of equipment (for example
between PCBs), there is usually no
overall sheath. They are
often used with
See also Loom.
that is constructed using a ribbon-like
diaphragm which itself forms
the conductor in which
voltage is generated
(there being no separate
This type of microphone can give a very high quality of
sound, but is extremely fragile and sensitive to
and conducted vibrations; is therefore
most useful for studio recording applications, where the
working environment is more controlled.
Never blow into a ribbon microphone, as this will
almost certainly cause serious permanent damage.
Because the ribbon has a very low
impedance (typically less
than 5 ohms), the microphone
usually incorporates a
transformer to increase
the output impedance of the microphone to a value that is
compatible with mixer inputs.
Far from protecting the microphone against damage by the
phantom power, this
transformer can cause a damagingly high burst of
current to flow through
the ribbon if the phantom power is mis-connected,
e.g. due to a faulty cable.
Therefore, it is best to avoid the connection of this
type of microphone to equipment that supplies phantom
See the Microphones page for more
information on microphones.
To keep a finger on a fader
so as to make continuous adjustments to compensate
for variations occurring in the
level of a
Usually, this fader would be a
Such compensation may be necessary, for example, if a
vocalist continually makes unintentional movements towards
or away from a microphone.
Essentially, this is a form of manual
The procedure may also be termed 'gain riding', but this
is somewhat misleading because it is the fader that
is adjusted − not the associated
A type of cymbal.
See also Crash.
See Technical rider.
A type of microphone
designed specifically for picking up sound at a large
distance (typically 1 to 5 metres) from its
source. Although some microphones having a
may be referred to as rifle microphones, more properly
speaking the rifle microphone has an even narrower
polar response than the hyper-cardioid.
Sometimes sub-classified into 'short' and 'long' types,
it gets its name from its narrow rifle-like barrel
(the interference tube), which may be up to
60 cm (2 feet) or so in length.
As the polar response of such a microphone includes many
lobes (predominantly towards
the rear), it is also called a lobar microphone.
It is rarely used in music PA,
finding its main PA application in theatrical work.
Also called a shotgun microphone or a line microphone.
uni-directional polar responses illustration
A set of interconnected equipment, consisting of
several items operating togther as a
complete system to provide a specific function.
Most usually refers to a complete
PA system, or to any
other complete system such as a stage lighting system.
Or, as a verb, to assemble such a system −
especially one erected temporarily, specifically to meet
the requirements of a particular event.
See also Load-in.
A procedure by which a sound engineer establishes
that all elements of a PA
system are correctly interconnected and are operating
A person who carries out
rigging tasks, often working
The process of constructing and
flying frameworks such
as trusses to which
lanterns (and sometimes
speakers) are attached or
suspended. See also the next definition and
An electric motor that is used in the process of
rigging, typically to hoist
heavy items such as trusses,
into position. See also Fly.
Ring (1) (Jack plugs)
An enclosed space, or an object, is said to 'ring' when
it exhibits noticeable
resonance at one or more
frequencies. See also
Ring circuit or Ring main
A wiring arrangement for
mains power distribution
to permanently fixed
BS 1363 outlets
(commonly referred to as '13
The ring main arrangement is commonly employed in
fixed electrical installations in the UK. Its purpose
is to enable several (sometimes many) socket outlets
to share a single
device, on the assumption that not all outlets will be used
at maximum loading
simultaneously. These outlets may be localised (e.g. to a
single room) or may be widely spaced throughout the building.
The wiring is generally 2.5 mm2
CSA 'twin and earth'
2.5 mm2 single cables in
The circuit protection device is either a 32 amp
or a 30 amp fuse;
it is because of this high
plugs are equipped with
an internal fuse,
limiting the current that may be taken from each
outlet to a value that is safe for the
and for the particular type of plug-cable in use
(a maximum of 13 amps, with appropriate cable).
When using a socket outlet supplied from a ring main,
in order to avoid an
it is essential to ensure that the total load
on the distribution circuit supplying it −
i.e. the sum of the
all the socket outlets on that same circuit −
does not exceed the rating of the circuit protection
device. (This can be difficult, as outlets are
rarely marked to indicate which circuit they are
supplied from.) As with all fixed mains
wiring in the UK, ring mains must be installed in
accordance with BS 7671.
A PA system (or a room
or other space in which one is used) is said to be
'ringing' when the system is operating close to the
point of acoustic
particularly when this causes amplified
sounds to persist
in the space for longer than
would be expected from the effects of the space's
The term is used by analogy to the behaviour
of a bell, whose sound continues for some time after
the bell is struck − a PA system operating close
to feedback may be heard to
in a manner similar to a bell. See also
and the next definition.
The deliberate creation of acoustic
feedback during the setting up
of a PA system,
to enable the predominant feedback
frequencies to be discovered
and manually compensated for, usually by use of a
This can be a useful approach to setting the graphic
equaliser(s) for stage
monitors, because these are
the most likely source of feedback in most
performance-related systems and prevention of
feedback is one of the prime reasons for equalising them.
In this case, the on-stage
microphones are typically
turned up one by one, in-situ, until feedback just
occurs, and cut is applied to
the relevant graphic equaliser frequency
bands until no particular
feedback frequency dominantes.
If the ringing out method is used to set the
graphic equaliser, there are two possible approaches:
Using the on-stage microphones in-situ.
This method is very inadvisable, because FOH sound
quality is of prime importance, and equalising the
acoustic feedback path from the FOH
speakers back to
the stage microphones situated
behind them is likely to result in a very unequalised
forward-facing path to the audience.
- Using a microphone at FOH, i.e. at one or
more locations of the audience, pointed towards the
FOH speakers. This should be a microphone with a suitably
[Using a vocal microphone (e.g. a Shure SM58) is
not recommended, because these typically have an
intentionally shaped response − in particular one
or more presence peaks
and a bass
roll-off (to counteract
The effect of this during ringing out
would be to cause the FOH graphic settings to incorporate
a curve that is approximately the opposite of the response
curve of that particular type of microphone, which would
be very undesirable.]
For a recommended approach to setting the FOH graphic,
see Stage 3 of the
methodology on the Getting
Started for Mixing Engineers page.
Note that the feedback created during ringing out should be
kept to a minimum SPL and duration,
to avoid possible damage to the speakers. See also
To copy a music or video track from a compact
disc or a DVD onto a
computer's hard drive or other storage device.
Variations, usually in a quantity that would ideally be
free from any variation. The term is most often used in
reference to regular variations that are small relative
to the average value of the quantity concerned. The most
common uses are a) in relation to a regular variation
of the output
voltage of a
generally at a frequency
double that of the AC mains
input, and b) in relation
to variations in the passband
response of filters −
A moveable platform placed on the stage in order to
elevate a particular area. Usually employed towards the
rear of the stage (e.g. for the drum kit), to avoid
performers there being hidden by performers nearer the
front. See also Up-stage
Rows of 'banked' audience seating, i.e. each
row is higher than the row in front. Often used in the
plural. An alternative term, more common in the USA,
is bleacher (again, often used in the plural).
The likelihood of an event or situation
occurring, that could cause injury or death,
multiplied by the possible severity of such a consequence.
For example, a 'high' risk could arise
from a substantial probability that one person
might suffer minor injury, or from a lesser
probability that many people might be so affected
or that one person might be killed.
Risks must be assessed and reduced as necessary
so as to prevent
N.B. This definition of the term may
differ from officially recognised definitions.
For further information on safety see the
See the previous definition and the
Safety section on the
cable, commonly used for
between computers and computer-related equipment.
('RJ' is an abbreviation for 'registered jack'.)
An abbreviation for 'return merchandise authorisation'.
The permission that is needed from an equipment supplier
(or manufacturer) before equipment can be returned to them
(e.g. for repair, or if purchased in error).
Usually the authorisation takes the form of a unique number
or other code that must be marked on the outside of the
packaging of the returned item, and also referenced on the
covering letter. An RMA may still be needed even if the
equipment is being returned under warranty.
An abbreviation for 'root-mean-square'. A means of
quantifying the level of an
or current such as an audio
signal (whether the level be
steady or fluctuating), by expressing it in terms of the
level of the DC voltage or current
that would be required to provide the same value of
average power, when connected
to a resistive
load, as the AC quantity in question
would produce when connected to that same load.
(Because of this close relationship between RMS
values and power, the term 'RMS power' is often used − this
however is a complete misnomer, as in reality what is being
referred to is not an
RMS value but an average value of power;
the correct term is therefore 'average power'.)
In the case of AC voltages and currents, the use of RMS
values is frequently taken for granted, and not explicitly
specified. Similarly, it is often overlooked that
sound pressure level
values (whether expressed in
Pascals or in dB SPL) are also normally
In general, the RMS value of a
is derived by taking the average
(or 'mean') of the squares of the
occurring throughout a complete cycle of the waveform, and then
taking the square root of the result (hence the name).
In the special case of a
sine wave, following this
procedure gives the result that the RMS value is approximately
0.707 times the most positive or negative value that the
waveform reaches in each cycle, but note that different
multiplying figures apply to other waveform shapes.
In the case of non-repetitive waveforms, such as occur in
music and speech signals, different RMS value indications
will be obtained depending upon the time period over which the
averaging is done (sometimes called the 'integration time').
Shorter intervals will tend to give a wider and more rapid
variation in the indicated level, and higher maximum
See also Music power,
A slang term for a person involved with the transport
of equipment to and from performance venues, and often
also with the load-in
and load-out of equipment.
See also Crew.
A progressive decrease in response, occurring either as the
frequency rises above
some specific value or as it falls below some specific value.
The term may be used of an unwanted decrease, which may
have an adverse effect on equipment performance, or of an
intentional decrease − whether fixed or
For example, a button on a mixer
might be provided to give the facility to roll-off
below 100 Hz,
in order to substantially
reduce the level of unwanted bass
frequencies on a microphone
channel − or a
microphone may have one or more integral switches for this
purpose. In either case, such a facility may described as
'bass roll-off'. Sometimes the term
'roll-off' is used in preference to 'cut' in order
to indicate specifically a more gradual decrease in
response; the 'steepness' of the decrease is specified
in dB per
octave − see
Slope. See also
High cut and
The paths that signals
take through an item of equipment, or through a system.
Or, the process of setting up such paths.
The term is most often used of paths that are readily
configurable or re-configurable, particularly of the
assignment of mixer
audio groups, and the
assignment of audio groups
and auxiliaries to
Switches that are used to configure the routing are
commonly referred to as 'assign' switches.
Note that in the USA this term is pronounced
whilst in the UK it is pronounced
See also Bus,
LCR (1) and
A term sometimes used by non-technical personnel
to refer to a microphone
that may be moved around over a relatively wide area in
use, most often to enable its use by several different
people. For example, a microphone that is taken into
the audience by a show host or performer, to allow use
by selected members of the audience. This may a wired
microphone with a long cable,
but is more likely to be a
This usage of a microphone may require particular
precautions, such as:
- Care to avoid
feedback when used
in front of the
- Readiness to adjust the
equalisation to cater
for different speaking levels,
voice types and pick-up distances.
- Readiness to mute the
channel in the event of unacceptable use.
- In the case of radio microphones, care to avoid
locations that are unsuitable for satisfactory operation
of the radio link.
- In the case of wired microphones, care to avoid a
An early form of low-speed serial
data interconnection between
items of equipment, most usually using the DE-9 (or,
originally, often DB-25) style of
Now almost entirely superceded by higher-speed
interconnections such as USB and
An abbreviation for
An abbreviation for
An abbreviation for 'return', most commonly in
reference to effect
An abbreviation for 'reverberation times', generally
referring to a set of
figures relating to a particular room or other space,
each figure being the reverberation time measured (or
desired) at a particular
An abbreviation sometimes (incorrectly) used to refer to
rack units. The correct abbreviation for these units is 'U'
− see U (1).
A flashing LED found on Mackie
mixers, serving to indicate
that one or more Solo PFL
buttons are activated.
An abbreviation for 'reception', or an identification
of the receive direction of communication.
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This page last updated 05-Sep-2017.