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The glossary pages provide definitions for over 2680 PA-related terms and abbreviations. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.

R * Rack tom * Rack unit * Radio-frequency * Radio-frequency interference * Radial * Radio microphone * Radius of reverberation * Rails * Rake * RAM * Rarefaction * Raster * RASTI * Rate * Rating * Ratio * Ratio control * RAVENNA * RC filter * RCA * RCBO * RCCB * RCD * RDM * Re-capping * Reactance * Real time * Real time analyser * Rear lobe * Reboot * Receptacle * Recone * Record deck * Rectifier * RED * Red Book * Red plating * Redundancy * Reel-to-reel * Reference level * Reflection * Reflection factor * Reflection filter * Reflex enclosure * Refraction * Refrain * Regenerator * Regulated frequency * Regulated power supply * Rejection * Relative humidity * Relay * Releasable cable tie * Release * Repeater * Repetitive waveform * Residual current circuit breaker * Residual current device * Resistance * Resistive * Resistor * Resolution * Resonance * Resonant frequency * Response * Return * Return leg * Return loss * Returns * Reverberant field * Reverb * Reverberation * Reverberation time * Reverse phase * Reverse phase-angle control * Reverse polarity * RF * RF capacitor microphone * RF condenser microphone * RF filter * RFI * RFP * RG cable * RGB * RGB * RGBW * RH * RHSL * Rhythm guitar * RIAA * Ribbon cable * Ribbon microphone * Ride * Rider * Rifle * Rig * Rig check * Rigger * Rigging * Rigging motor * Right * Ring * Ring circuit * Ring main * Ringing * Ringing out * Rip * Ripple * Ripple voltage * Riser * Risk * Risk assessment * RJ45 * RMA * RMS * RMS power * Roadie * Roll-off * Room acoustics * Routing * Roving microphone * RS-232 * RT * RT-60 * RTA * Rtn * RTs * RU * Rude solo * Rules − technical * Rx

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 channel'. See Stereo. Compare L.

Rack tom

Part of a drum kit, rack toms are medium and small-sized medium-pitched drums, several different sizes of which are attached to the framework of the kit (or, in small frameless kits, to the top of the kick drum). Sometimes referred to as 'mounted toms'. Compare Floor tom.

Rack unit

See U (1).


At right-angles to the direction of the axis. In the case of an electronic component or other part described as 'radial leaded', this refers it having leads that extend out sideways, in the same direction as the width or diameter of the component. Compare Axial.


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 Modulation. Compare Audio-frequency.

Radio-frequency interference

See RFI.

Radio microphone

A microphone that is not connected to the PA system by a cable, but which instead transmits a radio-frequency signal which is picked up by a receiver some distance away. The receiver converts the radio-frequency signal into an audio-frequency 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, Mute (PA), VHF, UHF, De-regulated frequency, Regulated frequency, ERP, Squelch, Diversity, Pilot tone, PMSE, ISM, BEIRG, ETS, White space, Carrier, Modulation, Roving microphone, Lavalier, Bodypack, Co-channel interference, LTE, DTT, Intermodulation, Image frequency, Channel 69, Channel 70, Channel 38 and In-ear monitoring.

Radius of reverberation

An imaginary surface at the boundary between free-field and diffuse-field 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 reverberation resulting from that source alone. See also Distance factor.


The DC power-supply connections inside an item of equipment (especially an amplifier), 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 Bus.


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 Up-stage and Down-stage.


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 'non-volatile'). Used extensively in computers and other equipment which requires the rapid storage and retrieval of data, such as DSP-based effects units.


A lowering of pressure, in relation to the propagation of sound through a gaseous medium such as air; the opposite of compression. 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 'longitudinal'.


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. (In LCD 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 frequency). If the raster lines are scanned alternately (called interlaced 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 interlaced system).

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 synchronisation is achieved by incorporating line sync and field sync pulses into the video signal.

After each individual line has been scanned, the spot must very 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 Composite video, CVBS and CVS.


An abbreviation for 'room acoustics speech transmission index'. For further information see STI. Compare ALCONS.


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 audio frequencies.)


A numeric value (with the relevant units stated) typically specifying the power-handling or current-handling capability of an item of equipment, or specifying its operating voltage. See also VA, RMS, Programme power, Music power, De-rating and the Power Ratings 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 matching transformer has an input impedance of 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 are equal.

Ratios of sound levels and signal levels are usually expressed logarithmically, as a value in decibels − see, for example, Signal-to-noise ratio. Note that ratios expressing an amount of compression or expansion of dynamic range are usually comparing decibel values − see Compression ratio (1) and Expansion ratio.

Ratio control

See Compression ratio (1) and Expansion ratio.


A specific proprietary audio over Internet Protocol (AoIP) system that is compliant with the AES67 standard for interoperability between AoIP networks. For further information see the RAVENNA website at ravenna-network.com (this link opens in a new window). Compare Dante.

RC filter

A intentional filter that is constructed using only resistors and capacitors, or an unintentional one that exists as a result of the resistance and capacitance incidentally present in a circuit. An example of an unintentional RC filter is the low pass filter created by the output impedance of an instrument output (assumed to be purely resistive) in combination with the capcitance of the instrument cable used to connect it to a backline amplifier. Some low cut filtering may result from the series coupling capacitors used at the input and/or output of amplifiers and between their internal stages. Calculators for simple RC low cut and high cut filters are available here on the Calculations page. 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 the distribution 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 dangerous value of mains current to flow to a safety earth.

RCDs are most commonly installed in order to provide supplementary protection agains electric shock hazards. Just 40 mA can be enough to kill a healthy adult, but supply MCBs and fuses typically 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 three-phase supplies.

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 trip current.

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 Live-to-Neutral short-circuits, 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 indirect contact 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 current-carrying capacity (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, Earth leakage, Fault protection, RCBO, Distro, Distribution board, and Power Breaker.


An abbreviation for 'remote device management', an industry-standard communications protocol which 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 rdmprotocol.org (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 equipment's operation.)


The current-opposing effect of the inductance and/or capacitance in a circuit or in an electrical component, at a particular frequency. It is measured in ohms. An inductor's reactance is given by π f L, where f is the frequency in hertz and L is the inductance in henrys. A capacitor's 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 the impedance.

Real time

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 latency. See also Live (3).

Real time analyser

See Spectrum analyser.

Rear lobe

See Lobe.


To restart a computer or a computer-based system such as a digital mixer (that is already running). See also Boot (2).

Receptacle (1)

A female connector.

Receptacle (2)

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 power outlet. Compare Plug.


The process of refurbishing a coned driver by replacing the cone and its attached voice coil. 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.

Record deck

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 Tone arm.


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 level meter or as part of a compression circuit). Rectifier circuits usually make use of one or more diodes.


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.

Red Book

See CD standards.

Red plating

A very undesirable effect that can occur within the output valves of an amplifier; usually applicable only to valve-based backline amplifiers. In this effect, the anode of the affected valve (or more usually just a section of it) glows red or orange; this may be a bright or dull glow. Note that the anode is the outermost part of the valve's internal structure, and red plating is not to be confused with the entirely normal orange glow of the heater located through the centre of an output valve, nor with the normal bluish glow that may be produced on the inside of the glass of some power valves during operation. The sound of the amplifier is usually adversely affected during red plating and possibly also afterwards, though the cause of this may not always be obvious at the time.

Red plating most commonly occurs when the output valve(s) have reached the end of their useful life and require replacement − if there are several then the complete set should always be replaced, even if only one of them has been red plating. However, certain other faults within the amplifier may also cause it, so when red plating has occurred it is strongly advisable for the amplifier to be checked for other faults before replacing the valves. If other faults are found, then after repairs to correct them, the output valves will usually still need to be replaced as they will most likely have been damaged by the red plating. See also Bias (2).


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:

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 Standby (1).


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 IPS). 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 'track', 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 signal-to-noise 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 level and bias to suit the particular type of tape being used (usually done using an alignment tape), and that the appropriate standard of tape equalisation is selected. See also Zenith and DAT.

Reference level

A signal 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, SOL, Programme level, Line-up (2), Tone (2) and Gain structure.


The phenomenon whereby sound waves bounce off objects they encounter on their path. This is a frequency-dependent effect, 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. Compare Diffraction. See also Absorption.

Reflection factor

In video projection, a more formal name for screen gain.

Reflection filter

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 RF (1)). 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.

Reflex enclosure

See Port.


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 Middle 8.


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 input signal 'from scratch' − as compared to simple filtering of the input signal to remove undesirable components. In the case of signal regeneration, the output signal level will also be corrected. Signal regenerators are often referred to as repeaters (though historically that term was used of devices providing only amplification and filtering − not true regeneration). They are sometimes referred to as digital amplifiers, though that term is much abused. Mains power regenerators are a particular class of the devices generally known as power conditioners. Sometimes abbreviated to 'regen'.

Regulated frequency

A radio frequency (e.g. for use by radio microphones or IEM equipment) that is subject to licensing. For further details see 'Wired or Radio' on the Microphones page. Compare De-regulated frequency.

Regulated power supply

A power supply that is designed to provide output(s) whose voltage(s) are maintained within close tolerances, under 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). Compare Unregulated power supply.


A reduction in response or in sensitivity, 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 directional microphone provides rejection of sound pick-up from particular angles, the controls of an equaliser may be adjusted to provide rejection of a particular band of frequencies and a balanced input provides rejection of interference on condition that it arrives at an equal level on the hot and cold conductors. 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 and Bandstop.

Relative humidity

See RH.

Relay (1)

The point-to-point transmission of live programme material, for an audience separated in distance from the location of the action. Or, the process of passing a received signal on to a further location. Or, a general term for any point-to-point signal link.

Relay (2)

An electromechanical device in which passing a current through a coil causes the closure and/or opening of electrical contacts, thereby enabling the switching on or off of one or more other currents or signals. Such devices have the particular advantages that 1) a small current through the coil may switch very much larger currents through the contacts and 2) the coil circuit may be galvanically isolated from the circuits that are switched. However, they can be bulky. Typical applications of relays in power amplifiers are in the speaker protection and inrush current limiting circuits.

Releasable cable tie

See Cable tie and Velcro tie.

Release (1)

A control that adjusts the delay before a compressor removes compression, or the rate at which a noise gate closes, after the expiry of the hold time (if any). Sometimes this control is labelled 'decay'. Compare Attack (2).

Release (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 re-generating the digital signal with its original amplitude and timing. (Note that this is not just an amplifier.) Such a device will generally be specific to a particular type of digital signal. They are sometimes referred to as digital amplifiers, though that term is much abused.

Repetitive waveform

A waveform that has a continuously repeating shape. This shape constitutes one cycle of the waveform, and the cycles are exactly repeated at an essentially constant frequency. Any changes in the waveform's level occur at a very slow rate in comparison with its frequency. For example, a sine wave, a square wave, etc. Examples of non-repetitive waveforms are audio programme signals and pink noise. See also Fundamental.

Residual current circuit breaker

See RCD.

Residual current device

See RCD.


The property of any electrical conductor or component which causes it to oppose the flow of a DC 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 impedance circuits (less than 50 ohms), such as in the case of the cables connecting speakers to amplifiers.

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 is made.

The passage of a current through a resistance will result in a voltage drop across the resistance, which can be calculated using Ohm's law. As a result of this, power will be dissipated in the resistance (calculated by multiplying the current flowing by the voltage dropped), causing its temperature to rise. For other electrical calculations, see the Calculations page.

Whilst resistance is generally considered to be a nuisance in the case of cable conductors, in other situations (such as in attenuators) it is sometimes required to be deliberately introduced. This is done using a component called a resistor.

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 inductance 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 and AWG.


Describes an electrical device or circuit which possesses only resistance, having a negligible amount of inductance and capacitance. See also Impedance.


An electrical component whose purpose is to introduce resistance into a circuit. In PA work, they are most often encountered in passive crossovers and attenuators (pads), but are used in every type of electronic equipment.

The nominal resistance value and the tolerance of fixed resistors of low-power rating (typically up to watts) is usually indicated by a colour code. Resistors are manufactured in standard nominal resistance values − see Tolerance for full details. The exact power rating of colour-coded types is not usually indicated and 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 (see Tolerance). 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):

When two or more resistors are connected in series, their combined value of resistance is the sum of the individual resistor values. 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. The formula and calculators are available on the Calculations 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 Capacitor.


A measure of the detail with which something is represented. In digital audio work the term generally refers to the number of bits used to represent each sample (see Analogue to digital conversion), however the term is more commonly used in reference to video projection and displays, in which it refers to the number of pixels making up the image horizontally and vertically. For information on the most common video resolutions see VGA. See also Aspect ratio.


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 frequencies, 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 equaliser). 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 feedback. See also the next definition, Damping, Q (3) and Ringing out.

Resonant frequency

The natural frequency of vibration of an enclosed space, an object, or a mechanical or electrical system; the frequency at which resonance occurs. Note that although there will typically be many resonant frequencies, the term usually refers to the dominant one (often the lowest, or 'fundamental' resonant frequency).


See Sensitivity.


An input; a connection point for a signal entering an item of equipment, e.g. from an effects unit. See also Returns and Effects loop. Compare Send.

Return leg

See Cold.

Return loss

A measure of the degree to which equality of the source impedance, load impedance and characteristic impedances is achieved in a particular impedance-matched interconnection, usually expressed in decibels. A higher figure indicates a greater loss on the return pathi.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.)


The circuits that carry the mixed line-level signals from the mixer to the power amplifiers, 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 monitor mix. The multicore normally provides balanced connections for the returns using XLR connectors. A stagebox is typically provided with male XLRs for balanced returns, and these may be connected to balanced female-XLR inputs of the power amplifiers using standard balanced microphone cables.

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 unbalanced mode, which can give rise to earth loop problems. Stageboxes are sometimes equipped with jack returns connectors to facilitate unbalanced connections to the amplifier(s). See also Powered multicore and Line (1). Compare Return.

Reverberant field

See Diffuse field.

Reverberation (Reverb)

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 reflections 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 acoustics. These units are useful in reducing the "dryness" of a sound. See also Gated reverberation, Absorption, Pre-delay, Dead (1), Lively, Tail, Critical distance (1) and the next definition.

Reverberation time

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, Dead (1), Lively and Tail. Compare Early decay time.

Reverse phase

See Phase reversal.

Reverse phase-angle control

See Phase-angle control.

Reverse polarity

See Polarity reversal.

RF (1)

An abbreviation for 'radio-frequency'.

RF (2)

An abbreviation for 'reflection filter'.

RF condenser microphone (RF capacitor microphone)

A special type of condenser microphone in which, instead of the capacitor plates being given a DC charge, the capacitor controls the frequency of a radio-frequency oscillator within the microphone. So, as the sound waves impinge upon the diaphragm and thereby affect the capacitance of the capacitor, the frequency of the oscillator is modified in sympathy. The audio output signal 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.

RF filter

A filter whose purpose is to substantially reduce the level of radio-frequency (RF) content that is present in a signal, while leaving the wanted content of the signal essentially unaffected. Such a filter is most commonly used to reduce radio-frequency interference − see the next definition.


An abbreviation for 'radio-frequency 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, EMC and Common mode interference.


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.

RG cable

See Coaxial cable.

RGB (1)

An abbreviation for 'red, green and blue', the three additive primary colours that may be combined in the necessary proportions to create any required colour. In the context of video, RGB usually refers to an interconnection in which the levels of these three colours are conveyed by separate signals. Although the signals are physically separate, their respective conductors are 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. Compare Composite video and S-video.

RGB (2)

An abbreviation for 'red, green and blue', the three additive primary colours that may be combined in the necessary proportions to create any required colour. In the context of stage lighting, RGB usually refers to an LED lantern which has separate LEDs for the production of red, green and blue light. In a typical lantern of this type, the levels of these three colours may be separately controlled using DMX. Other features, such as strobing and/or automatic sequencing may also be provided. Compare RGBW.


An abbreviation for 'red, green, blue and white'. In the context of stage lighting, RGBW usually refers to an LED lantern which has separate LEDs for the production of red, green, blue and white light. In a typical lantern of this type, the levels of these three colours may be separately controlled using DMX. Other features, such as strobing and/or automatic sequencing may also be provided. Compare RGB (2).


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 Absorption. RH should preferably be between 40% and 70%. Troublesome static discharges are more likely at RH values significantly below 40%.


An abbreviation for 'rehearsal'.

Rhythm guitar

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 pre-amplifier circuitry specially 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 improvement in signal-to-noise ratio. See also Phono (2).

Ribbon cable

A cable consisting of three or more conductors, the 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 insulation displacement connectors. See also Loom.

Ribbon microphone

A dynamic microphone that is constructed using a ribbon-like diaphragm which itself forms the conductor in which an audio signal voltage is generated (there being no separate voice coil). This type of microphone can give a very high quality of sound, but is extremely fragile and sensitive to breath blasts 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 application of 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 power. See the Microphones page for more information on microphones.

Ride (1)

To keep a finger on a fader so as to make continuous adjustments to compensate for variations occurring in the level of a signal. Usually, this fader would be a channel fader. 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 compression. The procedure may also be termed 'gain riding', but this is somewhat misleading because it is the fader that is adjusted − not the associated gain control.

Ride (2)

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 hyper-cardioid polar response 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.

View 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. Compare De-rig.

Rig check

A procedure by which a sound engineer establishes that all the constituent parts of a PA system are correctly interconnected and are operating satisfactorily. Compare Sound-check.


A person who carries out rigging tasks, often working at height.


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 SWL.

Rigging motor

An electric motor that is used in the process of rigging, typically to hoist heavy items such as trusses, speakers etc. into position. See also Fly.

In a stereo system, the channel that is destined for the speakers located at the right as viewed by the audience (i.e. those located at stage left).

Ring (1) (Jack plugs)

See TRS.

Ring (2)

An enclosed space, or an object, is said to 'ring' when it exhibits noticeable resonance at one or more frequencies. See also Ringing out.

Ring circuit or Ring main

A wiring arrangement for mains power distribution to permanently fixed BS 1363 outlets (commonly referred to as '13 amp sockets'). 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 circuit protection 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' cable or 2.5 mm2 single cables in trunking or conduit. The circuit protection device is either a 32 amp MCB or a 30 amp fuse; it is because of this high rating that BS 1363A 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 connectors 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 overload condition it is essential to ensure that the total load on the distribution circuit supplying it − i.e. the sum of the currents drawn from 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 resonant effect. The term may be used in relation to natural room reverberation having a single dominant frequency, but in PA work it is most commonly used to refer to a system that is operating close to the point of acoustic feedback, particularly when this causes amplified sounds in a narrow range of frequencies to persist in the space for longer than would be expected from the effects of the space's acoustics alone. The term is used by analogy to the behaviour of a bell, whose sound has a dominant single frequency and continues for some time after the bell is struck − a PA system operating close to feedback may often be heard to resonate in a manner similar to a bell. See also Oscillation and the next definition.

Ringing out

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 graphic equaliser.

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 front-of-house (FOH) graphic equaliser, there are two possible approaches:

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 Voicing (1).


To copy a music or video track from a compact disc or a DVD onto a computer's hard drive or other storage device. Compare Burn.


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 mains-powered DC power supply, generally at a frequency double that of the AC mains input, and b) in relation to variations in the passband response of filters − see Butterworth, Chebyshev and Bessel.

Ripple voltage

See Ripple.

Riser (1)

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 and Down-stage.

Riser (2)

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 danger. N.B. This definition of the term may differ from officially recognised definitions. See also Hazard. For further information on safety see the Safety page.

Risk assessment

See the previous definition and the Principles of Safety section on the Safety page.


A standardised type of connector. The most common variant is the 8p8c, which provides 8 poles, sometimes referred to as an Ethernet connector. It is usually used with 4-pair twisted pair data cable, most commonly the UTP type, for connections between computers and computer-related equipment − but now also for many other purposes such as VGA and HDMI point-to-point extender systems and digital multicore systems. It is important that the appropriate category of connector is selected, to suit the category of the cable it is used with − see Category cable − and also according to whether the cable conductors are solid or stranded and whether or not the cable is screened (shielded). ('RJ' is an abbreviation for 'registered jack'.)

Conventionally, panel-mounted RJ45 sockets are orientated with their contacts at the top, and viewing the front of a socket orientated this way the contacts are numbered 1 to 8 from left to right. Similarly, the contacts of an RJ45 plug, held with its contacts up and the end facing away from you (cable towards you), the contacts are again numbered 1 to 8 from left to right. There are two different standards for terminating the cable pairs to these connectors, known as EIA/TIA 568A and 568B (though the 568B wiring is very much more common). It is not necessary (though helpful) for the same termination standard to be used throughout a system of RJ45 interconnections, however it is very important that every cable having an RJ45 connector at both ends uses the same standard at both ends. The cable conductors connect to the contacts by means of insulation displacement when the cable is terminated. See also Ethernet cable and QTP. Compare GG45.

View RJ45 image


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 AC voltage 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 RMS values.

In general, the RMS value of a repetitive waveform is derived by taking the average (or 'mean') of the squares of the instantaneous values 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 indications. See also Music power, PMPO, Form factor, Crest factor, PPM and VU.

RMS power

A term that is frequently, but incorrectly, used to refer to what should be termed 'average power'. This most usually occurs in specifications of power ratings, especially of speakers and power amplifiers. Often, the intention is to refer to a continuous average sine wave power rating, as compared to other less conservative ratings such as programme power ('music power'), PMPO or pink noise based ratings such as IEC 60268-5 (previously known as IEC 268-5) − see IEC noise. See also Power Ratings on the Amplifiers and Speakers page. For information on the real meaning of RMS values, see the previous definition.


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 switchable/adjustable.

For example, a button on a mixer might be provided to give the facility to roll-off signals 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 Frequency response, Low cut, High cut and Shelving response.

Room acoustics

See Acoustics.


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 channels to audio groups, and the assignment of audio groups and auxiliaries to matrix outputs. Switches that are used to configure the routing are commonly referred to as 'assign' switches. Note that in the USA this term is pronounced "r-owt-ing", whilst in the UK it is pronounced "r-oot-ing". See also Bus, Main mix, Pan, LCR (1), Signal path and Patch bay.

Roving microphone

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 radio microphone. This usage of a microphone may require particular precautions, such as:


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 D-sub connectors. Now almost entirely superseded by higher-speed interconnections such as USB and IEEE 1394.


An abbreviation for 'reverberation time'.


See Reverberation time.


An abbreviation for 'real time analyser'.


An abbreviation for 'return', most commonly in reference to effect return inputs.


An abbreviation for 'reverberation times', generally referring to a set of reverberation time figures relating to a particular room or other space, each figure being the reverberation time measured (or desired) at a particular frequency.


An abbreviation sometimes (incorrectly) used to refer to rack units. The correct abbreviation for these units is 'U' − see U (1).

Rude solo

A flashing LED found on Mackie mixers, serving to indicate that one or more Solo PFL buttons are activated.

Rules − technical

See 3-to-1 rule and Pan rule.


An abbreviation for 'reception', or an identification of the receive direction of communication. Compare Tx.

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This page last updated 29-Jan-2021.