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Glossary of PA Terms - R

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The glossary pages provide definitions for over 1900 PA-related terms. 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.

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 * RCA * RCBO * RCCB * RCD * Reactance * Real time * Real time analyser * Rear lobe * Record deck * Reel-to-reel * Reference level * Reflection * Reflex enclosure * Refraction * Regenerator * Regulated frequency * Regulated power supply * Release * Repeater * Residual current circuit breaker * Residual current device * Resistance * Resistive * Resistor * Resonance * Resonant frequency * Return * Return leg * Return loss * Returns * Reverberant field * Reverb * Reverberation * Reverberation time * Reverse phase * Reverse polarity * RF * RF capacitor microphone * RF condenser microphone * RFI * RGB * RH * Rhythm guitar * RIAA * Ribbon microphone * Ride * Rider * Rifle * Rig * Rig check * Rigger * Rigging * Ring * Ringing * Ringing out * Ripple * Ripple voltage * Riser * Risk * Risk assessment * RJ45 * RMA * RMS * Roll-off * Room acoustics * Routing * Roving microphone * RT-60 * RTA * RU * Rude solo * 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.

R
An abbreviation for 'Right channel'. See Stereo. Compare L.

Rack tom
Part of a drum kit; 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).

Radial
At right-angles to the direction of the axis. In the case of a component or other part, this usually means "across the width" of the component. Compare Axial.

Radio-frequency
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, VHF, UHF, De-regulated frequency, Regulated frequency, Squelch, Co-channel interference, Diversity, Pilot tone, ISM, ETS, Carrier, Modulation, Roving microphone and Intermodulation.

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 the sound source. Or, another term for that distance. At any point on this surface, sound waves arriving direct from the sound source are at the same level as the reverberation resulting from that source alone. See also Distance factor.

Rails
The DC power-supply connections inside an item of equipment (especially an amplifier), or (more usually) the voltage on those connections. See also Overload and Bus.

Rake
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.

RAM
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.

Rarefaction
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'.

Raster
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 scanning movement is not visible because of the speed of the dot relative to the persistence of vision (and, in the case of the CRT display, 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 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. See also Composite video and CVS.

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

Rate
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.)

Rating
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 and the Power Ratings section on the Amplifiers and Speakers page.

Ratio
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.

RCA
Another name for a phono connector, so-named after the company which originally designed it (Radio Corporation of America).

RCBO
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.

RCCB
Another name for an RCD.

RCD
An abbreviation for 'residual current device'. A safety device, connected in the mains supply in order to provide a degree of protection against electric shock in the event of a fault which causes a potentially dangerous mains current to flow to a safety earth − even when this current is not high enough to cause the fuse to blow. (Just 40 milliamps (mA) can be enough to kill a healthy adult, whereas supply fuses are often rated at more than 100 times that value.)

The device 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 particular amount for a sufficient length of time (generally 30 mA for around 30 milliseconds (ms)). This difference in currents is known as the residual current.

Even when a 30 mA RCD is in use, any lower levels of earth leakage current passing through the body may still give a very serious shock − but for a healthy adult this is unlikely to be severe enough to cause death by electrocution. Any RCD used for shock protection must be manufactured to the relevant standards and be regularly tested to ensure correct operation.

Note that the nominal rated 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 nominal rated current of an MCB. 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 which incorporates an RCD is called an SRCD ('socket RCD'). Note that an RCD does not provide any protection against overload, nor against shocks caused by simultaneous contact with the Live and Neutral conductors. See also RCBO, Indirect contact, Direct contact and Power Breaker.

Reactance
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. 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'. See also Live (3).

Real time analyser
See Spectrum analyser.

Rear lobe
See Lobe.

Record deck
A device 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.

Reel-to-reel
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. See also 0 dB, Zero level, SOL, Programme level, Line-up (2), Tone (2) and Gain structure.

Reflection
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.

Reflex enclosure
See Port.

Refraction
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.

Regenerator
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. 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). Mains power regenerators are a class of the devices generally known as power conditioners.

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.

Release (1)
A control which adjusts the rate at which a compressor removes compression, after the expiry of the hold time. 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.

Repeater
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.

Residual current circuit breaker
See RCD.

Residual current device
See RCD.

Resistance
The property of any conducting electrical device which causes it to oppose the flow of a DC current to some extent. This property is measured in ohms. The thinner or longer the conductor of a cable, the greater its resistance is. In practice, the resistance of a 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).

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 device 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.

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

Resistor
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 resistance value and the tolerance of fixed resistors of low-power rating (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.

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):

• Why did the original resistor fail? This may be due to prior failure of one or more other components (often transistors).
• Use the same resistance value as the original.
• Ensure that the tolerance of the replacement 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 pre-amplifiers, the stability and noise level may be important.
• In certain safety-critical applications such as power supplies, factors such as flammability may be important.

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.

Resonance
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).

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

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 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.)

Returns
The circuits which carry the mixed line-level signals from the mixer to the 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 is often a separate cable. The multicore normally provides balanced connections for the returns using XLR connectors, but 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 this mode of operation. The term applies both to the front-of-house mix and to the monitor mix. See also Return and Powered multicore.

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 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.

Reverse phase
See Phase reversal.

Reverse polarity
See Polarity reversal.

RF
An abbreviation for 'radio-frequency'.

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 is dependent upon frequency. The Sennheiser MKH series of microphones employ this principle of operation.

RFI
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.

RGB
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 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.

RH
An abbreviation for 'relative humidity', a measure of the amount of water vapour present in the air. Along with ambient temperature, the relative humidity is important in providing a suitable storage and operating environment for equipment, and also because it affects the transmission (or 'propagation') of sound waves − see Absorption.

Rhythm guitar
A guitar on which chords are played so as provide a rhythmic instrumental backing to the overall sound. Compare Lead (2).

RIAA
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 microphone
Describes 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.

Rider
See Technical rider.

Rifle
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

Rig
A complete PA system, or any other complete system (e.g. a stage lighting system). Or, as a verb, to assemble such a system. See also Load-in. Compare De-rig.

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

Rigger
A person who works on rigging, often at height.

Rigging
The framework, usually of metal, which is located above a stage or auditorium and to which lanterns (and sometimes speakers) are attached or suspended. See also Fly and SWL.

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.

Ringing
A PA system is said to be 'ringing' when it is operating close to the point of acoustic feedback. See also Resonance and the next definition.

Ringing out
The deliberate creation of acoustic feedback during system set-up, to enable the predominant feedback frequencies to be discovered and manually compensated for, usually by use of a graphic equaliser.

This can be useful for setting the graphic equaliser(s) for the monitors, because prevention of feedback is one of the prime reasons for equalising the monitors − the most likely source of feedback. However, it is not the recommended method of setting the front-of-house (FOH) graphic equaliser, because here sound quality is of prime importance, and equalising the acoustic feedback path from the FOH speakers back to the stage microphones behind them is likely to result in a very unequalised forward-facing path to the audience. For the preferred approach for 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.

Ripple
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 the 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
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.

Risk
The likelihood of an event or situation occurring, that could cause injury or death, multiplied by the extent of such a consequence. For example, an equally high risk could arise from a substantial probability that one person might suffer injury or death, or from a smaller probability that many people might be so affected. Risks must be assessed and reduced as necessary so as to prevent danger. N.B. My 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.

RJ45
An 8-pole connector for UTP data cable, commonly used for Ethernet connections between computers and computer-related equipment. ('RJ' is an abbreviation for 'registered jack'.)

View RJ45 image

RMA
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.

RMS
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. See also PMPO.

Roll-off
A decreasing in response, which occurs either as the frequency rises above some specific value or as it falls below some specific value. 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 − this would be referred to as 'bass roll-off'. The 'steepness' of the decrease in response is specified in dB per octave. See also Slope and Shelving response.

Room acoustics
See Acoustics.

Routing
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, LCR (1) 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:

• Care to avoid feedback when used in front of the front-of-house speakers.
• Readiness to adjust the channel gain and/or 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 trip hazard.

RT-60
See Reverberation time.

RTA
An abbreviation for 'real time analyser'.

RU
An abbreviation for 'rack units'. See U (1).

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

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

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This page last updated 14-Nov-2009.