The PAforMusic glossary of PA and related terminology

www.paformusic.info
Glossary of PA Terms - I

0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

If you have arrived here by clicking on a linked term on another page of this site, it may take a moment before your browser jumps to the definition of the term that you clicked on; thank you for your patience. (If there's still no movement after a few seconds, you may have encountered a broken link; please report it.)
If you have arrived here from a search engine, or by clicking on an alphabet letter on another page of the Glossary, then click on your required term in the list below. The most commonly looked-up terms are in bold.

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.

I.Link * I/O * I/P * IATSE * IC * IDC * IEC * IEC 118-4 * IEC 268-5 * IEC 320 * IEC 60268 * IEC 60268-5 * IEC 60268-10 * IEC 60268-17 * IEC 60268-18 * IEC 60309 * IEC 60320 * IEC 60958-3 * IEC 60958-4 * IEC 61672 * IEC 61938 * IEC 651 * IEC connector * IEC noise * IEC noise weighting * IEE * IEEE * IEEE 1394 * IEEE 802.11 * IEM * IET * IF * IFB * IIR * ILS * IM * Image * Image frequency * Image rejection * IMD * Impedance * Impedance-balanced output * Impedance-matched * Impulse impedance * In-ear monitoring * In-phase * Incandescent lamp * Indirect contact * Indirect sound * Induced interference * Inductance * Induction loop * Inductive coupling * Inductor * Infinite baffle * Infinite impulse response * Infrasonic * Ingress protection * Input * Input impedance * Input stage * Inrush current * Insert * Insertion loss * Installation cable * Installation speaker * Instantaneous value * Instrument * Instrument cable/lead/cord * Instrument-level * Instrumental break * Insulating tape * Insulation * Insulation displacement * Insulator * Integrated circuit * Intelligibility * Intensity * Interface * Interference tube * Interlace * Intermediate frequency * Intermission * Intermodulation * Interpolation * Interval * Intraaural or Intraural * Intro * Inverse square law * Inverter * IP * IP address * IPA * IPS * IRE * IRT * IRT norm * ISCE * ISM * ISO * Isolated ground * Isolating transformer * Isolation * ISP * ISRC * ISU * ITE * I.T.E. * ITU * ITU-R * ITU-R 468 * ITU-R BS.1770 * ITU-R weighting

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.

I.Link

See IEEE 1394.

I/O

An abbreviation for "input / output". A designation commonly used for an interface, cable, connector pole, etc. that may be used either as an input or an output, whether simultaneously or alternately. For example, see SCART. See also I/P, O/P and Duplex.

I/P

An abbreviation for 'input'. See also I/O. Compare O/P.

IATSE

An abbreviation for 'International Alliance of Theatrical Stage Employes' (yes that's how they spell 'employees'), a stage-workers' union. Their website is www.iatse.net.

An abbreviation for 'integrated circuit'.

IDC

An abbreviation for 'insulation displacement connector'; any type of connector that uses insulation displacement technology.

IEC

An abbreviation for 'International Electrotechnical Commission', an organisation which defines standards for use in the electronics industry. In PA work, the term 'IEC' is most commonly used (as an abbreviation for 'IEC 320') to refer to a particular common type of 3-pole mains equipment connector. For further information see IEC 320.

Note, however, that the IEC produce very many other specifications, including ones for other types of mains power connectors (such as the CEE-form). Some of the specifications most relevant to PA work are listed in the definitions following this one. See also DIN, CCIR and CENELEC.

IEC 118-4

The international equivalent of UK standard BS EN 60118-4 for induction loop performance.

IEC 268-5

See IEC noise.

IEC 320

The original designation of the IEC standard covering a common type of 3-pole mains equipment connector, most frequently used for the connection of mains supply cables to equipment. It is commonly referred to as an 'IEC connector'.

The IEC 320 standard is now redesignated IEC 60320 (or EN 60320), and details several incompatible types of mains connector (2-pole as well as 3-pole types). The specific 3-pole connector most commonly referred to by the terms 'IEC' or 'IEC 320' has two sub-designations: C13 for the female (outlet connector, usually cable-mounted) and C14 for the male (e.g. for equipment power inlets).

However, some items of equipment that become hot in use, such as some types of lantern, are fitted with a heat-resistant type of IEC 320 connector, properly termed a 'hot condition' type. Such items frequently have a special type of IEC 320 inlet connector (C16 or C16A), which is able to accept only hot condition outlet-cable connectors (C15 or C15A respectively) that are identified by the presence of a keyway in their widest side. The 'A' versions are additionally keyed in their opposite side.

IEC 320 C13/C14 connectors are rated at either 6 or 10 amps. As there is no obvious physical difference between the 6 amp and 10 amp versions, and there is no built-in means to prevent use of the wrong type, it is important to ensure that equipment rated at more than 6 amps is always used with a 10 amp rated connector and cable.

Items of equipment that draw currents higher than 10 amps, such as some power amplifiers, use a higher-current version of IEC 320 connector, designated C19 for the female (outlet, usually cable-mounted) and C20 for the male (inlet, usually equipment-mounted). These are usually rated at either 16 or 20 amps.

Other common types of IEC connectors are the 2-pole 'figure-of-8' (C7 female and C8 male) and the 3-pole 'cloverleaf' (C5 female and C6 male), both of which are rated at 2.5 amps.

Warning: All the above types of connectors are used on both 230 V and 110 V equipment (and on equipment that is adjustable for operation on either of these voltages). Before connecting, always be sure that the mains supply voltage is suitable for the equipment and/or that the equipment is set for the mains voltage to be used.

In the UK, a mains cable fitted with an IEC connector at one end is frequently fitted with a BS 1363A ('13 amp') plug at the other end. If the IEC connector is a 6 amp type then the BS 1363A plug should be fitted with a 5 amp fuse; if it is a 10 amp type then the plug may be fitted with a 13 amp fuse (provided that the cable's current rating is adequate for these fuse values).

IEC 320 connectors may, in non-professional circles, sometimes be referred to as a 'kettle connector', and a mains cable fitted with it referred to as a 'kettle lead'; both are deprecated slang terms. [Kettles (and other such domestic equipment that becomes hot) are generally fitted with a 'hot condition' inlet connector − often the C16 type is used, requiring use of a keyed C15 female cable connector.]

See also PowerCon and CEE-form.

View IEC (IEC320 mains) image

IEC 60268

An international standard for sound system equipment. It is split into many parts, each covering a specific topic and designated by a numerical suffix. For example, IEC 60268-1 and -2 are general sections, -3 covers amplifiers, -4 covers microphones and -5 covers speakers. See also the following definitions.

IEC 60268-5

See IEC noise.

IEC 60268-10

See PPM.

IEC 60268-17

See VU.

IEC 60268-18

See PPM.

IEC 60309

See CEE-form.

IEC 60320

See IEC 320.

IEC 60958-3

See SPDIF.

IEC 60958-4

See AES3.

IEC 61672

A standard for the performance and testing of sound level meters. It replaces the older standard DIN/IEC (60)651. IEC 61672-1 specifies the performance requirements, while IEC 61672-2 and 61672-3 are concerned with testing. The standard designates two 'Classes' of meter, Class 1 being of greater accuracy than Class 2. These Classes are essentially equivalent to the Type 1 and 2 designations of the earlier standards. (Type 0 designated laboratory-standard meters, while Type 3 designated very low standard ones.)

IEC 61672 is often quoted in reference to the weightings it specifies. These include A-weighting, which is very popular (especially in the USA) for noise level measurements. In this context, compare ITU-R 468.

IEC 61938

See Phantom power.

IEC 651

An early standard for sound level meters, but sometimes still quoted to refer to A-weighting. IEC 651 is alternatively designated 'DIN 651'. The current standard for sound level meters and the specification of A-weighting is IEC 61672-1. See also Weighting.

IEC connector

Strictly, any connector complying with an IEC standard. However, in the context of PA work the term is most often used to refer to an IEC 320 connector.

IEC noise

A type of noise intentionally generated for the purpose of testing audio equipment, and especially for specifying the power rating of speakers. It is basically similar to pink noise, but is specially filtered in order to provide a better match to the power/frequency distribution of actual programme material. The IEC standard for this noise is defined in IEC 60268-5 (previously known as IEC 268-5), which is very similar to the DIN 45573 noise specification. Speakers must withstand the quoted IEC 60268-5 power level for 100 hours continuously. See also Power Ratings on the Amplifiers and Speakers page and White noise.

IEC noise weighting

See Weighting.

IEE

An abbreviation for the 'Institution of Electrical Engineers', a UK-based organisation which set standards for electrical installations. The IEE no longer exists as an institution, having become part of the IET when it was formed in 2006. However, 'IEE' is still sometimes incorrectly used to refer to the IET Wiring Regulations (see BS 7671). See also PAT.

IEEE

An abbreviation for the 'Institute of Electrical and Electronics Engineers', an American organisation which sets standards for electronic equipment (e.g. see the following definitions). See also National Electrical Code.

IEEE 1394

A standardised interface for high speed digital communications, especially with computer equipment. Also known as FireWire (a trade mark of Apple Computers) and I.Link (a trade mark of Sony). It can operate using two types of connector, a flat 6-pole type which carries power, and a smaller 4-pole type which does not.

An updated version of IEEE 1394 is known as IEEE 1394b, or FireWire800; so the original version is now sometimes referred to as IEEE 1394a, or FireWire400. (400 and 800 relate to the approximate maximum possible bit-rate in Mbit/s.) The updated version uses a 9-pole connector.

The bit-rate in use over a particular interconnection is identified by 'S100', 'S200', 'S400', etc, where 100, 200 and 400 relate to the approximate bit-rate in Mbit/s. (The corresponding actual bit-rates are nominally 98.304, 196.608 and 393.216 Mbit/s.) See also USB and Ethernet.

View Firewire (4 pole) image View Firewire (6 pole) image View Firewire (9 pole) image

IEEE 802.11

A standardised interface for digital wireless communications, especially with computer equipment. Also known as Wi-Fi and AirPort. See also Bluetooth.

IEM

An abbreviation for 'in-ear monitoring'.

IET

An abbreviation for the 'Institution of Engineering and Technology', a UK-based organisation which (amongst other activities) sets standards for electrical installations. Their website is www.theiet.org (opens in a new window or tab). The IEE became part of the IET when it was formed in 2006. See also PAT and BS 7671.

An abbreviation for 'intermediate frequency'.

IFB

An abbreviation for 'interruptible fold back'. A term used mostly in a broadcast context to refer to a facility enabling live presenters to hear cues and other directions from the producer and possibly to hear other remote audio (such as an incoming telephone caller, an outside broadcast or another studio), without that audio being picked-up by the presenter's studio microphone. In radio broadcast the presenter will typically wear headphones for this purpose, while in television in-ear devices are worn − these are often wired earpieces but may be wireless types.

The term arises from the need to provide the person's own voice (or a full station mix) to them most of the time (to assist in natural speech), but to interrupt that foldback in order for the producer's voice to be clearly heard. However, this term is sometimes used to refer to a general-purpose wireless in-ear monitoring system, as used by many on-stage performers.

IIR

An abbreviation for 'infinite impulse response'. Describes a time-domain digital filter which incorporates one or more feed-back paths. This enables complex filtering functions to be readily implemented, however care must be taken to ensure stability and to avoid the effect of compounded rounding errors in the computations; the phase response can also be problematic. These difficulties are avoided with FIR types. The name arises because the filter's response to an impulse input is not time-limited. Compare FIR.

ILS

An abbreviation for 'induction loop system'.

An abbreviation for 'intermodulation' or for 'intermodulation distortion'. See Intermodulation.

Image (stereo)

See Stereo image.

Image frequency

A frequency at which a radio receiver exhibits some unwanted sensitivity as a result of the conversion from received radio frequency (RF) to intermediate frequency (IF) that takes place within the receiver. The image frequency will be either twice the IF higher than, or twice the IF lower than, the intended RF reception frequency. The extent of the receiver's sensitivity at the image frequency is specified in terms of its 'image rejection', which is a function of the quality of RF filtering employed within the receiver.

Image rejection

See Image frequency.

IMD

An abbreviation for 'intermodulation distortion'. See Intermodulation.

Impedance

The value of the ratio of voltage to current of a signal at some point in a system, at a particular frequency. Or, a measure of the opposition to the flow of current, due to the combined effects of resistance and reactance, at an input or output of an item of equipment, or through an electrical component, at a particular frequency.

As the impedance value generally varies across the frequency range of interest, a nominal value is usually quoted, being an 'average' or 'typical' value over the relevant range of frequencies. For a DC circuit, it is the same as the resistance of the circuit. Like resistance and reactance, it is measured in ohms. It is given the symbol 'Z'.

Impedance is important in various situations, for a number of different reasons:

Load impedance governs how much current is drawn by a load, for a given applied voltage, and therefore affects how much power will be dissipated by the load, for a given applied voltage (because power = voltage times current, provided that they are in phase). For example, the load impedance of a passive speaker is usually 4, 8 or 15 ohms (except for 100 V line types, which have a much higher load impedance). For further information on speaker impedance, see the Impedance section on the Amplifiers & Speakers page.
In a voltage-matched interconnection, the output impedance of a signal source and the input impedance of the equipment that it connects to are co-ordinated so that the source voltage is not significantly reduced as a result of making the interconnection. For example, a 'low impedance' microphone having an output impedance of, say, 150 ohms would typically be connected to a mixer or pre-amplifier input having an input impedance of around 2 kilohms, but a 'high impedance' source such as an electric guitar would need to be connected to equipment (such as a combo or DI box) having a much higher input impedance − typically at least 200 kilohms.
In an impedance-matched interconnection, the output impedance of a signal source and the input impedance of the equipment that it connects to must be the same, in order to maximise the transfer of power between source and load. For impedance-matched interconnections that use radio-frequency signals, or which are very lengthy, the characteristic impedance of the interconnecting cable and connectors must also be the same as the output impedance and input impedance values, in order to maximise the return loss.

For illustrative purposes, some typical impedance values are summarised as follows:

Speakers (not 100 V line types): 8 or 4 ohms (sometimes 15 ohms).
Headphones and earphones:
- Early consumer types: 8 ohms.
- Modern consumer types: Usually in the range 24 to 40 ohms; most commonly 32 ohms.
- Professional types: Usually 250 or 400 ohms.
Microphones:
- Low-impedance: Usually 150 to 300 ohms (otherwise 50 to 600 ohms).
- High-impedance: Usually 5 to 15 kilohms.
Equipment inputs:
- Low-impedance (e.g. microphone inputs on mixers): Usually around 2 kilohms.
- High-impedance (e.g. line inputs on mixers and inputs of power amplifiers): 10 to 50 kilohms.
- Inputs of instrument amplifiers (e.g. guitar amps): 200 kilohms to 1 megohm.
- Inputs of passive DI boxes: 10 to 50 kilohms.
- Inputs of active DI boxes: 200 kilohms to 1 megohm.
Equipment outputs:
- Passive guitars and basses: 20 to 150 kilohms (usually varies heavily according to the setting of the instrument's controls; also often frequency-dependent).
- Active instruments, including guitars and basses (unbalanced outputs): 100 ohms to 20 kilohms.
- Line outputs: 50 to 600 ohms.
- Speaker outputs (not 100 V line): 0.005 to 0.1 ohm.
Impedance-matched interconnections: See Impedance-matched.

The term 'impedance' is frequently used as short-hand for 'load impedance', 'output impedance', 'input impedance' and 'characteristic impedance'; in most cases the intended meaning is indicated by the context of use.

Impedance-balanced output

A term that is usually used as an alternative name for a semi-balanced output.

Impedance-matched

Describes an interconnection in which the load impedance is equal to the source impedance. This arrangement is employed where radio-frequency signals and/or long lengths of cable are involved, such as in analogue video, digital audio and DMX lighting control interconnections. In such cases, it is vital that cable and connectors of the correct characteristic impedance are used in order to maximise signal transfer and minimise troublesome signal reflections. It is also important to avoid double terminations.

The impedance values used for some common types of impedance-matched interconnections are as follows:

SPDIF digital audio: 75 ohms (unbalanced).
AES3-id digital audio: 75 ohms (unbalanced).
AES3 digital audio: 110 ohms (balanced).
DMX: 120 ohms (balanced).
Analogue video: 75 ohms (unbalanced).
Antenna connections (e.g. of radio microphone receivers): 50 ohms (occasionally 75 ohms) (unbalanced).

Note that, in general, analogue audio interconnections are not impedance matched, but rather the load impedance is significantly higher than the source impedance; this is sometimes referred to as a voltage-matched interconnection. Exceptions to this rule are telecommunications circuits and the connection of valve-output amplifiers to speakers.

Impulse impedance

For most practical purposes, an alternative term for characteristic impedance.

In-ear monitoring

A system which provides a performer with monitor sound through one or two earphones. Often abbreviated to 'IEM', or else the slang term 'ears' is used. Sometimes referred to as 'EWM' (ear-worn monitoring). Most usually the earphones are connected to a battery-powered portable radio receiver in order to provide wire-free operation, but sometimes they are instead wired to a headphone amplifier connected to monitor output(s) of the mixer.

In simple wired set-ups a headphone distribution amplifier is used, providing the same monitor mix to several performers, while in more complex set-ups each performer may be provided with a separate mix via individual headphone amplifiers or via a multi-channel unit. In the case of wireless systems, the receiver receives from a transmitter which is connected to a monitor output of the mixer. If required, several receivers can be used with each transmitter.

The use of IEM is generally superior to the use of monitor speakers − usually floor monitors (wedges) − because it avoids the monitor sound being heard by the audience or by other performers, and avoids leakage from monitor speakers into the microphones (often a major source of potential feedback problems). Additionally, in the case of wireless systems, it enables the performer to move freely around the stage without loss of monitor sound.

Many wireless types offer two-channel operation, which allows the performer's receiver to be supplied with both an overall monitor mix and just the performer's own sound; the performer can then adjust the balance between these to suit his or her own preference, using a control on the receiver. However, the two channels are instead often used to provide the performer with a stereo mix, as this helps to give a sense of space and so helps to reduce the disorientation that can sometimes occur when using in-ear monitoring. It can also be helpful in reducing this latter effect if an ambient microphone is included in the performer's mix.

These systems must only be operated on the frequencies that are permitted for use in the area concerned. This means either using de-regulated frequencies or else licensing the equipment to use a licensed frequency. The frequencies used must also be compatible with the frequencies used by any radio microphones in use at the same time. (See the radio mic information on the Microphones page for information on frequencies.)

Wireless IEM systems can be particularly prone to signal integrity problems such as drop-out, partly because their receivers are usually non-diversity types. A directional transmit antenna such as an LPDA or helical type is therefore sometimes used to increase the effective signal power transmitted in the direction of the receiver(s). See also IFB and ERP.

In-phase

Describes the situation in which a signal's instantaneous voltage (or current) changes at essentially the same time and in the same direction (i.e. positively or negatively) as that of some reference signal at the same frequency or carrying the same information, i.e. there is no phase difference between them. (They may, however, be of different level.)

Or, the situation in which the changes in a sound wave's instantaneous pressure occur at essentially the same time and in the same direction as those of a reference sound wave carrying the same information, at specified location(s). If two in-phase sound waves combine then they will reinforce one another, giving an increase in sound pressure level; this is termed constructive interference. In order to avoid the opposite effect (destructive interference), it is important to ensure that when two or more speakers cover the same area they are operating in-phase − unless deliberate timing differences are being used to provide a specific directional coverage pattern. There is therefore a need to ensure correct polarity of all speaker connections (and indeed of all audio connections).

In an AC circuit, when the voltage and current are in phase then the average power dissipated can be calculated by multiplying the RMS voltage by the RMS current, a situation referred to as 'unity power factor'. Compare Out of phase and Anti-phase.

Incandescent lamp

A lamp in which the light is produced as a result of raising the temperature of a metal filament to a suitable point by passing an appropriate amount of electric current through it. Such lamps are therefore also known as filament lamps. This method of producing light is very inefficient; most of the electrical power consumed gets converted to heat rather than to light. Therefore, other types of lamp such as LEDs or discharge types are often used where appropriate.

Mains-powered incandescent lamps are commonly used in lanterns for stage lighting, and such lanterns are generally suitable for dimming by phase-angle control. However, the increased efficiency and convenience of LED-based lanterns (which often incorporate their own brightness and colour control) means that they are increasingly used instead. Incandescent lamps are sometimes used in speakers to provide driver protection − for further information see Protection lamp.

Indirect contact

In electrical safety, the potentially lethal situation where a person comes into contact with an accessible conductive part that is not intended to be live at a dangerous voltage (e.g. at mains voltage), but that has become live at a dangerous voltage because of a fault condition. Such a conductor would typically be the metal case of an item of mains-powered equipment, or something connected to it (such as the strings of a guitar), or some other item of metalwork (such as a steel gantry).

This situation is protected against by the connection of accessible conductive parts to a safety earth ('earthing' or 'bonding'), or by the provision of additional insulation between such conductors and parts that are intended to be live ('double insulation'). Further protection against indirect contact may be provided by a suitable RCD, but this should not be the sole means of protection against indirect contact. These measures are collectively referred to as 'fault protection' against electric shock. See also Class I, Class II and PAT. Compare Direct contact.

Indirect sound

Sound that has travelled from its source to the listener, or to a microphone, by a path(s) other than a single essentially straight line; or sound that has undergone one more reflections along its path. Compare Direct sound. See also Free field, Diffuse field and Radius of reverberation.

Induced interference

See Inductive coupling.

Inductance

The property of a conductor which causes it to oppose the flow of a changing current (AC) to a greater extent than it opposes the flow of a steady current (DC); this property is measured in henrys. More importantly, the higher the frequency of the current, the greater the opposition caused. Therefore, this property can be used to construct filters, selectively passing some frequencies and blocking others.

The inductance of a conductor depends not only on its dimensions but also on its shape. This is because the inductive effect has a magnetic origin: coiling a conductor considerably increases its inductance because it concentrates the magnetic field, and when such a component is deliberately formed it is called an inductor.

The inductance which exists, undesirably, in the conductors of a cable (or other wiring) may be a cause of high-frequency attenuation in the cable. The inductance of a straight piece of normal single-core wire is around 2 µH per metre of length (0.6 µH per foot). Note, however, that the round-trip inductance of a 2-core cable is less than its end-to-end inductance, because the magnetic fields of each conductor are in opposition and partially cancel one another. See also Back-emf, Reactance and Impedance. Compare Capacitance.

Induction loop

An assistive listening facility which provides an audio-frequency magnetic field suitable for pick-up by hearing aids equipped with a T setting. Such systems are frequently used to satisfy the UK legislation requirements of the Equality Act or, in Northern Ireland, the DDA. In the UK, BS 8300 includes requirements for the provision of assistive listening facilities, and code of practice BS 7594 specifies requirements for the installation of induction loop systems.

The system consists of (usually) a single loop of single-core cable installed at a constant height and arranged so as to surround the area to be served. There is an 'ideal' height for installation of the cable (which depends upon the shape of the loop), but because of practical considerations it is usually installed either at ground or ceiling level (or at around 2.5 metres height in the case of high ceilings). The two ends of the loop cable are connected to a purpose-designed loop-driver amplifier, which is fed from an output of the PA system.

The magnetic field created by the loop is not contained entirely within the area enclosed by the loop cable, but also extends some distance outside of (and above and below) that area. This fact can sometimes enable the area of the loop to be reduced, but also means that loop systems can cause interference with PA (and other) equipment even when located outside the loop (more details below), and that multiple loop systems in close proximity cannot be used simultaneously.

Loop systems in the UK must meet the requirements of standard BS EN 60118-4 (IEC 118-4 internationally). This requires the complete system to have a flat (within +/−3 dB) frequency response from 100 Hz to 5 kHz, and to include compression in order to adapt the signal to the limited dynamic range of hearing aids. In practice, loop amplifiers generally have a frequency response of around 80 Hz to 10 kHz. However, the frequency response actually achieved will be influenced by the inductance of the loop, which depends on its size and shape and on the presence of nearby ferrous materials such as structural steelwork or reinforcing meshes embedded in concrete floors and ceilings. Some induction loop amplifiers have the facility to provide adjustable compensation for the presence of such materials, usually referred to as a 'metal compensation' control.

The average magnetic field strength inside the loop, at listener height, is required to be 0.1 amps per metre (A/m) RMS. Although compression is applied, its time constant must be sufficiently long to preserve the essential dynamics of speech, so as not to impair intelligibility. Allowing 12 dB of headroom for such dynamics implies that it must be possible to achieve a peak field strength of 0.4 A/m RMS. The amount of current around the loop that is needed to create this peak level of magnetic field depends upon a number of factors, particularly the physical dimensions and layout of the loop; typically it will be in the region of 6 to 12 amps RMS.

The required value of peak current is the main factor in the selection of an appropriate loop amplifier, although it must also be able to produce sufficient voltage to force this amount of current through the highly inductive impedance of the loop, at the highest frequency at which such high peaks of current may occur (typically 1.5 to 2 kHz). The loop cable gauge must be selected to be suitable for carrying the required current, and also in order to ensure an appropriate value of loop impedance. Consult the loop amplifier manufacturer’s handbook to determine the required gauge for your loop dimensions and total cable length. (As a guide, it is common for modern loop amplifiers to require a loop resistance of between 0.5 and 1 ohm for correct operation.)

Loop amplifiers must also avoid the radiation of electromagnetic interference (EMI) from the loop, usually achieved by the application of appropriate output filtering and by incorporating limiting to avoid the possibility of clipping.

A possible problem for PA systems is inductive coupling from the loop into the magnetic pick-ups commonly used in musical instruments such as electric guitars and basses (especially if the pick-up is not of the humbucker type). Such unwanted coupling may also occasionally occur into other system components such as dynamic microphones and audio transformers, if they have poor magnetic screening. Coupling into an instrument pick-up will cause any source (including vocals) that is being fed to the loop to be heard in that instrument's backline system (after processing by any pedals etc.). Those sources will also be fed from the backline back into the PA system via the instrument's backline microphone (and by any other mic(s) that are picking up the backline's sound) and/or via the instrument's DI connection. If any PA channels that are receiving the instrument's signal or picked-up sound (whether intentionally or not) are fed into the induction loop, then a circular signal path is created that may result in the occurrence of feedback. (This feedback sometimes happens only at low input levels into the induction loop amplifier, when that amplifier is not compressing the signal, and/or only happens when the instrument is held in certain orientations relative to the loop cable). To assist in the avoidance of these problems, it is highly preferable for the loop area to be kept well clear of the stage area occupied by the band. (It is not sufficient to avoid the loop area encompassing the stage, due to the fact that the magnetic field from the loop extends some distance outside the loop area.)

Inductive coupling

The phenomenon whereby a signal or power current that is flowing on one conductor impresses itself to some degree on a nearby conductor because of the mutual inductance that exists between the two conductors. Such coupling is usually undesirable (except internally within a transformer, and from an induction loop to a hearing aid), and is normally avoided by use of balanced interconnections. In such interconnections, the two signal conductors are usually twisted around each other along the length of the cable, in order that each conductor experiences a near-identical inductive pick-up from interfering sources. The remaining induced interference is then mostly common mode interference.

Undesirable inductive coupling can also be caused by the close proximity of items of equipment which use transformers − for example if a transformer balanced DI box were placed on top of a combo then it may pick up an unwanted hum. (Note, however, that a similar effect can also be caused by earth loops, and under certain circumstances such multiple interfering sources of hum may partially cancel one another when mixed.) See also Crosstalk. Compare Capacitive coupling and Common impedance coupling.

Inductor

An electrical component whose purpose is to introduce inductance into a circuit. In PA work, they are most usually encountered in passive crossovers. Sometimes called a 'choke'. Inductors most commonly use enamelled copper wire in their windings. Audio-frequency inductors are most usually air-cored, while mains-frequency components use a laminated core.

Inductors inevitably also introduce a certain amount of resistance into the circuit, but this is usually arranged to be small compared with their inductive reactance at the frequencies of interest; the amount of resistance introduced is often specified by the Q of the inductor. See also Resistor and Capacitor.

Infinite baffle

See Sealed box.

Infinite impulse response

See IIR.

Infrasonic

Describes a sound whose frequency is below the generally accepted audio-frequency range, i.e. is below 20 Hz. (The term 'subsonic' is sometimes incorrectly used for this meaning.) See also Sub-bass. Compare Ultrasonic.

Ingress protection

See IP.

Input

A connection point, e.g. on an item of equipment, intended to accept a signal from elsewhere, typically from an output connection of some other item of equipment (or, rarely, from an output of the same item of equipment). Or, a similar connection point of a stage, module, or component internal to an item of equipment. Inputs of modules within digital processing systems may be virtual connection points, physically unidentifiable in the hardware.

Or, the signal that is applied to such a connection point.

The term is sometimes used quantitatively of a signal in reference to its level, e.g. "How much input does that equipment need?" Electrically, an audio input may be classified in many ways, e.g. as balanced or unbalanced, line level or microphone-level, low impedance or high impedance, analogue or digital, etc. Often abbreviated to 'I/P'. See also Connector, Level, I/O and the next definition. Compare Output.

Input impedance

The impedance that a signal input connection on an item of equipment presents to an applied signal. This is a measure of the current that will be drawn by the input, for a given applied voltage, and therefore indicates the degree to which this input will load the output to which it is connected. For examples of typical input impedance values see Impedance.

The degree to which the level of an output's signal will drop, as a result of it being loaded, is dependent upon the output impedance of the signal source and upon the combined input impedance of all the loads connected to that same output (the overall load impedance).

As with all impedances, the value of an input impedance may well change significantly with frequency − this is particularly evident in the case of speakers. See also Low impedance, High impedance and Characteristic impedance. Compare Output impedance.

Input stage

The circuitry, within an item of equipment, which initially processes the signals that are applied to the input connection(s) of the equipment. For example, the input stage must present the appropriate input impedance to input signals, be able to handle the expected range of input signal levels and provide any necessary initial interference filtering. See also Pre-amplifier and Stage (2). Compare Output stage.

Inrush current

The larger-than-normal current that initially flows for a very short period (typically less than 1 or 2 seconds) when a supply voltage is first applied to an item of equipment. The term is most commonly used in reference to switch-on of mains-powered equipment that is particularly prone to this effect, such as power amplifiers and some types of lanterns.

In the case of power amplifiers, the inrush current is caused largely by the initial charging of the internal power supply reservoir capacitors and (in the case of linear power supplies) the initial energisation of the transformer. In the case of filament lamps it is caused by the resistance of a cold filament being very much less than that of one at operating temperature.

The inrush currents of some equipment can be as much as 50 times the normal operating supply current, so fuses and circuit breakers for this equipment need to be arranged so as to not spuriously operate during a normal inrush current, but only when there is a genuine excess-current situation of a duration likely to cause danger or damage. For example, the mains input fuse of a power amplifer is typically a temporised fuse or a thermal circuit breaker.

When inrush current problems manifest themselves in spurious operation of protective devices, it is often the case that such operation occurs on apparently random instances of switch-on of the equipment concerned. This is generally due to random variations in the point, within the cycle of the AC supply voltage, at which the power switch is operated. On occasions when the switch is closed while the supply voltage cycle is near a maximum of its instantaneous value, the instantanous current will commence at a higher value. On such occasions a higher inrush current will result than is the case on occasions in which the switch is operated near a minimum of instantaneous supply voltage. This makes the likelihood of spurious operation of protective devices variable between different occasions of switch-on.

Equipment such as high-power power amplifiers, whose inrush current would (by default) be so high as to make spurious operation of fuses etc. a likely issue, typically incorporate a design feature specifically intended to reduce their inrush current to an acceptable value and so avoid such problems. Such a feature is commonly referred to as 'soft start'.

Insert

A connection, on a mixer or amplifier, which allows the connection of a serial effects unit into the internal signal path of the equipment, by interrupting that path when a plug is put into the connector. It is usually a ¼″ (6.35 mm) TRS ('stereo' jack) connection. The tip is generally used for the send connection and the ring for the return connection (pre-1990 Soundcraft mixers used the opposite convention). Inserts may be provided for individual channels, audio groups, the main mix, auxiliary mixes and/or matrix outputs.

The insert points are usually pre-EQ, but are sometimes post-High pass (check the block diagram in the handbook!) − this means that effects units connected this way are handling the un-equalised signal. This may be an important issue in some cases, such as with dynamics processors (but using their side chain EQ can help).

A channel insert connection may usually also be used as a direct output connection, either by partially inserting the jack plug (not good practice), or by using a special lead which has the tip and ring of the jack connected together. See also Normalling.

Insertion loss

A measure of the reduction in level that is caused in a signal chain by the presence of a particular item of equipment (such as a filter or an earth isolator.) Numerically, it is the reduction in level (usually expressed in decibels) that occurs when the item in question is connected ('inserted') into the signal chain, as compared to the level obtained when the item is absent − continuity of the chain then being made by a direct connection between the previous and following items of equipment.

Installation cable

A cable that is designed to be fixed in place within a building, auditorium, etc., rather than for short-term use. Such cables do not have the requirement to withstand frequent flexing or mechanical abrasions and stresses, and so are usually of lower cost and are less thick and of lighter weight. Such cables are usually installed within trunking or are secured with cable clips. See also CPR (2) and LSF.

Installation speaker

A speaker that is designed for use as a long-term fixture in a building, auditorium, etc., rather than for portable or mobile use. Such speakers are styled to be visually discrete, and are usually provided with a means of fixing to a wall or ceiling, rather than being floor-standing or pole-mounted. (Note, however, that any bracket(s) required may have to purchased as a separate item.) Some models have various degrees of weather resistance, for outdoor use. Types intended for use with public address systems are frequently of the 100 volt line variety. See also A & E specifications.

Instantaneous value

The value of a signal's voltage, current or power at a single moment in time. Or, the value of sound wave pressure at a single moment in time. Note that this value continually varies, even for a signal or sound of constant level. Compare RMS and Peak.

Instrument

In lighting, the American name for a lantern.

Instrument cable/lead/cord

A cable intended for connection to a musical instrument, generally referring to a cable intended to carry instrument-level audio signals from the instrument to equipment such as a combo, head, effects pedal, DI box or instrument pre-amplifier. Such cables are unbalanced coaxial cables, most usually fitted with 2-pole ¼″ (6.35 mm) jack connectors at both ends. They are also often used for other unbalanced instrument-level audio interconnections, such as connections between the items listed above.

As interconnections between instruments and the type of equipment listed above usually operate at high impedance, most instrument cables are specifically designed to minimise the undesirable effects sometimes encountered with high impedance interconnections. Such effects include microphony and a loss of treble due to the cable's capacitance, and can be particularly troublesome when the impedance is very high − as is usually the case with passive guitars and basses. Enhanced mechanical flexibility and resistance to kinking are also often important design features of such cables. Note that unbalanced cables fitted with 2-pole jack plugs are frequently referred to as jack-to-jack cables, however such a cable is not necessarily of a type specifically designed for high impedance instrument connections.

Instrument-level

A rather imprecise term referring to the output signal level that can typically be expected from musical instruments − especially electric guitars and acoustic guitar pick-ups. The term is imprecise because different types and makes of instrument are likely to produce substantially different typical output levels, and because the level obtained from a particular instrument at any point in time will usually depend on the settings of its controls and on the way it is being played. However, the term is generally used to refer to a nominal level of around 100 mV (−18 dBu or −20 dBV). Such a signal is likely to peak at around 250 mV (−10 dBu or −12 dBV), but again this will depend on the instrument and on how it is played. Compare Microphone-level and Line-level.

Instrumental break

A section of a song, during which no words are sung. There is often a solo part by one or more lead instruments during this section.

Insulating tape

A flexible tape that is specifically designed to have very good insulating properties. The most common type of insulating tape is PVC tape, though this may not be suitable for use in some situations.

Insulation

That part of a cable, wire, connector, or equipment, which is intended to present a barrier to an electric current and so prevent an unwanted flow of it from a conductor. As such a flow of current could cause a fatal electric shock or a fire, it is essential for safety purposes that insulation remains intact and in good condition. Insulation is frequently colour-coded to enable identification of the associated conductor. See also Insulator, Dielectric, PAT, Short circuit, Direct contact, Megger and PVC tape. Compare Sheath.

Insulation displacement

Describes a connector or a cable termination point having one or more metal blades that slice through the insulation of the individual wires of the cable, so making contact with the conductors of the wires. Such a connection may be referred to as a 'punchdown connection'. A special tool, designed to suit the particular terminal type in question, is required to insert the wires correctly.

This method of connecting cables is popular with certain types of cables which have to be terminated in large quantities (e.g. at patch bays or in computer networking), as, when the wire size is compatible with the connector, it can give a rapidly-made connection of high quality without the use of solder or screws. However, some types of insulation displacement connector cannot be re-used, once terminated. Insulation displacement connections are most frequently used with solid-cored (rather than stranded) cables, such as are often used in fixed installations. See also Krone.

Insulator

Any material which presents a very high resistance to the flow of current. Or, a component part whose purpose is to prevent electrical contact between conductors. See also Insulation and Semiconductor.

Integrated circuit

A semiconductor-based electronic circuit which is fabricated on a single piece of silicon, and packaged as a single component. Commonly abbreviated to 'IC'. Also referred to as a 'silicon chip', or just a 'chip' (a slang term). There are very many different types of ICs, each for a specific purpose. Their internal circuitry ranges from fairly basic (typically a few tens of transistors) to extremely complex (many millions of transistors). Many types fall into general categories such as operational amplifiers, CPUs, etc.. Like most components, many types of ICs are available in both surface-mount and through-hole mounted variants.

The design of an item of equipment may be referred to as 'integrated' if it makes use of ICs, even though other kinds of components will usually also be present. See also Solid state and Hybrid (1). Compare Discrete circuit.

Intelligibility

The clarity of a sound. In particular, the degree to which spoken words are correctly recognised or (in a negative sense) the degree to which deficiencies in a PA system or in a room's acoustics impact unfavourably on such recognition. Good intelligibility of speech is heavily dependent upon the degree to which consonants can be clearly distinguished from one another. Several methods are available to assess intelligibility, including ALCONS, STI and RASTI. In general, an assessment of intelligibility is meaningful only when it takes into account the characteristics of both the specific room and the PA system in use, given a specific set of system adjustments. Note, however, that such standardised assessments do not take into account such factors as the clarity or accent of the talker, or the quality of the listener's hearing. See also Distortion, Aliasing, Sibilance and Audiology.

Intensity

For intensity of light, see Candela (see also Lux and Lumens).
For intensity of sound, see Sound intensity level (see also Sound pressure level).

Interface

A point on an item of equipment or system, at which connection is made with other item(s) of equipment or system(s). Typically an input or an output, though many types of computer equipment interfaces (such as Ethernet, USB and Firewire) are bi-directional. Or, an item of equipment that allows the interconnection of two otherwise incompatible items of equipment. See also Connector, I/O, Duplex, Protocol, Proprietary and Bridge (2).

Interference filter

An item of equipment, or an arrangement of components within an item of equipment, whose purpose is to substantially attenuate unwanted intereference signals that are present along with the wanted signal(s). Some basic interference filtering is often provided as part of the input stage of items of equipment such as instrument amplifiers. Such a filter usually operates by providing substantial attenuation over just the frequency ranges occupied by the interfering signal(s). This is of course not possible if the wanted signal(s) occupy the same range(s).

Interference tube

See Rifle.

Interlace

In a video display system, a technique for the reduction of flicker at low frame rates. It operates by arranging for alternate lines of the raster to be scanned on successive fields; two such fields being required in order to complete a whole frame. So, the odd-numbered lines are first scanned (forming an odd-numbered field), followed by the even-numbered ones (forming an even-numbered field), then the odd ones again, etc. This arrangement suffers from the disadvantage that adjacent lines are scanned at substantially different times, giving a blurring effect to objects in the picture that have a large horizontal velocity across the screen. See also Field sync. Compare Progressive scan.

Intermediate frequency

A fixed radio-frequency to which the radio signal picked up by a receiver is converted prior to amplification and subsequent demodulation. Amplifying and demodulating the signal at an intermediate frequency (rather than at the received frequency) has the advantages that these processes may be performed at a fixed frequency regardless of any changes to the received frequency, and at one which is (usually) lower than the received frequency. Commonly abbreviated to 'IF'. See also Image frequency.

Intermission

An American term for an interval, i.e. a period of time between the major sections of a performance.

Intermodulation

The process, usually undesirable, whereby two signals, when passing through a system component that is not perfectly linear, interact to produce new signal(s) with frequencies that are either the difference or the sum of the frequency of the original two signals or of their harmonics. For example, signals at frequencies 'A' and 'B' may interact to produce signals at frequencies A+B, A−B, 2A+B, 2A−B, A+2B, etc.

When this effect occurs between two audio frequencies, resulting in the generation of unwanted new audio frequencies, the result is termed 'intermodulation distortion' (IMD).

When this effect occurs between two radio frequencies, resulting in the generation of unwanted new radio frequencies, the result is termed 'intermodulation interference'. This may be experienced when two or more radio microphone systems are operated together, on frequencies that are not members of a compatible set for that model of system.

Interval (1)

A period of time (typically around 20 minutes) between the major sections of a performance, during which no acts are on stage and the audience move about to take refreshments etc. See also Intermission.

Interval (2)

A musical term for the difference in pitch between two notes, corresponding to the ratio of their frequencies. See, for example, Semitone, Tone, Third, Fifth, Octave and Cent. Note that, in terms of frequency, intervals multiply rather than add. So since an octave corresponds to a frequency ratio of 2, three octaves correspond to a ratio of 8 (not 6).

Interpolation

The process of calculating an intermediate value between two given values, usually on the assumption of a straight line joining all three points (strictly, "linear interpolation"). Although not a true error correction system, this process may be used as a crude method for improving the (apparent) quality of digital signals, by inserting an interpolated value when an error detection system (such as parity) indicates that the received value is invalid. However, it can only work effectively for occasional single-word errors. See also Bit error rate.

Intraaural or Intraural

Describes headphones that are worn inside the ear canals. See also Earphones. Compare Circumaural and Supraaural.

Intro

An abbreviation for 'introduction'. The very first section of a song; the section that begins it. Compare Outro.

Inverse square law

The rule that describes the way in which dispersion causes sound intensity levels to change with changing distance between the source and the listener (or microphone). It states − assuming a point source, free dispersion (i.e. a free field) and no absorption by the medium − that the sound intensity decreases proportionally with the square of the factor by which the distance has increased. For example, if the distance increases by a factor of 3, then the sound intensity level (in watts per square metre) reduces by a factor of 9, because the area through which the same amount of radiated direct sound power is flowing will have increased by a factor of 9.

In terms of decibel (dB) measurements of sound intensity, the change in level is 10 times the log of the factor by which the intensity has changed. Therefore, due to the necessary squaring of the distance factor, the change in level is −20 (rather than −10) times the log of the factor by which the distance has increased. Put more simply, we can say that the level decreases by (very nearly) 6 dB for every doubling in distance. For our example of a tripling in distance, the change in level is approximately −10 dB (which will sound about 'half as loud'). [All references to logs here are to the base 10.]

However, for several important reasons, in PA work sound levels are measured and expressed in terms of sound pressure level (SPL) rather than sound intensity level. Contrary to popular belief, sound pressure does not follow the inverse square law − it decreases proportionally with the factor by which the distance has increased (not with the square of that factor). For example, if, in the same free field, the distance increases by a factor of 3, the sound pressure level (in Pascals) also reduces by a factor of 3.

Now when we express this change in sound pressure as a value in dB, in accordance with dB conventions we have to make the calculation with regard to the effective change in sound power (or intensity, which is power per unit area), which requires us to square the factor by which the pressure has changed (because sound power changes as the square of sound pressure). This means that the dB value changes by 20 (not 10) times the log of the factor by which the pressure has changed, which in turn means that the SPL has changed by −20 times the log of the factor by which the distance has increased. Put more simply, we can say that the SPL decreases by (very nearly) 6 dB for every doubling in distance. For our example of a tripling in distance, the change in level is again approx −10 dB (which will sound about 'half as loud'). N.B. This is the same change in dB as we calculated for changes with distance in sound intensity with distance. i.e. although SPL 'itself' doesn't follow the inverse square law, when SPL is expressed in dB the figures give the appearance that it does. We could, perhaps misleadingly, consider that SPL values in dB do follow 'an' inverse square law, but not 'the' inverse square law.

The inverse square law applies only within the free field of the source, and indicates the reduction in level due to the effect of dispersion only. The presence of natural reverberation, absorption, refraction, grazing and, in outdoor situations, wind speed and direction will usually modify this basic law substantially.

Note that a line array is not a point source and therefore does not follow this law (within the distance and frequency ranges over which it effectively operates as a line source − see Critical distance (2)).

Below is indicated the change in direct sound level (to the nearest dB) according to the inverse square law, with reference to the level at a distance of 1 metre from a point source speaker. (1 metre is the distance at which speaker sensitivities are usually quoted.) But, as stated above, this will rarely be the actual reduction in overall sound level that occurs in practice, because of the effects of natural reverberation, absorption etc.

1 m ........ 0 dB
2 m ........ −6 dB
5 m ........ −14 dB
10 m ...... −20 dB
15 m ...... −24 dB
20 m ...... −26 dB
25 m ...... −28 dB
30 m ...... −30 dB
35 m ...... −31 dB
40 m ...... −32 dB
50 m ...... −34 dB
75 m ...... −38 dB
100 m .... −40 dB

For guidance on how to use this law to calculate the input power required to a speaker, see Speaker Sensitivity on the Amplifiers and Speakers page. See also Needed acoustic gain.

Inverter

An item of equipment which produces mains voltage from a low-voltage DC supply (usually a 12 V or 24 V battery). These devices are ideal for powering small mobile systems (especially when vehicle-mounted), but are suitable only for relatively low-power applications (up to about 1.5 kW mains power requirement). It is very important to ensure that neither the continuous nor the short-term power ratings of the inverter are exceeded.

For higher-power systems, or when long-term use is required and connection to the battery of a vehicle with a running engine is not practicable, a better approach is to use a suitable generator set.

IP (1) (followed by 2 digits)

An abbreviation for 'ingress protection', a classification which is used to indicate the degree to which the enclosure of an item of equipment is resistant to the infiltration of solid particles and moisture.

The first of the two digits following 'IP' indicates the resistance to ingress of solid particles and objects, as follows:

0 (or X) − no protection
1 − 50 mm diameter and greater
2 − 12.5 mm diameter and greater
3 − 2.5 mm diameter and greater
4 − 1 mm diameter and greater
5 − Dust-protected
6 − Dust-tight

The second of the two digits following 'IP' indicates the resistance to ingress of moisture, as follows:

0 (or X) − no protection
1 − Water droplets falling vertically
2 − Water droplets falling vertically, when the enclosure is tilted at up to 15º from the vertical
3 − Water spraying at an angle of up to 60º from the vertical
4 − Water splashing from any direction
5 − Water jets from any direction
6 − Powerful water jets from any direction
7 − Temporary immersion in water
8 − Continuous immersion in water

An optional additional or supplementary letter may also be added. If in any doubt about the precise meaning of any of the above classification codes for a particular product, check with the manufacturer.

IP (2)

An abbreviation for 'Internet protocol', the networking protocol that is used for data transfer within the Internet and also by most local data networks and some point-to-point links. See also AoIP, AVoIP, AES67, RAVENNA and Dante.

IP address

A number indicating the logical identity of a network node in a data transfer network that uses Internet protocol. It may be made up of a subnet ID section and a node ID section, the boundary between which is indicated by a subnet mask. Both IP addresses and subnet masks are 32 bits long and are usually written either in 'dotted decimal' form (e.g. 192.168.15.132) or in hexadecimal. In a subnet mask, the number of '1' bits, which are all positioned to the left, indicate the length of the subnet ID section of the IP address.

IPA

An abbreviation for 'iso-propyl alcohol', a cleaning agent commonly used for electronic equipment as it is harmless to metals and to most plastics, and dries rapidly leaving no residue. Definitely NOT to be confused with the beer 'IPA', India pale ale!

IPS (1)

An abbreviation for 'inches per second', a measure of the speed of recording tape during direct recording and playback of analogue audio (now very rarely employed). The quality of the reproduction improves with increasing speed. The standard speeds are:

17⁄8 (1.875) IPS for cassette tapes,
33⁄4 (3.75) IPS for consumer reel-to-reel,
71⁄2 (7.5) IPS for semi-professional reel-to-reel and
15 IPS for professional reel-to-reel.

IPS (2)

An abbreviation for 'in-plane switching', a type of LCD display panel offering a wider viewing angle than earlier types. See also TFT.

IRE

An abbreviation for 'Institute of Radio Engineers' (of America). This no longer exists, as it merged with the IEEE. However, the term remains in use to refer to a scale of relative luminance, such as is represented by a video signal. This scale, formalised by the IRE, specifies relative luminance values as a percentage of peak white level, so 'IRE 0' refers to black and 'IRE 100' refers to peak white. See also Grey scale.

IRT

An abbreviation for 'Institut für Rundfunktechnik', the German Institute for Radio Technology.

IRT norm

Describes a product or an equipment interface that is compliant with the relevant IRT standard(s).

ISCE

An abbreviation for 'Institute of Sound and Communications Engineers'. Their website is: www.isce.org.uk. See also PLASA and AES.

ISM

An abbreviation for 'industrial, scientific and medical', a description applied to several bands of the radio-frequency spectrum in order to indicate the use to which they have been allocated. One of the ISM bands is the so-called "863-865 MHz" band (also known as "Channel 70") allocated for licence-free use of radio microphone and in-ear monitoring systems. See also De-regulated frequency and ETS.

ISO

An abbreviation for the 'International Standards Organisation', a body which defines standards for (amongst other things) audio systems, formats, etc..

Isolated ground

A specifically US term, describing the particular type of 'domestic style' fixed mains outlet (or 'receptacle') having a safety earth pole that is electrically separate (i.e. galvanically isolated) from the metallic frame of the outlet. This allows the outlet to provide a safety earth that is connected via a dedicated insulated earth wire (along with the other dedicated wiring supplying that outlet) to the distribution board, independent of the safety earth carried by the metallic conduit or trunking to the outlet. In some cases, such a separate dedicated safety earth connection may provide reduced common impedance coupling of interference into the equipment supplied from the outlet, avoiding other potentially 'dirty' earth currents that may be flowing in the conduit system. However, such problematic earth currents may nevertheless couple into the dedicated safety earth conductor by means of inductive or capacitive coupling.

This distinction between types of 'domestic style' (13 amp, BS 1363 outlets) does not apply in the UK in the same way, because:

In non-industrial premises it is now relatively unusual for these outlets to be fixed to a metallic back-box that is earthed by means of metallic conduit or trunking. The majority of metallic back-boxes are earthed via a circuit protective conductor that is either insulated or is contained within sheathed cable, and is dedicated to the circuit concerned.
These outlets are not normally individually cabled from the distribution board, but rather several are supplied from the same circuit (a ring or radial circuit). Therefore, even with a dedicated earth connection for that circuit, interfering earth currents from equipment connected to other outlets may still be a problem. (In contrast, the CEE-form outlets typically used in entertainment situations are usually individually supplied from the distribution board and independent earth connections to them are readily provided, as standard types have no inherent link between their earth pole and any metallic back-box or fixing point.)

Nevertheless, BS 1363 outlets with a so-called 'clean earth' capability are available in the UK for specialised applications where a safety earth must be provided that is independent of that provided by the metallic back-box or fixing point, such as in high-risk medical environments.

Not to be confused with the term ground isolator, which relates to signal earth isolation. See also Technical earth.

Isolating transformer

A transformer used with the main purpose of providing electrical separation, rather than for other purposes such as voltage conversion, impedance matching or balancing/unbalancing (for the latter see Balun). The term is used for two entirely different kinds of transformer, used respectively for:

Power applications. Here the purpose is usually to provide a local mains supply that is not referenced to earth potential, either to reduce problems from earth loop currents or for safety purposes. Some types are fitted with an inter-winding screen that should be connected to safety earth. Warning: An isolating transformer does not provide isolation in the sense of Isolation (2). Such transformers usually provide an output voltage that is nominally the same as its input voltage, i.e. at a mains voltage constituting a potential shock hazard. Supplies taken from the transformer output must be considered to be as hazardous as those from the original supply, and all appropriate safety measures must be applied to both supplies. (Unless, exceptionally, the output voltage of the transformer is at a safe level − e.g. less than 50 volts).
Signal applications. Here the purpose is usually to provide galvanic isolation so as to reduce problems from earth loop currents, while still passing the required signal without significant degradation. Some types are fitted with an inter-winding screen that should be connected to signal earth.

Isolation (1)

Separation measures taken to substantially reduce unwanted transmission or coupling. Such measures are typically provided at a suitable location on the route(s) of coupling between a source of interference and another point or location where that interference might otherwise cause a problem. For example:

Use of a booth, drum screen or sound insulating material to reduce acoustic transmission.
Use of a shock mount to reduce mechanically-coupled vibration to a microphone.
The avoidance of a galvanic connection between electrical conductors, either by means of insulation or, in cases where the flow of a signal or power must nevertheless be maintained, most commonly by use of an isolating transformer. (In these cases, the intention is typically in order to reduce problems from earth loop currents.)