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Glossary of PA Terms - I

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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.

I.Link * I/O * I/P * IATSE * IC * IEC * IEC 268-5 * IEC 320 * IEC 60268-10 * IEC 60268-17 * IEC 60268-18 * IEC 60309 * IEC 60958-3 * IEC 60958-4 * IEC 61938 * IEC 651 * IEC noise * IEC noise weighting * IEE * IEEE 1394 * IEEE 802.11 * IEM * IET * IF * IIR * ILS * Image * Image frequency * Image rejection * IMD * Impedance * Impedance-balanced output * Impedance-matched * In-ear monitoring * In-phase * Indirect contact * Indirect sound * Induced interference * Inductance * Induction loop * Inductive coupling * Inductor * Infinite baffle * Infrasonic * Ingress protection * Input * Input impedance * Input stage * Insert * Insertion loss * Installation cable * Installation speaker * Instantaneous voltage * Instrument * Instrument cable/lead/cord * Instrument-level * Insulating tape * Insulation * Insulation displacement * Insulator * Integrated circuit * Interface * Interference tube * Interlace * Intermediate frequency * Intermission * Intermodulation * Interpolation * Interval * Intraaural or Intraural * Inverse square law * Inverter * IP * IPA * IPS * IRT * IRT norm * ISCE * ISM * ISO * Isolation * ITU

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 Duplex.

I/P
An abbreviation for 'input'.

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-intl.org.

IC
An abbreviation for 'integrated circuit'.

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', which is the IEC standard for a common type of 3-pole mains equipment connector − 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 268-5
See IEC noise.

IEC 320
The IEC standard for 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'.

IEC 320 connectors are rated at either 6 or 10 amps. As there is no obvious physical difference between these two types, 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.

Warning: These connectors are used on both 230 V and 110 V equipment (and on equipment that may be set 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).

This type of connector 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. [A cable intended for use with kettles, heaters and the like is usually fitted with a heat-resistant type of IEC 320 connector, properly termed a 'hot condition' type. Such appliances frequently have a special type of IEC 320 socket, which is able to accept only hot condition plugs that are identified by the presence of a keyway in their widest side.]

See also PowerCon and CEE-form.

View IEC (IEC320 mains) image

IEC 60268-10
See PPM.

IEC 60268-17
See VU.

IEC 60268-18
See PPM.

IEC 60309
See CEE-form.

IEC 60958-3
See SPDIF.

IEC 60958-4
See AES3.

IEC 61938
See Phantom power.

IEC 651
See Weighting.

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 IEC 268-5, which is very similar to the DIN 45573 noise specification. Speakers must withstand the quoted IEC 268-5 power level for 100 hours continuously. See also White noise and Power Ratings on the Amplifiers and Speakers page.

IEC noise weighting
See Weighting.

IEE
An abbreviation for the 'Institution of Electrical Engineers', a UK-based organisation which sets standards for electrical installations. The IEE no longer exists, having become part of the IET when it was formed in 2006. See also PAT and BS 7671.

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 maximum bit-rate in Mbit/s.) The updated version uses a 9-pole connector. 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 sets standards for electrical installations. The IEE became part of the IET when it was formed in 2006. See also PAT and BS 7671.

IF
An abbreviation for 'intermediate frequency'.

IIR
An abbreviation for 'infinite impulse response'. Describes a time-domain digital filter which incorporates feed-back paths. This enables complex filtering functions to be readily implemented, however care must be taken to ensure stability and to avoid the effect of compounded rounding errors in the computations; the 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.

ILS
An abbreviation for 'induction loop system'.

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 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.

• Speakers (not 100 V line types): 8 or 4 ohms (sometimes 15 ohms).
• Headphones:
  • Early consumer types: 8 ohms.
  • Modern consumer types: 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:
  • 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).

In-ear monitoring
A system which provides a performer with monitor sound through an ear-piece connected to a portable radio receiver. The receiver receives from a transmitter which is connected to a monitor output of the mixer. This arrangement is generally superior to the use of a monitor speaker, because it enables the performer to move freely around the stage without loss of monitor sound, avoids the monitor sound being heard by the audience or by other performers, and avoids feedback caused by microphones picking up the monitor sound. If required, several receivers can be used with one transmitter.

Many 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.

The radio frequencies used by these systems must be compatible with the frequencies used by any radio microphones in use at the same time, and must be either de-regulated frequencies or be properly licensed. (See the radio mic information on the Microphones page for information on frequencies.) Often abbreviated to 'IEM', or else the slang term 'ears' is used.

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 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 connections.

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

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" switch. Such systems are frequently used to satisfy the requirements of the DDA (UK legislation).

The system consists of a single loop of single-core cable installed so as to surround the area to be served (usually at ground level or at about 2.5 metres height). 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 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 must meet the requirements of standard IEC 118-4 (EN 60118-4 in Europe). This requires the complete system to have a flat frequency response from 100 Hz to 6.5 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 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 handbook to determine the required gauge for your loop dimensions and total cable length. (As a guide, it is common for modern loop amplifiers to require a loop resistance of between 0.5 and 1 ohm for correct operation.)

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

A possible problem for PA systems is inductive coupling from the loop into guitar pickups (especially if not of the humbucker type) and occasionally into other components such as dynamic microphones and audio transformers having poor magnetic screening. Coupling into a guitar pickup will cause any source that is being fed to the loop to be heard in that guitar's backline (after processing by any guitar pedals etc.), and to be fed from there back into the PA system via the backline microphone and/or DI. If the guitar channel is itself fed into the induction loop, then a circular signal path is created that can result in the occurrence of feedback. (This feedback sometimes happens only at low input levels into the induction loop amplifier, when that amplifier is not compressing the signal, and/or only happens when the guitar is held in certain orientations relative to the loop cable). To assist in the avoidance of these problems, it is highly preferable for the loop area to be kept well clear of the stage area occupied by the band. (It is not sufficient to avoid the loop area encompassing the stage, due to the fact that the magnetic field from the loop extends some distance outside the loop area.)

Inductive coupling
The phenomenon whereby a signal that is present 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 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.

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, on an item of equipment, intended to accept a signal from an output connection of some other item of equipment (or, rarely, from an output of the same item of equipment). Or, the signal that is applied to such a connection point. Electrically, an audio input may be balanced or unbalanced. Often abbreviated to 'I/P'. See also Connector, Level, I/O and the next definition.

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. The degree to which the level of an output's signal will drop, as a result of this loading, 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 Characteristic 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. See also Pre-amplifier and Stage (2). Compare Output stage.

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.

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 voltage
The value of a signal's voltage at a single moment in time. Note that this value continually varies, even for a signal 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. It is usually a coaxial cable, often fitted with a 2-pole jack connector at both ends, providing an unbalanced interconnection between the instrument and a high impedance input of equipment such as a combo, DI box or instrument pre-amplifier.

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.

Insulating tape
See PVC tape.

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

Insulation displacement
Describes a connector having one or more metal blades in electrical contact with each of its poles, for the purpose of slicing through the insulation of the individual wires of the cable when it is attached to the connector, so making contact with the conductors of the wires. This method of connecting cables to connectors is popular with certain types of cables which have to be terminated in large quantities, as, when the wire size is compatible with the connector, it can give a good quality connection without the use of solder or screws. However, many types of insulation displacement connector cannot be re-used, once terminated. Such connectors are most frequently used with solid-cored (rather than stranded) cables, such as are often used in fixed installations.

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

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. See also Connector, I/O, Duplex and Proprietary.

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 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, Semi-tone, Tone, Third, Fifth and Octave.

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. Compare Circumaural and Supraaural.

Inverse square law
The rule that describes how sound levels change with changing distance between the source and the listener (or microphone). It states that, assuming a point source, the level of direct sound drops by a proportion equal to the square of the proportion by which the distance has increased.

For example, if the distance increases by a factor of 3, the direct sound level reduces by a factor of 9. In terms of decibels, this means that the sound level changes by −20 times the log (to the base 10) of the factor by which distance increases or, put more simply, decreases by 6 dB for every doubling in distance. For our example of a tripling in distance, the sound level change is approximately −10 dB.

The inverse square law applies only within the free field of the source.

Note that a line array is not a point source and therefore does not follow this law (over the distance and frequency range 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) with reference to the level at a distance of 1 metre from a point source speaker, the distance at which speaker sensitivities are usually quoted. Note, however, that this assumes the absence of effects such as reverberation, absorption, refraction, grazing and, in outdoor situations, wind speed and direction.

• 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

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

IP (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.

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.

IPS
An abbreviation for 'inches per second', a measure of the speed of recording tape during recording and playback. The quality of the reproduction improves with increasing speed. The standard speeds are:

• 1 7/8 (1.875) IPS for cassette tapes,
• 3 3/4 (3.75) IPS for consumer reel-to-reel (now rarely encountered),
• 7 1/2 (7.5) IPS for semi-professional reel-to-reel and
• 15 IPS for professional reel-to-reel.

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: http://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..

Isolation (1)
Measures taken to substantially reduce the transmission of adverse interfering effects, particularly mechanically-coupled vibration and earth loop currents. See also Anti-shock mount and Earth isolator.

Isolation (2)
The complete disconnection of a mains supply, such that equipment normally powered by it may be safely worked on for maintenance or repair purposes, or in an emergency situation.

ITU
An abbreviation for 'International Telecommunications Union', an organisation which sets technical standards in telecommunications and related fields. See also EBU and AES.

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