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Welcome to Sound On Sound's indispensible, regularly updated, explanations of technical terms from the fields of Recording, Audio Production, Music Technology, MIDI, Music Software, Audio Plug-ins, Mac and PC Computing, Live Sound, Acoustics, Electronics and more...

If we do not explain a particular term below, please email and we will add it to our next update.

Last updated: 01/02/24

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The input/output connections of a system.


An abbreviation of Integrated Circuit, a collection of miniaturised transistors and other components on a single silicon wafer, designed to perform a specific function.

IEM (In-Ear Monitor)

A wirelessly-connected foldback monitoring system, often used by musicians on stage with in-ear earpieces.


The ‘resistance’ or opposition of a medium to a change of state, often encountered in the context of electrical connections (and the way signals of different frequencies are treated), or acoustic treatment (denoting the resistance it presents to air flow). Although measured in Ohms, the impedance of a ‘reactive’ device such as a loudspeaker drive unit will usually vary with signal frequency and will be higher than the resistance when measured with a static DC voltage.

Signal sources have an output impedance and destinations have an input impedance. In analogue audio systems the usually arrangement is to source from a very low impedance and feed a destination of a much higher (typically 10 times) impedance. This is called a ‘voltage matching’ interface. In digital and video systems it is more normal to find ‘matched impedance’ interfacing where the source, destination and cable all have the same impedance (eg. 75 Ohms in the case of S/PDIF).

Microphones have a very low impedance (150 Ohms or so) while microphone preamps provide an input impedance of 1,500 Ohms or more. Line inputs typically have an impedance of 10,000 Ohms and DI boxes may provide an input impedance of as much as 1,000,000 Ohms to suit the relatively high output impedance of typical guitar pickups.

Impulse Response (IR)

An impulse respsonse (IR) is the time-domain equivalent of the much more familiar frequency (and phase) responses in the frequency-domain. A very brief click (technically, a Dirac delta function) which theoretically contains all frequencies at equal amplitude, is passed through the device under test. The resulting output is the 'impulse response' of that device and uniquely describes its signal processing behaviour. Impulse responses are very convenient for digital signal processing applications as the source impulse is very similar to a single digital sample value.


A reactive component that presents an increasing impedance with frequency. (Also see Capacitor.)


The frequencies of overtones in a normal harmonic series are directly related to integer multiples of the fundamental frequency. In contrast, inharmonicity is the extent to which the frequencies of individual overtones deviate from integer multiples of the fundamental frequency.

In a musical context, percussion instruments such as chimes or cymbals often create strong inharmonic overtones, but the effect is also present in many other instruments, most notably the piano, and is directly responsible for the concept of stretch-tuning.


Resetting a device to its 'start-up' state. Sometimes used to mean restoring a piece of equipment to its factory default settings.

Input Impedance

The input impedance of an electrical network is the ‘load’ into which a power source delivers energy. In modern audio systems the input impedance is normally about ten times higher than the source impedance - so a typical microphone preamp has an input impedance of between 1500 and 2500 Ohms, and a line input is usually between 10 and 50k Ohms.

Insert Points

The provision on a mixing console or ‘channel strip’ processor of a facility to break into the signal path through the unit to insert an external processor. Budget devices generally use a single connection (usually a TRS socket) with unbalanced send and return signals on separate contacts, requiring a splitter or Y-cable to provide separate send (input to the external device) and return (output from external device) connections . High end units tend to provide separate balanced send and return connections. (cf. Effects Loop)

Instrument Level

The nominal signal level generated by an electric instrument like a guitar, bass guitar or keyboard. Typically around -25dBu. Instrument signals must be amplified to raise them to line-level.


A material that does not conduct electricity. (Also see conductor)


A device that acts as an intermediary to two or more other pieces of equipment. For example, a MIDI interface enables a computer to communicate with MIDI instruments and keyboards.

Interference Tube Microphone

A microphone — typically with a hypercardioid capsule — which has a perforated tube fitted in front of the diaphragm to try and create a substantially narrowed polar pattern.

Sound waves directly on-axis pass straight down the tube to reach the diaphragm without being affected in any significant way. However, this tube also has a number of slots or holes along its length which allow off-axis sound waves to enter at different points. The resulting different path lengths to the diaphragm create phase shifts resulting in partial cancellation and thus reduced sensitivity to sounds from the side of the tube. Hence the design's name of an 'interference tube microphone'. Alternative names include 'rifle' or 'shotgun' mic, principally because in the film and TV industries the mic is often fitted to a handle, resembling a firearm.

The cancellation of off-axis sounds is rarely perfect or consistent, resulting in varying amounts of attenuation for sound sources of different frequencies and at different angles of incidence. This means that off-axis sounds, while genreally attenuated, are also substantialluy coloured , and if the angle between mic and sound source is varied the cancellation dips move up and down the spectrum to give a phaser-like colouration or whoosing effect.


Something that happens occasionally and unpredictably, typically a fault condition.

Intermodulation Distortion

A form of non-linear distortion that introduces frequencies not present in and musically unrelated to the original signal. These are invariably based on the sum and difference products of the original frequencies.


Inches Per Second. Used to describe tape speed. Also the Institute of Professional Sound (


Interrupt Request. Part of the operating system of a computer that allows a connected device to request attention from the processor in order to transfer data to it or from it.

Isolation Room

A separate room or enclosure, often called a 'Booth' designed to provide acoustic isolation from external noise. Often used alongside a studio's main live room to record vocals or drums, for example, without spill from other instruments.

Isolator (also decoupler)

A device intended to prevent the transmission of physical vibrations over a specific frequency range, such as a rubber or foam block. The term can also be applied to audio isolation transformers, used to provide galvanic isolation between the source and destination, thus avoiding ground loops.

Isopropyl Alcohol

A type of alcohol commonly used for cleaning and de-greasing tape machine heads and guides.