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Dictionary of Audio Terminology - I


I - The symbol for current.

IACC (interaural cross-correlation coefficient) - A measure of the difference in arrival times between the ears of a listener. It is expressed in values ranging from -1 (arriving signals equal in magnitude but exactly out of phase) to 0 (arriving signals have no similarity) to +1 (identical arriving signals, i.e. same amplitude & phase).

IATSE - The International Alliance of Theatrical Stage Employees, Moving Picture Technicians, Artists and Allied Crafts of the United States, Its Territories and Canada.

IBOC (in-band on-channel) - Original name for the digital radio technology that allows simultaneous analog and digital broadcasting using existing band allocations.

IC (integrated circuit) - A solid-state device with miniaturized discrete active components on a single semiconductor material, invented by Jack Kilby.

ICIA (International Communications Industries Association) - The founder of InfoComm.

ID3 tag (Identification 3 tag) - Adds a small chunk of extra data ("tag") to the end of an MP3 file to carry information about the audio and not just the audio itself. Developed by Eric Kemp in 1996.

IDMA (International Dance Music Award) - The premier awards show for the dance and electronic music industry worldwide sponsored by the Winter Music Conference.

IEC (International Electrotechnical Commission) A European organization (headquartered in Geneva, Switzerland) involved in international standardization within the electrical and electronics fields. The U.S. National Committee for the IEC operates within ANSI.


IECEE - IEC System for Conformity Testing and Certification of Electrical Equipment.

IEEE (Institute of Electrical and Electronic Engineers) - The largest professional organization for electrical engineers. Primarily concerned with education and standardization.

IEEE-488 (also referred to as the general purpose interface bus (GPIB)) - Most common parallel format computer interface for simultaneous control of up to 15 multiple peripherals.

IEEE 754-1985 - Standard for binary floating-point arithmetic often referred to as IEEE 32-bit floating-point. A standard that specifies data format for floating-point arithmetic on binary computers. It divides a 32-bit data word into a 24-bit mantissa and an 8-bit exponent.

IEEE 802.3af or PoE (Power over Ethernet) - The name for the technology defined by IEEE 802.3afthat allows Ethernet appliances to receive power as well as data over existing LAN CAT 5 cabling.

IEEE-1394 (aka Firewire) - A joint Apple and TI implementation of the IEEE P1394 Serial Bus Standard. It is a high-speed (100/200/400 Mbits/sec now, with 1 Gbit/s on the horizon) serial bus for peripheral devices.

IEM - In-ear monitor.

IETF (Internet Engineering Task Force) - "The mission of the IETF is to make the Internet work better by producing high quality, relevant technical documents that influence the way people design, use, and manage the Internet." [from IETF website]

IEV (International Electrotechnical Vocabulary) - A valuable database, made available on-line by the IEC. It contains over 18,500 electrotechnical concepts divided into 73 subject areas (IEV parts). Each concept contains equivalent terms in English, French and German.

IFB (interrupted foldback) (aka talent cueing) - An audio sub-system allowing on-air personnel ("talent") to receive via headphones, or ear monitors, the normal program audio mixed with audio cues from the production director, or their assistants.

IFPI (International Federation of Phonographic Industry) - The organization representing the international recording industry. It comprises a membership of 1500 record producers and distributors in 76 countries.

IGBT (insulated-gate bipolar transistor) - A hybrid form of a MOSFET and bipolar transistor producing an electrically insulated gate instead of a base connection combined with a robust bipolar output. It combines MOS gate control with bipolar current control.

IGFET (insulated-gate field-effect transistor) - Formal name for MOSFET or an FET with one or more gate electrodes electrically insulated from the channel. [IEEE]

IGTP (isolated ground technical power) - Audio/Video Electrical Power Systems.

IHF (Institute of High Fidelity) - The old organization of North American hi-fi manufacturers that created voluntary industry standards for testing and specifying consumer electronics. The IHF merged with the EIA in 1979. The IHF worked closely with the IRE. Today the AES is responsible for setting audio standards for the United States.

IIR (infinite impulse-response) filter - A commonly used type of digital filter. This recursive structure accepts as inputs digitized samples of the audio signal, and then each output point is computed on the basis of a weighted sum of past output (feedback) terms, as well as past input values.

ILD (interaural level difference) - Different arrival intensities due to the diffraction or shadowing caused by the head as an obstacle.

IM or IMD (intermodulation distortion) - An audio measurement designed to quantify the distortion products produced by nonlinearities in the unit under test that cause complex waves to produce beat frequencies, i.e., sum and difference products not harmonically related to the fundamentals. For example, two frequencies, f1 and f2 produce new frequencies f3 = f1 - f2; f4 = f1 + f2; f5 = f1 - 2f1; f6 = f1 + 2f2, and so on.

Numerous tests exist, each designed to "stress" the unit under test differently. The most popular follow:

SMPTE/DIN IMD - The most common IMD measurement. SMPTE standard RP120-1994 and DIN standard 45403 are similar. Both specify a two-sine wave test signal consisting of a large amplitude low-frequency tone linearly mixed with a high-frequency tone at ¼ the amplitude of the low frequency tone. SMPTE specifies 60 Hz and 7 kHz mixed 4:1. The DIN specification allows several choices in both frequencies, with 250 Hz and 8 kHz being the most common.

ITU-R (old CCIF), Twin-Tone, or Difference-Tone IMD, or Difference Frequency Distortion (DFD) All these terms refer to the same test and are used interchangeably. The test specifies two equal-amplitude closely spaced high frequency signals. Common test tones are 19 kHz and 20 kHz for full audio bandwidth units. While all combinations of IM distortion products are possible, this test usually measures only the low-frequency second-order product falling at f2-f1, i.e., at 1 kHz. The principal standard is IEC 60268-3.

DIM/TIM (dynamic/transient intermodulation distortion) - A procedure designed to test the dynamic or transient behavior, primarily, of audio power amplifiers. The other IM tests use steady-state sine wave tones, which do not necessarily reveal problems caused by transient operation. In particular, audio power amplifiers with high amounts of negative feedback were suspect due to the inherent time delay of negative feedback loops. The speculation was that when a rapidly-changing signal was fed to such an amplifier, a finite time was required for the correction signal to travel back through the feedback loop to the input stage and that the amplifier could be distorting seriously during this time. The most popular test technique consists of a large amplitude 3 kHz square wave (band-limited to ~20 kHz). [Historical Note: This test proved that as long as the amplifier did not slew-limit for any audio signal, then the loop time delay was insignificant compared to the relatively long audio periods. Thus, properly designed negative feedback was proved not a problem. Subsequently, this test has fallen into disuse.]

IMA (International MIDI Association) - The original association that developed MIDI, now defunct and replaced by the MMA.

image impedances - The impedances that will simultaneously terminate all of a network's inputs and outputs in such a way that at each of its inputs and outputs the impedances in both directions are equal. In this manner the input and output impedances "see" their own "image." [IEEE]

imaging or stereo imaging - Usually refers to the localization of sounds in a two-channel (normally) stereo sound system, i.e., left-to-right (or vice versa) apparent performer positions.

image parameters - Fundamental network functions, namely image impedances and image transfer functions, used to design or describe a filter. [IEEE]

imaginary number - A number whose square equals minus one, or, alternatively, a number that represents the square root of minus one.

imaginary part - Any number of the form a + bj, where a and b are real numbers and j is an imaginary number whose square equals -1 [AHD]; and a represents the real part (e.g., the resistive effect of a filter, at zero phase angle) and b represents the imaginary part (e.g., the reactive effect, at 90 degrees phase angle).

impact-echo technology - A process where sound waves are sent into concrete and the rebound is measured for nondestructive evaluation of concrete and masonry.

impedance - A measure of the complex resistive and reactive attributes of a component in an alternating-current (AC) circuit. Impedance is what restricts current flow in an AC electrical circuit; impedance is not relevant to DC circuits. In DC circuits, resistors limit current flow (because of their resistance). In AC circuits, inductors and capacitors similarly limit the AC current flow, but this is now because of their inductive or capacitive reactance. Impedance is like resistance but it is more. Impedance is the sum of a circuit, or device's resistance AND reactance. Reactance is measured in ohms (like resistance and impedance) but is frequency-dependent.

impedance matching - Making the output driving impedance and the next stage input impedance equal, often requiring the insertion of a special impedance matching network.

impulse response - A theoretical impulse has an amplitude vs. time response that is infinitely high and infinitely narrow -- a spike with zero duration and infinite amplitude, but finite energy. This means the energy is spread over a very large frequency range, making impulses an ideal source for acoustic measurements. Real world use of the mathematical impulse consists of a test impulse that has a very short time duration and whose amplitude is limited to whatever will not overload the system components. The Fourier theorem tells us that this rectangular pulse is nothing more than a sum of sine and cosine functions with known amplitudes and phases, therefore the impulse response of a linear system occurs in the time domain, but also contains all of the frequency information. By capturing the system impulse response with a digital storage scope and then performing a Fast Fourier Transform (FFT) analysis, the frequency-domain response (amplitude and phase) is obtained.

inch Abbr. in or in. - (1) A unit of length in the U.S. Customary and British Imperial systems, equal to 1/12 of a foot (2.54 centimeters). (2) A unit of atmospheric pressure that is equal to the pressure exerted by a one-inch column of mercury at the earth's surface at a temperature of 0°C. [AHD]

incident sound - Sound heard directly from the source, i.e., first arriving sound without reflections.

inductance - A force that resists the sudden buildup of electric current (as opposed to capacitance which resists the sudden buildup of electric voltage). [IEEE]

inductive loop or inductive coupling - The association of two or more circuits with one another by means of inductance mutual to the circuits or the mutual inductance that associates the circuits. [IEEE]

inductive reactance - The opposition to the change of current on a element. Inductive reactance is proportional to the signal frequency and the inductance .

inductor - Circuit symbol: L. - A device consisting of one or more windings, with or without a magnetic core, for introducing inductance into an electric circuit. [IEEE]

in-ear monitor (IEM) - Devices used by musicians, audio engineers and audiophiles to listen to music or to hear a custom crafted mix of vocals and stage instrumentation for live performance or recording studio mixing. They are often custom fitted for an individual's ears and provide a high level of noise reduction from ambient surroundings.

inert - In reference to electronics, inactive; not requiring power.

infinite baffle - A separator allowing no path or acoustical crosstalk between the front and rear of a loudspeaker, i.e., it provides complete isolation between the back and front. An infinitely large flat mounting board is an example; another is a sealed box.

InfoComm International® - The leading nonprofit association serving the professional AV communications industry worldwide. Founded in 1939, the association offers industry expertise and market research serving press and others seeking information about the industry. [from Infocomm website]

infrasonic - Generating or using waves or vibrations with frequencies below that of audible sound.

infrasonic filter (aka rumble filter) - A high-pass filter used with phonograph turntables to reduce the effects of low frequency noise and vibration, called rumble, caused by imperfections in turntable performance and warped records. Often mistakenly called subsonic filter. Since typical rumble frequencies occur in the 3-10 Hz area, most infrasonic filters have a corner frequency of around 15 Hz, with a steep slope, or rolloff rate, of 18 dB/octave, and a Butterworth response.

inharmonic - Not harmonic; discordant. [AHD]

Inharmonicity - The discrepancy between the actual overtones produced by a vibrating sting and the theoretical overtones, which are whole-number multiples of the fundamental (lowest) frequency of vibration. (from Electronic Musician, October 2006, p. 56.)

inherent noise or residual noise - Noise level of a microphone when no signal is present. Microphone inherent self-noise is usually specified as the equivalent SPL level which would give the same output voltage, with typical values being 15-20 dB SPL.

initial time-delay gap (ITDG) - The difference in time between the first arrival of direct sound and the first arrival of reflected sound at the listener. The sensation of intimacy is quantitatively measured by the ITDG. First defined in 1962 by Beranek in Music, Acoustics & Architecture.

inline mixer - Console where the monitor and record path can be monitored via each channel strip. This is in contrast to a split mixing console where the monitor and record path are designated their own respective channels on the console. This is usually done where one half of the console. [WikiAudio]

in-phase - Phase in a synchronized or correlated way.

input impedance - The input impedance of a device, usually high in the 2k - 100k ohm range. Input impedance can be frequency dependent and may vary with circuit feedback, therefore the value given should state the frequency range it covers.

input referred noise or EIN (equivalent input noise) - Output noise of a system or device referred to the input. Done by modeling the object as a noise-free device with an input noise generator equal to the output noise divided by the system or device gain.

insertion loss - The loss of voltage (or power), as measured in dB, resulting from placing a pad (or other power absorbing network) between a voltage (or power) source and its load impedance. It is the ratio of the voltage (or power) absorbed in the load without the pad (or network) to that when the network is inserted.

insert loop - The preferred term for a specialized I/O point found on mixers utilizing a single 1/4" TRS jack following the convention of tip = send, ring = return, & sleeve = signal ground. Used to patch in an outboard processor using only one cable, with unbalanced wiring. A balanced insert loop requires two jacks utilizing two 1/4" TRS jacks, two XLR jacks or any number of numerous interconnections. Used to patch in an outboard processor using only two cables, with balanced wiring.

in situ - In the original position. [AHD]

intensity stereo - Another name for the X-Y microphone technique.

intermittor or current intermittor - Antiquated name for a snap-action switch.

internal voice blindness - "The near universal inability of people to articulate the tone and personality of the voice that forms their interior monologue." [A Dictionary of the Near Future by Douglas Coupland, NY Times, September 12, 2010.]

instrument-level - Nominal signal from musical instruments using electrical pick-ups. Varies widely, from very low mic-levels to quite large line-levels.

integrated circuit or IC - A solid-state device with miniaturized discrete active components on a single semiconductor material, invented by Jack Kilby.

intensity - The external measured level of a sound, i.e., the sound pressure level. Note that intensity is an objective measurement; contrast with loudness which is a subjective measurement.

interaural - Literally "between the ears," it is the comparison of sound heard by one ear verses the same sound heard by the other ear. Specific terms include interaural time difference (ITD) (different arrival times due to the distance between the ears) and interaural level difference (ILD) (different arrival intensities due to the diffraction or shadowing caused by the head as an obstacle).

interference - In reference to acoustics, anything that hinders, obstructs, or impedes sound travel, including another sound wave.

interferometer - Any of several optical, acoustic, or radio frequency instruments that use interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to determine wavelengths and wave velocities, measure very small distances and thicknesses, and calculate indices of refraction. [AHD]

Interharmonics - IEC-1000-2-1 defines it this way: "Between the harmonics of the power frequency voltage and current, further frequencies can be observed which are not an integer of the fundamental. They can appear as discrete frequencies or as a wide-band spectrum."

interlayer-transfer or print-through - The name for the magnetic tape recording phenomena where the act of layering, or winding layer upon layer of tape causes the flux from one layer to magnetize the adjacent layer, thus printing through from one layer onto another layer.

Interleaving - The process of rearranging data in time. Upon de-interleaving, errors in consecutive bits or words are distributed to a wider area to guard against consecutive errors in the storage media.

intermodulation distortion or IMD - An audio measurement designed to quantify the distortion products produced by nonlinearities in the unit under test that cause complex waves to produce beat frequencies, i.e., sum and difference products not harmonically related to the fundamentals. For example, two frequencies, f1 and f2 produce new frequencies f3 = f1 - f2; f4 = f1 + f2; f5 = f1 - 2f1; f6 = f1 + 2f2, and so on.

International Music Products Association - A professional trade organization for people working in the music business -- primarily in retailing and manufacturing of music making products.

International System of Units or SI - The International System of Units, universally abbreviated SI (from the French Le Système International d'Unités), is the modern metric system of measurement. SI is the dominant measurement system not only in science, but also in international commerce.

interonset time or interonset interval - The time between the onsets of adjacent events in an ordered event series. [Greenebaum]

Interpolate - To estimate a value of (a function or series) between two known values. [AHD]

interpolating response - Term adopted by Rane Corporation to describe the summing response of adjacent bands of variable equalizers using buffered summing stages. If two adjacent bands, when summed together, produce a smooth response without a dip in the center, they are said to interpolate between the fixed center frequencies, or combine well.

interrupted foldback or (IFB) (aka talent cueing) - An audio sub-system allowing on-air personnel ("talent") to receive via headphones, or ear monitors, the normal program audio mixed with audio cues from the production director, or their assistants.

intimacy - The sensation of intimacy is quantitatively measured by the ITDG. First defined in 1962 by Beranek in Music, Acoustics & Architecture.

IntMDCT (integer MDCT) - A lossless form of MDCT.

Intonation - The opening phrase of a plainsong composition sung as a solo part. [AHD]

inverse square law In reference to Sound Pressure Level, sound propagates in all directions to form a spherical field, thus sound energy is inversely proportional to the square of the distance, i.e., doubling the distance quarters the sound energy (the inverse square law), so SPL is attenuated 6 dB for each doubling.

inverter - A device that converts direct current into alternating current.

I/O (input/output) - Equipment, data, or connectors used to communicate from a circuit or system to other circuits or systems, or the outside world.

IP (intellectual property) - Referring to protected proprietary information, usually in the form of a patent, maskworks (integrated circuits or printed circuit boards), a copyright, a trade secret, or a trademark.

IP (internet protocol) - IP is the most important of the protocols on which the Internet is based. Originally developed by the Department of Defense to support interworking of dissimilar computers across a network, IP is a standard describing software that keeps track of the Internet work addresses for different nodes, routes outgoing messages, and recognizes incoming messages. It was first standardized in 1981. This protocol works in conjunction with TCP and is identified as TCP/IP.

IPA (International Phonetic Alphabet) - A phonetic alphabet and diacritic modifiers sponsored by the International Phonetic Association to provide a uniform and universally understood system for transcribing the speech sounds of all languages. [AHD]

IP address - Another name for an Internet address. A 32-bit identifier for a specific TCP/IP host computer on a network, written in dotted decimal form, such as 209.221.147.50, with each of the four fields assigned 255 values, organized into hierarchical classes. Whenever you click on a name address like http://www.tubeequipment.com/, this creates a path to the domain naming system (DNS) that translates the name into the IP address, which is used to connect.

ips (inches per second) - A measure of tape speed.

IR (infrared) - Standard abbreviation found A/V remote control units, among many other things.

IRE (Institute of Radio Engineering) - Now defunct, the organization merged with IEEE.

IRMA (International Recording Media Association) - An advocacy group for the growth and development of all recording media and is the industry forum for the exchange of information regarding global trends and innovations.

iron vane - Rugged design and construction, used primarily in AC voltage, current and power measurements. It features an accurate true rms measurement capability when measuring distorted or non-sinusoidal waveforms.

ISBN (International Standard Book Number) - In bibliography, a 13-digit number assigned to a book which identifies the work's national, geographic, language, or other convenient group, and its publisher, title, edition and volume number.

ISDN (Integrated Services Digital Network) A high-capacity digital telecommunication network (mainly fiber optic) based on an international telephone standard for digital transmission of audio, data and signaling -- all in addition to standard voice telephone calls. A cost-effective alternative to satellite links.

ISE (Integrated Systems Europe) European trade show for professional AV and electronic systems integration.

iSMO (Independent Music Store Owners) or MSO (Music Store Owners) - This organization exists to serve as a link between dealers, manufacturers and the public.

ISO (International Standards Organization or International Organization for Standardization) - Founded in 1947 and consisting of members from over 90 countries, the ISO promotes the development of international standards and related activities to facilitate the exchange of goods and services worldwide.

isochronous (pronounced "i-sok-ronus") ("iso" equal + "chronous" time) - A term meaning time sensitive; isochronous transmission is time sensitive transmission.

isolation - The isolation of sound is the process by which sound energy is contained or blocked as opposed to being converted into heat.

Isotropic - Identical in all directions; invariant with respect to direction. [AHD]

ISRC (International Standard Recording Code) - The international identification system for sound recordings and music video recordings.

ISWC (International Standard Musical Work Code) - A unique, permanent and internationally recognized reference number for the identification of musical works per International Standard ISO 15707.

IT (information technology) - Defined by the Information Technology Association of America (ITAA) as "the study, design, development, implementation, support or management of computer-based information systems, particularly software applications and computer hardware."

ITD (interaural time difference) - Different arrival times due to the distance between the ears.

ITDG (initial time-delay gap) - The difference in time between the first arrival of direct sound and the first arrival of reflected sound at the listener. The sensation of intimacy is quantitatively measured by the ITDG. First defined in 1962 by Beranek in Music, Acoustics & Architecture.

ITF (International Turntablist Federation) - Founded in 1996, an organization dedicated to the spread of turntablism.

ITU (International Telecommunications Union) - Headquartered in Geneva, Switzerland, ITU is an international organization within which governments and the private sector coordinate global telecommunication networks and services. The ITU is divided into three sectors: radiocommunications (ITU-R), telecommunications development (ITU-D), and telecommunications standards (ITU-T).

ITVA (International Television Association) A global community of professionals devoted to the business and art of visual communication. Now renamed Media Communications Association - International (MCA-I).

iyailu - The "mother drum" of the talking drum ensemble.





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HistoryOfRecording.com acknowledges the Elsevier, Inc. publication, Audio Engineering know it all, the University of Washington Press publication, The Audio Dictionary, second edition, the Howard W. Sames & Co., Inc. publication, Audio cyclopedia, the Cambridge University Press publication, The Art of Electronics, Rane Corporation (Dennis A. Bohn, CTO), Houghton Mifflin Company publication, The American Heritage Dictionary of the English Language, Fourth Edition, the IEEE publication, IEEE 100: The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition and Wikipedia in the preparation of this Dictionary of Audio Terminology.

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