IAC (a.k.a. Interapplication Communication). A Macintosh driver that allows different programs running on the same computer to synchronize to one another and transfer data back and forth.
IACC. 
See interaural cross correlation.
IC. Integrated Circuit, a.k.a. microchip or chip. A semiconductor electronic component that contains thousands of miniaturized components, such as transistors, capacitors, and resistors. A single IC can replace a large circuit board full of discrete components, resulting in the ability to design much more efficient, compact circuits containing the functionality of thousands of components.
icon. A graphic or symbol in a GUI (graphical user interface) that represents a file, application, storage device, peripheral device, or other item.
IDE. Integrated Drive Electronics. See ATA.
IEC. International Electrotechnical Commission. An international organization founded in 1906 that develops standards for electrical and electronic safety and performance. www.iec.ch.
IEC 958. The standard developed by the IEC that contains the specifications for AES/EBU (IEC 958 part 4) and S/PDIF (IEC 958 part 3) digital transfer protocols.
IEEE. Institute of Electrical and Electronics Engineers. A trade organization founded in 1884 that develops standards for electronics. www.ieee.org.
IEM. In-Ear Monitor. An earpiece designed to be worn by a musician when recording in a studio or performing onstage. The in-ear monitor is designed to replace headphones and other types of monitors by providing a monitor feed while also isolating the musician from noise in the room.
IID. See interaural intensity difference.
IIR filter. Infinite Impulse Response filter. IIR filters emulate the way capacitors and other analog components function. IIR filters are most often used in digital equalizers (primarily plug-ins) that are designed to function in similar fashion to analog equalizers. See also FIR filter.
iLok. A dongle—a hardware copy protection device—developed by PACE that stores authorizations that allow software programs to run. An iLok connects to a computer’s USB port and can store authorizations for more than 100 programs, which can be loaded, moved to another iLok, or deleted.
image. A file that contains an exact replica of the data on a floppy or hard disk or a CD or DVD along with the structure of the data on the disk. Image files are often used by CD and DVD burning programs to hold all the data that will be burned to the disc. Image files are also used for backup, archiving, and disk cloning purposes, as well as for data recovery when there is a catastrophic problem with a disk.
image copy. A type of copy that is made without regard for the content being copied. The device or program doesn’t even access the data in the file being copied; it simply makes a bit-for-bit copy. Making this type of copy is faster and does not require that the software be compatible with the data file.
image file. See image.
imaging. The ability of a speaker system to provide sufficient directional cues to allow a listener to pinpoint the position of a sound in a stereo or surround mix. Imaging depends on the speakers being precisely matched in level, response, and phase, as well as the acoustic environment of the listening room.
IMD. See intermodulation distortion.
impedance. The resistance of a circuit to alternating current. Impedance is the combination of resistance and reactance (an effect of inductance or capacitance). It is important to match the output impedance of a device to the correct input impedance of the device it is feeding in order to optimize performance or to prevent damage in some cases (such as with speakers and amplifiers). Most modern devices have very high input impedance so they can be driven by the low-impedance outputs of other devices.
import. A software function that allows a program to load, translate, and use data and files created in another program.
impulse. A “spike” of sound of very short duration used to perform acoustical measurements.
impulse response. Literally, how a device or space responds to an impulse. By capturing or recording the response of a room or device to an impulse signal, FFT (Fast Fourier transform) analysis can be performed to obtain frequency-, phase-, transient-, and time-related response information that can be used to manipulate other data in a process called convolution. See Figure I.1.
Figure I.1. A convolution reverb such as Trillium Lane Labs, TL Space can use an impulse response created from the sound of a real room to add reverb to a track or signal.
in the box. A work flow or production in which all processing and mixing takes place inside a DAW, without using external hardware processors, recorders, or mixers.
inductor (a.k.a. coil). An electronic component consisting of a coil of wire wrapped around a core. Inductors pass direct current but resist the flow of alternating current—higher frequencies are resisted more than lower frequencies.
infant failure. The proven tendency of electronic equipment to fail very early in its life—within the first day or two of use. Some manufacturers attempt to combat this tendency with an extended “burn in” period before their products are shipped to retailers or end users.
infrasonic. Frequencies below the range of human hearing.
initialize. To restore a device to its default state. Initializing often erases any user data or settings.
inline mixer. A mixer in which each channel has two paths—one for the normal incoming signals from microphones and other sources, and the other for returning signals from a multitrack tape recorder. See also split mixer.
input. A connection or jack used to send signal into a device.
input monitoring. A function on some software and hardware recorders that routes the signal coming in the input connections directly to the outputs so it can be monitored.
input sensitivity. See sensitivity.
input transformer. A transformer used to match the input of a device to the device that is feeding it. See also transformer.
insert. See insert point.
insert point (a.k.a. patch point). A jack or jacks on a mixer that break into the signal path for a channel, allowing a processor, such as a compressor, limiter, or equalizer, to be inserted. Inserts can also often be found on the master outputs of a mixer. See Figure I.2.
Figure I.2. An insert point can be used to interrupt a mixer channel’s signal path so that an external processor can be applied to the signal.
insert slot. A point in a virtual mixer in a DAW where a plug-in can be loaded. See Figure I.3.
Figure I.3. DAWs feature insert slots—analogous to insert points in hardware mixers—that can accept plug-in processors.
installer. A piece of software that automatically installs another piece of software. Installers are used to ensure that all the various drivers and other parts associated with an application are installed to the proper locations in the system.
instance. Loading a plug-in into a track in a host DAW creates a single instance of that plug-in. An advantage of software plug-ins over hardware processors or instruments is that multiple instances of the same plug-in can be loaded and active at once, versus a hardware processor, which can only be used on one track at a time.
instantiate. To open or load a plug-in within a host program.
instrument input. A specialized input on a preamp or audio interface intended to accept signals from electric guitars or basses. An instrument input will be set up to accept instrument levels and may have an impedance optimized for instrument pickups.
instrument level. A signal level of around –20 dBu as generated by an instrument, such as an electric guitar or bass. In most cases, a direct box, or a preamp with an instrument-level input, is required to raise the instrument’s signal to a level other devices can deal with.
instrument track. A specialized track in a DAW that combines a MIDI track for controlling a virtual instrument with an audio track that can accept the output signal from the instrument. Before the development of instrument tracks, DAWs used two tracks for each instrument—one for MIDI, and one for the instrument’s audio. Instrument tracks provide a more convenient way to deal with virtual instruments.
insulator. A material that does not conduct electricity. Insulators are used to keep signal paths from contacting one another and signals from interacting, and as protection from potentially dangerous voltages.
integrated circuit. See IC.
intelligibility. The ability to discern aural details in a given space.
interapplication communication. See IAC.
interaural cross correlation (a.k.a. IACC). The measure of the similarity of a signal received by the two ears of the listener. Interaural cross correlation indicates how much “envelopment” a listener will feel when listening to a sound.
interaural intensity difference (a.k.a. IID). The difference in the volume in a sound as it reaches the right ear versus the left ear or vice versa, created by “head shadow,” or the interference of the listener’s head in the sound’s path. Interaural intensity difference helps the listener identify the direction of a sound source.
interaural phase difference (a.k.a. IPD). The difference in phase in a waveform as it reaches the right ear versus the left ear or vice versa. Interaural phase difference is affected by the signal’s frequency and the interaural time difference. Interaural phase difference helps the listener identify the direction of a sound source and the sound’s frequency.
interaural time difference (a.k.a. ITD). The difference in arrival time between a listener’s right ear and left ear or vice versa. Though this time is extremely short, it is enough for the human ear to identify the direction of a sound source.
interface. 1. A device that allows other devices to communicate. In the audio/music world, there are two main types: MIDI interfaces, which allow MIDI-compatible devices to communicate with a computer (direct USB connections have made MIDI interfaces obsolete for many users), and audio interfaces, which allow a computer to record and play back high-quality multichannel audio. A variety of other types of interfaces are in use as well: FireWire connects storage devices and other hardware devices with computers, GUIs (graphical user interfaces) allow a computer to communicate with users, and so on. 2. To connect or communicate.
interleaved stereo file. An audio file in which two channels of data are combined together for storage as one unit. (The two audio channels remain discrete; only the data is interleaved.)
interleaving. A system of data storage designed to reduce or eliminate errors and to increase read speeds. Interleaving works by breaking up a piece of data and distributing it among other pieces of data so that it is spread across the storage media. Read speed is increased because the data is dispersed in a manner that is efficient for the computer and drive to handle as the disk rotates—the drive spends less time waiting for the correct sector to rotate under the read head.
intermodulation distortion (a.k.a. IMD). New sum and difference frequencies that result from the interaction of two or more other frequencies in a signal.
interpolate. To create a value between other values by estimating, calculating, or averaging.
interrupt. See IRQ.
intersymbol interference (a.k.a. ISI). Smearing of a digital clock pulse that may result in increased jitter. See also jitter.
inverse reverb. A reverb with an envelope that produces a slow buildup of reverberation with a quick cutoff.
Inverse Square Law. A physical law that says intensity is inversely proportional to the square of distance. In acoustic terms, this results in a 6-dB drop every time you double the distance from the source; 10 times the distance reduces loudness by 20 dB. Note that as with all decibel-related phenomena, this is all relative: Doubling a one-inch distance to two inches produces the same 6-dB drop that doubling a one-foot distance to two feet does.
I/O. Short for input/output. An abbreviation typically used to refer to the input and output connections on a piece of gear.
IPD. See interaural phase difference.
IPS. Inches Per Second. A measure of tape speed.
IR. See impulse response.
IRQ. Interrupt Request. A signal sent from a piece of software or a hardware peripheral in a PC that tells the operating system to stop or suspend an operation in order to start another operation. Interrupts are given priorities that help the operating system decide how and when to proceed.
ISA. 1. Industry Standard Architecture. An older type of computer expansion slot and bus. 2. Any of a variety of abbreviations and organizations, such as Internet Security and Acceleration, International Society of Arboriculture, International Society of America, International Student Association, Informix Server Administrator, International Summer Award program, and countless others.
ISO. International Organization for Standardization or Organisation internationale de normalisation. A non-governmental federation of standards agencies from 130 countries, founded in 1947. www.iso.org.
ISO 9660. A standard developed in 1988 by the International Organization for Standardization based on the older High Sierra file system (HSF). ISO 9660 specifies a file system for CD-ROMs. Various levels of the standard support different file name lengths. Level 1 allows for eight-character file names. Level 2 allows for 31-character file names.
isochronus. A transmission type in which the data must be delivered within certain time limits so that synchronization can be maintained.
isolation. Keeping sound from entering or escaping from a space.
isolation booth. A small room designed to contain, or isolate, the sound of a source so it can be recorded without bleed to or from other sound sources.
isolation cabinet. An enclosure intended to contain the noise from a computer, hard drives, and other equipment. Isolation cabinets are usually equipped with low-speed fans to help keep the equipment inside cool.
isolation transformer. A transformer that is used to electrically isolate one circuit from another circuit to prevent ground loops and other problems. See also transformer.
ISRC. International Standard Recording Code. A code defined by ISO (International Organization for Standardization) for identifying a sound recording or music video recording—an ISRC is issued by the RIAA per track, not for the media or collection the track is part of. The ISRC identifies the country of origin, the identity of the person or company registering the recording, the year of recording, and the serial number of the recording. Changes in ownership of the rights to a recording do not change the ISRC. ISRC codes can be embedded in Red Book CDs or encoded into MP3 and other file types. One way ISRC codes are used is for tracking songs on download services such as iTunes.
ITD. 1. Initial Time Delay. The time between the arrival of a direct sound and its first reflection. 2. See interaural time difference.
ITU. International Telecommunication Union. A United Nations agency charged with standardizing and developing telecommunications.www.itu.int.
ITU 775. International Telecommunication Union, Operational Bulletin No. 775. A document containing recommendations for positioning the speakers for a 5.1 surround sound system.