Dynamic Range
The amplitude range, usually expressed in decibels (dB), between the loudest signal that can be handled by a piece of equipment and the level at which small signals disappear into the noise floor. (See AES17)
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Equaliser
A device which allows the user to adjust the tonality of a sound source by boosting or attenuating specific ranges of frequencies. Equalisers are available in the form of shelf equalisers, parametric equalisers and graphic equalisers — or as a combination of these basic forms. (cf Filter)
Expander
A hardware or software plug-in device designed to increase the dynamic range, typically by reducing the volume of low level signals (below a set threshold), or to increase the volume of high level signals (above a threshold). (See Compressor.)
Foldback
A system for making one or more separate mixes audible to musicians while performing, recording and overdubbing. Also known as a Cue mix. May be auditioned via headphones, IEMs or wedge monitors (wedge-shaped loudspeakers placed on the floor).
Hertz (Hz)
The standard measurement of frequency. 10Hz means ten complete cycles of a repeating waveform per second. (Previously defined as 'cycles per second or CPS).
High-Pass Filter (HPF)
A filter which passes frequencies above its cut-off frequency, but attenuates lower frequencies. Also known as a 'low-cut' filter.
Impedance
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.
Loudness-Normalisation
The practice of matching the perceived loudness of different material to a given target loudness value. To accommodate varying peak levels, the medium requires an approporiate headroom margin. Loudness-normalisation is now the default form for HDTV broadcasts, as well as most audio streaming services, although the target loudness level currently varies between different platforms. Loudness-normalisation is measured using the LUFS or LKFS scale. (See LUFS, Peak-Normalisation, Mastering, Loudness Wars).
MADI
Multichannel Audio Digital Interface. Originally specified by the Audio Engineering Society (AES) as AES10 in 1991. This unidirectional digital audio interface shares the same core 24-bit audio and status data format as AES3, but with different 'wrapping' to contain 56 or 64 synchronous channels at base sample rates, or 28/32 channels at 96kHz. It can be conveyed over unbalanced coaxial cables, or via optical fibres.
MIDI Controller
A term used to describe the physical interface by means of which the musician plays the MIDI synthesizer or other sound generator. Examples of controllers are keyboards, drum pads, wind synths and so on.
MIDI In
The socket used to receive information from a master controller or from the MIDI Thru socket of a slave unit.
Mixer
A device used to combine multiple audio signals together, usually under the control of an operator using faders to balance levels. Most mixers also incorporate facilities for equalisation, signal routing to multiple outputs, and monitoring facilities. Large mixers are also known as ‘desks’ or ‘consoles’.
Parametric EQ
An equaliser with a bell-shaped frequency response curve, and separate controls for the centre frequency, the bandwidth (Q), and the amount of cut/boost. If a bell EQ omits the bandwidth (Q) control, it is known as semei- or quasi-paramteric.
Paraphonic
Often used to descibe a multi-oscillator mono-synth which can be configured to allow the oscillators to be controlled independently from the keyboard, allowing two or more notes to be played simultaneously.
PFL
Pre-Fade Listen. A system used within a mixing console to allow the operator to audition a selected channel signal, prior to the fader, and thus independent of the fader setting. Normally used to check the level of the signal through the channel to optimise the gain structure, or to confirm the presence of a signal before fading it up. (See AFL and Solo)
Port
A connection for the input or output of data or other signals. In the context of loudspeakers it refers to the vent in a bass reflex cabinet.