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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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Poly Mode

The most common MIDI mode that allows an instrument to respond to multiple simultaneous notes transmitted on a single MIDI channel.


In the context of musical instruments, the term polyphonic refers to the number of notes that an instrument can play simultaneously. Most poly-synths can play up to 6 or 8 notes at a time, some 16 or 32, and others many more depending on the technology involved.

Polyphonic Aftertouch

A means of generating multiple control signals in a synthesizer based on how much pressure is applied to individual keys of a MIDI keyboard. Only a few instruments curently generate polyphonic aftertouch, although more can respond to it. Aftertouch is typically used to control such functions as vibrato depth, filter brightness, loudness etc. See Aftertouch


The ability of an instrument to play two or more notes simultaneously. An instrument which can only play one note at a time is described as monophonic.


A synthesizer that can play/sound more than one note at a time (eg. eight or 16 notes), each with an independent signal chain of oscillators, filters, and envelope generators.

Pop Shield

A device placed between a sound source and a microphone to trap wind blasts - such as those created by a vocalist’s plosives (Bs, Ps and so on) - which would otherwise cause loud popping noises as the microphone diaphragm is over- driven. Most are constructed from multiple layers of a fine wire or nylon mesh, although more modern designs tend to use open-cell foam.


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.


A gliding effect that allows a sound to change pitch at a gradual rate, rather than abruptly, when a new key is pressed or MIDI note sent.


A signal derived from the channel path of a mixer after the channel fader. A post-fade aux send level follows any channel fader changes. Normally used for feeding effects devices.


Work done to a recording.

Potentiometer (Pot)

A form of electrical potential divider in which the ratio of the upper and lower resistances can be changed either with a rotary control or slider (eg. a fader).

Power Amplifier

A device which accepts a standard line-level input signal and amplifies it to a condition in which it can drive a loudspeaker drive unit. The strength of amplification is denoted in terms of Watts of power.

Power supply

A unit designed to convert mains electricity to the DC voltages necessary to power an electronic circuit or device.

Powered Loudspeaker or Monitor

A powered speaker is a conventional passive loudspeaker but with a single power amplifier built in or integrated with the cabinet in some way. The amplifier drives a passive crossover, the outputs of which connect to the appropriate drive units.


Peak Programme Meter. A meter designed to register the approximate peak amplitude of a signal, rather than the average level indicated by, for example, a VU meter. However, PPMs have a defined integration time (typically 10ms) which means that they actually under-read on the fastest transient peaks. (cf. VU Meter)


Pulsed Per Quarter Note. Used in the context of MIDI Clock derived sync signals.

PQ Coding

Process for adding Pause, Cue and other subcode information to a digital master tape in preparation for CD manufacture.

Pre-amp (Preamp)

Short for ‘pre-amplification’ : an active gain stage used to raise the signal level of a source to a nominal line level. For example, a microphone pre-amp (aka mic preamp).


A system for applying high frequency boost to a sound before processing. When the corresponding de-emphasis is applied any noise contribution from the processing is reduced.


A signal derived from the channel path of a mixer before the channel fader. A pret-fade aux send level is unaffected by channel fader changes. Normally used for creating Foldback or Cue mixes.


Pre-ringing refers to an inherent character of the impulse response of linear-phase filters employed in D-A or A-D converters. In a conventional analogue or minimum phase filter, an input impulse signal will generate an output impulse response with a strong initial spike followed by a string of ripples of decrasing magnitude. In a linear-phase filter the impulse spike is preceded by a build-up of ripples in advance of the impulse arriving, and a symmetrical string of decaying ripples afterwards.


An effects unit or synth patch that cannot be altered by the user.


An alternative term for Aftertouch.


The undesirable process that causes some magnetic information from a recorded analogue tape to become imprinted onto an adjacent layer. This can produce low level pre or post echoes.


A device designed to treat an audio signal by changing its dynamics or frequency content. Examples of processors include compressors, gates and equalisers.

Program Change

A MIDI message designed to change instrument or effects unit patches.

Project Studio

A relatively small recording studio facility, often with a combined recording space and control room.

Proximity Effect

Also known as ‘Bass tip-up’. The proximity effect dramatically increases a microphone’s sensitivity to low frequencies when placed very close to a sound source. It only affects directional microphones — omnidirectional microphones are immune, and the effect can be nullified on a cardioid mic if the close source is placed 90 degrees off-axis.

Public Address System

A system originally intended to amplify a human voice using microphones and loudspeakers in order to pass important information to the public over a large area, such as in transport hubs or sports stadia. The term now also applies to boosting the volume of instruments and vocals at musical concerts.  

Pulse Wave

Similar to a square wave but non-symmetrical. Pulse waves sound brighter and thinner than square waves, making them useful in the synthesis of reed instruments. The timbre changes according to the mark/space ratio of the waveform.

Pulse-width Modulation

A means of modulating the duty cycle (mark/space ratio) of a pulse wave. This changes the timbre of the basic tone; LFO modulation of pulse width can be used to produce a pseudo-chorus effect.


The action of placing an already recorded track into record at the correct time during playback, so that the existing material may be extended or replaced.


The action of switching a tape machine (or other recording device), out of record after executing a punch-in. With most multitrack machines, both punching in and punching out can be accomplished without stopping the tape.

PWM Compression

A form of audio compressor which uses Pulse Width Modulation to detedmine the energy in the audio signal over time. In essence, the audio signal is sampled at a very high rate, but the duration (width) of the samples (pulses) is adjusted (modulated) to control the average energy over time, and thus provide signal attenuation. When done well, this is the fastest form of compressor with the lowest distortion artefacts.