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Glossary of PA Terms - P

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The glossary pages provide definitions for over 1900 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

P/C * P48 * Pa * PA * PA amplifier * Pad * Pad switch * PAG * Pair * PAL * Pan * PAR can * Parallel * Parallel-series * Parallel effects unit * Parameter * Parametric equaliser * Parity * Pascal * Passband * Passive * Passive crossover * Passive DI box * Passive equaliser * Passive speaker * Passive pick-up * PAT * Patch * Patch bay * Patch cable * Pattern of response * pbar * P/C * PCB * PCM * Peak * Peak programme meter * Peaking response * Peaky * Pedal * Pedal board * PEQ * Perceived level * Perceived volume * Perceptual coding * Period * Peripheral * Peritel * Personal mixing * PF (p.f.) * pF * PFL * Phantom power * Phase * Phase conductor * Phase linear * Phase response * Phase reversal * Phase shift * Phase switch * Phat * Phon * Phone jack * Phono * Phono equalisation * Pick-up * Pick-up angle * Pick-up pattern * Pico... * Piezo * Pilot signal * Pilot tone * Pin * Pin 1 problem * Pin jack * Pink noise * Pitch * Pixel * Pk * Planar * PLASA * Plate * Plate microphone * Plexiglas * Plosive * Plot * Plug * Plug-in * PM * PMA * PME * PMP * PMPO * PMSE * Pneumatic stand * Polar pattern * Polar response * Polarity * Polarity reversal * Polarisation * Polarised * Pole * Polyphonic * Pop filter * Pop shield * Popping * Port * Portamento * Positive feedback * Post Office jack * Post-EQ * Post-fade * Pot * Potential * Potential acoustic gain * Potential difference * Potentiometer * Pouch * Power * Power amp * Power amplifier * Power Breaker * Power compression * Power conditioner * Power factor * Power filter * Power-off procedure * Power-on procedure * Power rails * Power supply * PowerCon * Powered board * Powered desk * Powered mixer * Powered monitor * Powered multicore * Powered speaker * Powerlock * PPM * Pre * Pre-amp * Pre-amplifier * Pre-delay * Pre-EQ * Pre-fade * Precedence effect * Presence * Presence peak * Pressure gradient microphone * Pressure microphone * Pressure zone microphone * Primary winding * Principle of superposition * Print-through * Printed circuit board * Processor * Processor-controlled * Processing * Production * Programme * Programme level * Programme power * Progressive scan * Propagation time * Proprietary * Prosumer * Protective bonding * Protocol * Proximity effect * Pseudo-balanced * PSU * Psychoacoustics * Psychology of hearing * PTT * Public address * Pulse code modulation * Pumping * Punter * Pure tone * PVC tape * PVR * Pyro * PZM

The definitions for these terms are given on the assumption of their use in the context of PA systems; many of the terms have more general meanings when used in a wider context. Where more than one definition is given for a term, the definitions are numbered (1), (2) etc.

Some of the definitions themselves use terms (such as "signal") in a specific way − most of these are links (just the first time they are used, in each definition), so just click on them to see the meanings that are intended.

P/C
An abbreviation for 'per channel', a term used in the specification of power amplifiers to indicate that the specified output power value is available from each individual channel of the amplifier (usually simultaneously), rather than being a total output power value.

Pa
An abbreviation for Pascal.

PA
Originally an abbreviation for 'public address'. However, in general usage the term 'PA' now refers to any system whose primary purpose is to make sound louder or to distribute sound over a wider area than would be possible without such a system (as distinguished from systems that are primarily intended for recording or for live broadcast purposes). This includes systems for the high-quality amplification of live music for a 'live audience'; these systems are more properly referred to as sound reinforcement systems, however the term 'PA' is very widely used for them simply because it is short and much more convenient to say, so we use 'PA' throughout this website.

Note that PA is also an abbreviation for many other unrelated terms, such as 'personal assistant' (to a manager, director, or other official) and 'personal appearance' (of a celebrity).

PA amplifier
See Power amplifier.

Pad
See Attenuator.

Pad switch
A switch whose operation introduces a defined amount of attenuation into the signal path. Such switches are most often found on mixer channels and condenser microphones, for the purpose of reducing the signal level in order to avoid overload of the channel input or overload of the microphone's internal pre-amplifier, respectively.

PAG
An abbreviation for 'Potential acoustic gain'.

Pair
The two signal-carrying conductors of a balanced circuit (or balanced line), sometimes called legs. The term 'pair' usually refers to the two physical wires that carry one such circuit within an audio multicore cable, in which case the two wires that make up each pair are twisted together over the length of the cable and are surrounded by a screen. The two legs of the pair are commonly referred to as the 'hot' and 'cold' legs, or as the '+' and '−' legs. Note that in a star quad cable two wires are used to make up each leg of the pair, making four wires in total. See also One-legged.

PAL
An abbreviation for 'phase alternating line'. The method used in the UK for coding the chrominance information prior to creating a colour composite video signal. Compare NTSC.

Pan
A facility of a mixer, allowing the sound being handled by an individual channel or group to be positioned at the desired location between the Left and Right speakers of a stereo system. It operates by controlling the proportion of the signal that is fed to the Left and Right mixes. For example, if the control is set to the left of its central position then more signal is fed to the Left speakers than to the Right, causing the sound to appear to be left-of-centre in the stereo image.

Note that there are several different styles of pan control − some types provide a reduction in the level of both the Left and Right signals when the control is in the central position, to compensate for the increased sound level that would otherwise result (for a centrally located listener) from both speakers reproducing the same sound. The term is short for 'panoramic'. A pan control is often fitted with a central detent.

PAR can
A lantern consisting only of a lamp and its lamp-holder in a metal can. The reflector and the lens are integral with the lamp. (PAR is an abbreviation for ‘parabolic aluminised reflector’.)

The following abbreviations are sometimes used to identify the beam-width of PAR lamps:

• VNSP − very narrow spot
• NSP − narrow spot
• SP − spot
• MFL − medium flood
• WFL − wide flood
• EWF − extra-wide flood

Parallel (1)
Describes a digital interface or communications link in which multiple electrical connections are required to carry the data, so that, at any one point in time, more than one bit of information is being conveyed. Note, however, that these multiple connections are usually made through a single cable, containing many conductors. Compare Serial.

Parallel (2)
Describes the interconnection of two or more items in such a way that the total circuit current splits into several paths, and a part of it flows through each of the items before recombining. When one item is connected in parallel with another, it is said to act as a shunt.

When two or more connectors on an item of equipment are said to be connected in parallel, this means that the corresponding terminals of each connector are wired together (inside the equipment), e.g. for a 3-pole XLR: pin l to pin l, pin 2 to pin 2 and pin 3 to pin 3.

To calculate the overall value of unequal resistances connected in parallel it is necessary to sum the reciprocals of the individual resistances and then take the reciprocal of that sum. The same rule applies to parallel values of reactance (taking into account their sign), but not to a mixture of parallel resistances and reactances. It also applies to parallel values of impedance, provided they all contain the same proportion of resistance and reactance. When a number of equal values of resistance, of reactance, or of impedance are connected in parallel, the overall value is simply the value of one of them divided by the number of them.

Values of capacitance connected in parallel may simply be added together to calculate their overall value, but for values of inductance connected in parallel it is necessary to sum the reciprocals of the individual inductance values and then take the reciprocal of that sum. See also Series-parallel. Compare Series.

Parallel-series
See Series-parallel.

Parallel effects unit
An effects unit whose output consists only of the processed signal; none of the original signal is mixed in by the unit. So, to achieve a balance of original and processed sounds, it is necessary for the output of the unit to be separately mixed with the original signal. Therefore, these units are ideal for use with the effect send and return connections of a mixer. They are not generally suitable for simply inserting into the signal path, or for connecting in a 'chain' with other effects units. Compare Serial effects unit.

Parameter
A particular characteristic, or aspect, of something − usually one that is controllable or adjustable in some way in order to change a detail of the thing's character or behaviour, without altering its fundamental nature or purpose. For example, the parameters of a delay (echo) effects unit would typically be delay time, number of repeats, and rate of decay. The main parameters of a speaker would be its power rating(s), frequency response, impedance, directivity, size and weight. For a further example see the next definition.

Parametric equaliser
A type of equaliser with a peaking response, most often used to cut or boost mid-range frequencies, in which several parameters of the filter are adjustable. One control determines the amount of cut or boost, another control determines the centre frequency of the range (or "band") of frequencies that is cut or boosted, and a third control (called the Q control) determines the width of the range of frequencies that is cut or boosted. In the case of some digital equalisers, it is also possible to change the shape of the response curve between a peaking response and a shelving response.

Confusion can arise over terms such as 'fully-parametric', 'semi-parametric' and 'quasi-parametric', as different people use these terms to mean different things. In particular, when it is the whole equalisation facility (e.g. of a mixer channel) that is being considered, the term 'fully-parametric' might be reserved for equalisers in which all frequency sections (HF and LF as well as Mids) are equipped with a Q control. Alternatively that term may be used when just the mid-range section(s) are so equipped, if the person is considering only the mid-range part of the equaliser. In that case, the term 'semi-parametric' would only be used if not even the mid-range section(s) were provided with Q control(s). See also Graphic equaliser. Compare Sweep EQ.

Parity
In a digital signal or stored digital information, additional bits that are incorporated to enable the receiving or reading equipment to detect the presence of data errors. Usually, one parity bit is added for each group of data bits (often for each word), and is arranged to make the total number of '1' bits in the group either always even ("even parity") or always odd ("odd parity"). This system is only able to detect an odd number of bit-errors occurring within the group. See also Bit error rate, Checksum and Cyclic redundancy check.

Pascal
A unit of pressure, equal to a force of one Newton per square metre. Abbreviation Pa. Used in the measurement of sound pressure level (SPL). As the Pascal is quite a large unit of SPL, equivalent to 94 dB SPL, units of µPa (one millionth of a Pascal) are frequently used. In terms of other units of pressure, the Pascal is equivalent to 10 dynes per square cm and to 10 µbar.

A sound pressure level of 20 µPa RMS, the nominal threshold of hearing, is used as the reference value for expressing sound levels in decibels. Normal atmospheric pressure at ground level is variable around a value of approximately 100,000 Pa (100 kPa, or 1 bar). For further information see Acoustic Sound Levels on the Decibels page.

Passband
The range of frequencies that is allowed to pass through a filter relatively unaffected in level. Compare Stopband.

Passive
Describes something that operates without the need for any source of electrical power. An example would be a passive DI box. Compare Active.

Passive crossover
See Crossover.

Passive DI box
See DI box.

Passive equaliser
An equaliser that consists entirely of inductors, capacitors and resistors, and which therefore requires no electrical power source.

Passive speaker
A speaker that contains no power amplifier(s) within the speaker enclosure. Therefore the use of external power amplifier(s) is required, which may consist of separate unit(s) or be incorporated within a powered mixer or mixer-amplifier. May also be called an 'unpowered speaker'. Compare Powered speaker.

Passive pick-up
See Pick-up.

PAT
An abbreviation for ‘portable appliance test’ − a rather imprecise term for a formal inspection and test procedure to verify the safety of in-service electrical equipment. Despite the name, it also applies to items that are not portable (i.e. fixed and stationary items). In particular, the procedure includes visual verification of the mechanical integrity of the equipment's enclosure and any associated supply cables and connectors, and (in the case of Class I equipment) verification of the electrical integrity of its safety earth conductor(s) and connections.

As a PAT test only checks the safety of the equipment on the date of the test, it is essential (and, in the UK, a legal requirement) that equipment is at all times properly used and maintained so as to prevent danger arising. Therefore, in addition to the formal PAT tests, equipment should be informally inspected before each use; suspect equipment should never be used.

The recommended interval between PAT tests is dependent upon the type of equipment and its operating conditions, or may be specified by regulatory authorities. For the UK, guidance on initial test intervals and other matters is given in the IET's publication: 'Code of Practice for In-service Inspection and Testing of Electrical Equipment'.

The inspections and tests do not include the permanent electrical installation of the building, which is subject to other requirements for inspection and testing. (See BS 7671.) For further information on safety see the Safety page. See also Class I, Class II, Direct contact and Indirect contact.

Patch (1)
To connect together two or more items of equipment, usually items located close together. Or, to make connections that incorporate additional item(s) of equipment into a system.

Patch (2)
In electronically generated sounds (e.g. electronic keyboards), a combination of settings that gives rise to a particular type of sound. Patches are usually stored in banks.

Patch bay
An item of equipment whose purpose is to provide a convenient and flexible method of arranging signal paths so as to make the desired interconnections between several items of equipment. They are most commonly used in large systems and in recording and broadcast studios. Each panel of an audio patch bay usually consists of two rows of jack sockets (often B-type ones), the upper row being fed from signal sources and the lower row feeding signal destinations. Examples of use would be to provide flexibility in the interconnection of a multicore to a mixer, or in the assignment of effects units to mixer channels.

When no jacks are inserted, usually each upper-row jack is automatically routed to the lower-row jack beneath it, but by use of patch cables this routing can be rearranged as required. There are several variations on what the effect of inserting a jack in the upper or the lower row will be − see normalling for details. A less common design of patch bay additionally has an intermediate 'monitor' row, into which jacks can be inserted without any effect on the signal routing.

However, the term 'patch bay' is also sometimes used to refer to the complete set of connectors (of any type) on a large item of equipment such as mixer.

Patch cable
A short cable for use with a patch bay, or for connecting adjacent items of equipment. Also referred to as a patch cord (US term).

Pattern of response
See Polar response.

pbar
See Bar.

PCB
An abbreviation for 'printed circuit board'. The flat surface, internal to an item of equipment, onto which the electronic components that make up the circuit are mounted.

Interconnections between the components are made by printed copper 'tracks' on the board, to which the components are connected by soldering. Single-sided PCBs have tracks on one surface only, double-sided PCBs have tracks on both surfaces, and multi-layer PCBs have internal layers of tracks, as well as tracks on the two surfaces. Components with leads are mounted on the 'top' surface of the board, the leads beings threaded through holes to secure the component and to connect with the tracks on the underside. Surface-mounted components have no leads, and may be mounted on any tracked surface of the board.

PCM
An abbreviation for 'pulse code modulation'.

Peak
In PA work, the highest level reached (or expected to be reached) by a sound or by a signal, or of which an item of equipment is capable of accepting or providing whilst continuing to work normally. The peak level of an audio programme signal will usually be significantly greater than its nominal level (which is normally the signal's average level). In common with other signal level measurements, peak levels are quoted as RMS values.

As real signals often contain very short duration peaks (see Transients), the peak value registered by any level meter will depend upon that type of meter's ability to respond to rapid increases in level. In practice, the accepted peak value of an audio programme signal is considered to be the maximum reading given by a PPM complying with an appropriate standard. The lighting of an indicator marked "Peak" (or "Pk") on a mixer or an amplifier indicates that the equipment is nearing overload. See also Headroom, VU and Dynamic range. Compare Trough.

(Note that the meaning given above is quite different to that in electrical engineering, where the peak value of the signal voltage is the highest instantaneous voltage, positive or negative (measured from zero), occurring in each and every cycle of a repetitive waveform − see also Crest factor.)

Peak programme meter
See PPM.

Peaking response
Describes an equaliser facility that provides control over a range of frequencies, limited by both a lower and an upper boundary, but have little effect on frequencies outside that range. For example, a peaking response is provided by a parametric equaliser, by a sweep EQ facility and by each band of a graphic equaliser. Such equalisers are sometimes said to exhibit a 'bell curve' response, because of the bell-like shape of their frequency response curve. Some peaking response equalisers allow adjustment of the centre frequency, and some also of the Q. See also Bandpass. Compare Shelving response.

Peaky
Describes a signal whose peak level is much higher than its average level. See also Headroom, PPM, VU and Dynamic range.

Pedal
An effects unit intended to be placed on the floor and operated with the foot. Used mostly with electric guitars, but also with electric basses and with acoustic guitars that are equipped with pick-ups. See also the next definition.

Pedal board
An item of equipment, placed on the floor, which accommodates several pedals. It may also incorporate a power supply for the pedals, and/or provide a facility for the audio interconnections between them.

PEQ
An abbreviation for 'parametric equaliser', or, less often for 'preset equaliser'.

Perceived volume (Perceived level)
See Loudness (1).

Perceptual coding
A lossy compression algorithm which utilises the way in which sounds are perceived by the human auditory system (hearing) − particularly the masking phenomenon. Such techniques are heavily used in the MPEG-1 and MPEG-2 Layer 3 (MP3) and AAC compression standards. See also ATRAC.

Period
The time taken for one complete cycle of a repetitive waveform. This can be calculated (as a value in seconds) by dividing 1 by the frequency in Hz. For example, a 200 Hz signal has a period of 1/200th second, or 5 ms. See also Wavelength.

Peripheral
A supplementary item of equipment, connected to the main unit in order to provide additional or enhanced functionality. Most frequently used in the context of computer equipment. For example, a printer and an external hard drive are peripherals.

Peritel
See SCART.

Personal mixing
A facility that enables each performer to create their own individual monitor mix, for use with in-ear monitors, headphones, or floor monitors (wedges).

PF (p.f.)
An abbreviation for 'power factor'. (But see also the next definition.)

pF
An abbreviation for 'picofarad', a million-millionth (10⁻¹²) of a farad. (But see also the previous definition.)

PFL
An abbreviation for 'pre-fade listening'. A facility of a mixer, allowing the operator to listen, via headphones, to the sound being handled by an individual channel, selection of channels, or group, regardless of the setting of the channel or group fader. See also AFL and Solo.

Phantom power
An arrangement whereby DC power is supplied through a balanced interconnection between equipment, without disturbing the signal being carried by that connection. It is usually used to allow a mixer to supply the power required by condenser microphones and active DI boxes.

Mixers that provide phantom powering usually allow this facility to be switched on or off as required. On smaller mixers a single global switch is generally provided, whereas larger mixers often provide a separate switch for each channel. To avoid loud thumps which may damage speakers, such switches should not be operated while the affected channel(s) are routed to speakers (whether main or monitors). Furthermore, once phantom power is activated on a channel, microphone connections to that channel should not be made or disconnected at any point.

The positive side of the powering voltage is applied equally to the hot and cold signal connections of the balanced circuit (pins 2 and 3 of a standard XLR microphone connector), via independent feed resistors. The negative side of the voltage is applied to the screen connection (pin 1). (The term 'phantom power' arises because the powering voltage is 'invisible' to equipment connected across the two signal connections, because they both carry exactly the same voltage.)

So, it can be seen that any channel that has phantom power present on its input must only be driven from a balanced source. (If an unbalanced source were used, connected between pin 2 and pin 1, the phantom voltage would be applied across it − regardless of whether or not pin 3 is linked to pin 1.)

As some microphones are designed for low voltages of phantom power, to avoid damage always check the requirements of your microphones before connecting. The most usual specification, DIN EN 61938 (formerly known as DIN 45596 or IEC 61938), specifies a nominal voltage of 12, 24 or 48 volts DC, but the 48 volt version (known as P48) is by far the most popular. This allows a supply source voltage range of 44 to 52 volts (including the allowance for loading of the source by the devices connected). Whilst some microphones can accommodate a very wide range of supply voltages (e.g. 9 to 52) with little change in performance, others (especially professional studio mics) are intended to operate only within a much smaller range. (The current drawn by a microphone will inevitably reduce the voltage actually reaching it − usually to well below the 44 volt minimum source voltage for P48 − due to the voltage drop across the two 6.8 kilohm feed resistors, which are effectively in parallel. However, the design of the microphone should take this reduction into account, and the supply voltage range quoted in its specifications will normally be the source voltage range, unless stated otherwise.)

Also, care must be taken to ensure that the total current demand of all the phantom-powered devices connected to a mixer does not exceed the total phantom power current available from the mixer. The current demand of a single condenser microphone is typically 2 to 5 mA, but the P48 standard allows up to 10 mA per mic to be drawn.

As ribbon microphones can be seriously damaged by the mis-application of phantom power, it is best to avoid phantom power when using this type of microphone.

As a final word of warning, take care when using microphone splitters (or split outputs of mixers), that the intended mixer is providing the phantom power, and that the phantom power supplied by one mixer is not fed into the microphone input(s) of another if this could cause damage.

Phase (1)
The timing relationship between two or more varying quantities, usually the instantaneous voltages of two signals or the instantaneous wave pressures of two sounds, considered as a proportion of one complete cycle of variation. This definition means that, strictly, the term should only be used when comparing sine-wave signals or sounds of the same frequency; it is not strictly meaningful in reference to the complex multi-frequency audio signals and sounds generally encountered in practice.

However, in common usage, the expression 'in phase' is used to describe two complex quantities whose instantaneous variations happen at essentially the same time, at the relevant point(s) of observation. If, however, there is some relative time delay between their variations, then the quantities are said to be 'out of phase'; this is misleading because in such a case the extent of the phase difference between them (as a proportion of a cycle) will be different for each of their various frequency components.

Confusion often arises because the expression 'out of phase' (or 'in anti-phase') is also used to refer to a relative reversal of polarity, in the absence of any timing difference. In such a case, the variations between the two quantities are exactly opposite at all frequencies − i.e. as the instantaneous value of one quantity changes in a positive direction, the other changes in a negative direction. (This confusion of terminology arises because, in the case of a sine wave, a reversal of polarity is indistinguishable from a time delay equivalent to half a cycle of change.)

Phase is important in two respects:

• In a system having multiple speakers (covering the same frequency range), it is important that all the speakers operate in phase, so that their individual contributions to the overall sound add together rather than cancelling each other. (Usually, a positive drive signal is arranged to create a positive sound pressure in front of the speakers.) If the speakers are operating in anti-phase (e.g. due to one of them having been connected with reversed polarity), then a considerable loss of bass response will usually result. This is because bass frequencies are more susceptible to cancellation, as their long wavelength means that delays caused by small differences in distance etc are less significant in terms of phase.
• When two or more channels of a mixer are handling different versions of the same sound (for example when a guitar combo is used with a DI box as well as being miked-up, or when a snare drum is miked both above and below), it can be useful to be able to reverse the polarity of the signal on one or more of those channels in order to change the result of combining the various versions of the sound. Some mixers (especially larger ones) are provided with 'PHASE' switches to enable this to be done, on a per-channel basis. (Note that the labelling of these is strictly incorrect, as they reverse the signal's polarity rather than change its phase.)

Phase differences between the current and voltage of an AC mains supply are expressed in terms of power factor. See also Phase shift, Phase response and Group delay.

Phase (2)
An effect sometimes used with guitars to improve the "interest" of the sound. It operates by introducing a change in phase (1) to a narrow band of frequencies, the affected band being continuously varied through the required frequency range.

Phase (3)
For efficiency, the mains supply to large buildings is delivered on three live conductors rather than just one. This arrangement is termed a "3-phase supply", because there is a phase difference (of 120º) between the voltages on each of these three conductors.

However, each low-power circuit in the electrical installation (such as normal 'domestic type' socket-outlet circuits) takes its supply from just one of the three phases. The live conductor of the circuit is connected to the selected phase conductor of the incoming supply (via appropriate switching and circuit protection devices), and the neutral of the circuit is connected to the common neutral conductor of the incoming supply.

The main significance for PA systems is that if interference from the mains supply is a problem, and the building has a 3-phase supply, then sometimes a solution can be found by taking the power for the PA system from a socket outlet that is supplied from a different phase.

However circuits providing very large amounts of power, such as those supplying large stage lighting systems or very large PA systems, use all three phases − often connected through CEE-form connectors (always coloured red). For current ratings greater than 125 amps, single-pole connectors such as the Snaplock, Powerlock or Camlock types are usually employed.

In the UK, the three phases have, until 2004, been identified as the 'Red', 'Yellow' and 'Blue' phases, but with the change to use brown, black and grey colourings (to harmonise with Europe), the official identification is now L1, L2 and L3.

WARNING: The neutral conductor, previously identified by the colour black in the UK, is now identified by blue. This means that black may identify neutral or a live phase, and blue may also identify neutral or a live phase, depending on which colouring scheme is in use. When an installation uses a mixture of the old and new colouring schemes, extreme caution must be applied to ensure that all conductors are correctly identified.

For safety reasons, never interfere with an electrical installation or with power distribution equipment unless you are competent to do so safely, have the correct tools and test equipment and have the permission of the person responsible for it. WARNING: In the UK, where the normal mains supply is at 230 volts (previously 240), the voltage between any two phase conductors of this supply is nominally 400 volts (previously 415). This is a very dangerous voltage. Compare Single phase.

Phase conductor
In a mains electrical supply circuit, a UK technical term for a conductor that is at supply voltage with respect to earth. A single-phase supply has just one phase conductor, whereas a 3-phase supply has three independent phase conductors. More recently, this term has been replaced by 'line conductor'. Informally referred to as a live conductor.

Phase linear
See Linear phase response.

Phase response
An indication of the way in which the phase shift through a system varies according to the applied frequency. See also Linear phase response and Group delay.

Phase reversal
A term normally used to describe a signal changing from an in-phase condition to an anti-phase condition, or vice-versa, relative to some reference phase. Such a change will usually be the result of an intentional or accidental reversal of polarity in an interconnection, or of the operation of a 'Phase' switch on a mixer or other item of equipment. Another possible cause is the re-positioning of a microphone, for example from above to below a snare drum.

[Strictly, this term is a misnomer because in strict terms 'phase' relates to a timing difference, and not to the direction of the signal voltage. Further, it could be argued that phase (in its strict sense) cannot be 'reversed'. Or, that to perform a phase reversal can only mean to swap over two independent signals (e.g. two separate speaker feeds) that are identical except for some difference in timing − which is a completely different thing to the normal usage. The normal usage described above arose because it is useful to reserve the term 'polarity reversal' for the physical swapping of the poles or signal wires of an interconnection, requiring the need for another term to describe the equivalent 'voltage direction reversal' effect on a signal (which may have been caused electronically, without any such physical swapping of connections). The term 'phase reversal' is deemed to be appropriate because in the case of a symmetrical repetitive waveform such as a sine wave, a reversal of voltage direction is indistinguishable from a true phase shift of 180 degrees.] See also Balanced.

Phase shift
The difference in phase between the output and input of a system at a particular frequency, usually measured in degrees (360º being one complete cycle). See also Phase response.

Phase switch
A switch provided on a mixer channel to enable a deliberate polarity reversal to be introduced into the signal − for further details see Phase (1) (especially the 2nd bullet).

Phat
Describes (subjectively) musical sound that is rich in 'body' and 'depth'. Compare Thin.

Phon
A quantified measure of loudness − the level of sound as perceived by an average human ear. It is not the same as the sound pressure level (SPL), because the ear does not have a flat frequency response. It is expressed in decibels, by attributing the value of sound pressure level (in dB SPL) of a 1 kHz sine wave (or narrow-band noise centred on 1 kHz) to the perceived loudness of that tone and to that same perceived loudness of a sound of any other frequency.

So, whilst a 1 kHz tone at, say, 90 dB SPL will always have a loudness of 90 phons, an SPL level of more or less than this may be required to achieve a loudness of 90 phons at other frequencies. Furthermore, the amount by which the SPL differs from the loudness depends not only on the frequency but also on the sound level. The frequency at which the ear is most sensitive is around 2.5 to 3 kHz. See also A-weighting and Sone.

Phone jack
For the audio connector, see Jack. For the telephone connector, see BT431a.

Phono (1)
A type of connector sometimes used for unbalanced screened connections. Its main use is in line-level audio connections (but also for video and digital audio signals) on consumer 'Hi-Fi' and television equipment. Professional applications are limited. Beware − because of the consumer market usage, versions of very doubtful quality are widely available!

The following colour-coding conventions for phono connections are often (but not universally) used:

• White − Left audio.
• Red − Right audio (memory aid − think R for Red and R for Right), Red chrominance component of component video, or Red of RGB.
• Yellow − Composite video, or luminance component of component video.
• Green − Green of RGB, or luminance component of component video.
• Blue − Blue chrominance component of component video, or Blue of RGB.

Its name derives from its initial use as a means of connecting a "phonograph" (an antiquated term for a record deck, or turntable) to other equipment. Also called an RCA connector, after the company which originally designed it, and sometimes a Cinch connector, after one of the first companies to manufacture it. Occasionally called a pin jack (USA term).

View Phono image

Phono (2)
An input intended ONLY for the connection of a record deck (turntable) pick-up. Usually provided as a pair of inputs, for stereo pick-ups. The internal pre-amplifier circuitry to which they are connected inside the equipment provides the special fixed equalisation (known as RIAA equalisation) which is needed for the proper reproduction of recordings on vinyl.

Phono equalisation
See RIAA.

Pick-up
The part of a musical instrument (usually a guitar or electric bass) that converts the vibration of the strings (and body) of the instrument into an electrical signal, for connection to a means of amplification. There are two types:

A passive pick-up has no internal amplification of the signal, and so requires no power source. Some varieties have a very low output level or a high output impedance, and these types may be unsuitable for connection to a passive DI box without use of a pre-amplifier, otherwise a loss of sound quality will result.

An active pick-up provides some amplification of the signal internally, and so requires a source of power (usually a battery in a compartment built into the instrument, or in a bodypack). These types may usually be connected directly to any type of DI box without any problems. See also Humbucker.

Pick-up angle
The angle through which a microphone is usefully responsive to sounds, measured as the total angle from one side of the microphone to the other. This angle will be mostly governed by the polar response of the microphone at each of the frequencies present in the sound, but may also be affected by the distance between the microphone and the sound source. The limits of the pick-up angle are usually taken as being the points where the sensitivity of the microphone has dropped by 3 dB from the on-axis sensitivity, at some reference frequency. For some typical figures see Cardioid, Super-cardioid and Hyper-cardioid. Also called the 'angle of acceptance'.

Pick-up pattern
See Polar response.

Pico...
A prefix indicating one million-millionth (10⁻¹²). It is abbreviated to 'p'. For example, a picovolt (pV) is one million-millionth of a volt, and a picobar (pbar) is one million-millionth of a bar.

Piezo
An abbreviation for 'piezo-electric'. The effect exhibited by certain materials (especially ceramics) in which an applied voltage creates a physical force (and therefore the potential for movement) and in which an applied physical force generates a voltage. These two aspects of the piezo-electric effect may, respectively, be used to create drivers or microphones.

Such drivers are generally only useful at high audio (and ultrasonic) frequencies, but as they are limited in power output, and have a high minimum usable frequency, their use in professional full-range PA systems is generally restricted to speakers in which the mid-range is handled by one or more separate drivers. They are sometimes encountered in disco speakers, to extend the frequency range above that provided by horns having a limited upper frequency limit.

Piezo-electric microphones are generally unsuitable as conventional microphones for use in PA systems, and are restricted to specialist applications such as contact microphones.

Pilot tone (Pilot signal)
An inaudible supplementary signal that is transmitted by a radio microphone transmitter, along with the audio signal. Its purposes vary from system to system, but usually include conveying to the receiver information such as the presence of a compatible switched-on transmitter, the identity of the transmitter, the state of charge of the transmitter's battery and/or the position of the transmitter's mute switch. See also Squelch.

Pin
See Pole.

Pin 1 problem
A term that refers to the situation in which poor equipment design results in the introduction of noise (usually hum and/or radio-frequency interference) when, under particular circumstances, a connection is made to the signal earth terminal of a balanced signal connector on the affected item of equipment. This problem can arise when, internal to the equipment, the signal earth terminal of such a connector is wired to a signal earth point in the circuitry, rather than (or as well as) being connected directly to the metallic equipment chassis. This is poor design because:

• Firstly, the internal signal earth wire from the connector's signal earth terminal will inject any cable screen current into the equipment's circuitry at the point to which it connects, and this is likely to result in unwanted additions to the signal output(s) from the equipment, often due to common impedance coupling. Screen currents typically result from:
  • Differences in safety earth potentials between the chassis of the interconnected items of equipment (only relevant when the cable screen is connected to a safety earth at the far end of the cable, via the equipment at that end). This situation can result in a hum or buzz, common referred to as an 'earth loop' problem.
  • Inductive coupling of mains-frequency (and harmonic) currents into the cable screen from nearby mains cabling and/or mains-powered equipment (only relevant when the screen forms part of a complete earth path back to the equipment at the local end of the cable, via the equipment at the far end). This too can result in a hum or buzz.
  • Capacitive coupling of noise onto the screen from other nearby cabling (especially mains cabling) and/or equipment.
  • Capacitive coupling of other signals onto the screen from other circuits of the same cable (if not effectively screened). This can result in crosstalk.
• Secondly, the internal signal earth wire from the connector's signal earth terminal will have a relatively high impedance at radio frequencies, so at those frequencies the cable screen is not effectively earthed at the point of connection to the equipment. That signal earth wire will act as an aerial internal to the equipment, and is likely to radiate into the equipment some of the radio-frequency interference (RFI) picked up on the cable screen. This can result in the audible reception of radio transmissions (even from distant transmitters), and also other problems.

• Problems with pick-up of RFI.
• When the same cables are used in different places in in the system on different occasions, problems with hum and buzz due to the lack of a signal earth where one is needed.
• Non-functioning of phantom-powered condenser microphones and DI boxes.

The problem can be tested for by using a device to force an audio-frequency current to flow between the signal earth connection of each balanced audio connector on the equipment (in turn) and the equipment chassis, and monitoring the equipment output(s) to evaluate to what extent the applied signal is present there, at various settings of the equipment controls. A popular form of such a test device applies a current-limited low-voltage mains-frequency signal, and is therefore referred to as a 'hummer'.

When equipment is found to have this problem, and modification of the equipment is not a practical option, for XLR connectors one solution that sometimes works is connecting the cable screen to the shell of the connector that mates with the equipment, instead of to pin 1 of that connector. However, this method relies on a good electrical contact between the shells of the two connectors, and on the shell of the equipment's connector having good electrical contact with its earthed chassis. Also, remember that this approach is only a possible work-around; the "pin 1 problem" is not directly concerned with the wiring of the signal earth terminals or shells of the connectors that are attached to cables: the problem itself relates only to the wiring of such terminals inside items of equipment. [To try out this work-around conveniently, it can be useful to make a special short XLR extension cable that has the screen connected to pin 1 (only) at one end and to the shell (only) at the other end; be sure to label it accordingly! N.B. Two versions will be required: for inputs a version with the shell connection at the male end, and for outputs a version with the shell connection at the female end − be sure to label which cable is which!]

This issue was so-named because pin 1 of an XLR is used for the signal earth connection through that type of connector. It was first documented by Neil Muncy.

Pin jack
See Phono (1).

Pink noise
A particular type of noise, whose level decreases with increasing frequency in such a way that each octave of the audible frequency range contains the same amount of noise power. It is intentionally generated for use in assessing the acoustics of rooms and in assessing the power handling capability of speakers, because it has a power/frequency distribution that is similar to that of actual programme material. See also White noise and IEC noise.

Pitch
The musical term for the perceived fundamental frequency of a single note produced by a musical instrument or sung. Because the ear responds logarithmically to frequency (as well as to sound level), pitch is not measured in Hertz but on a logarithmic scale, using tones, semi-tones and octaves. See also Concert pitch, Middle C, Cent and Harmonic.

Pixel
An abbreviation for 'picture element' (or possibly for 'picture cell'). On a display screen, a pixel is the smallest area of the screen that may be independently controlled. Screen images are constructed of rows and columns of these units, each of which has a single brightness, colour and saturation. The greater the number of pixels that are accommodated by a display of a given size, the better the resolution of the images produced − see, for example, VGA. See also LCD and HDTV.

Pk
An abbreviation for 'peak'.

Planar
Describes something which is flat, especially a driver which uses a flat diaphragm rather than a cone. This type of driver is very rarely encountered in PA systems.

PLASA
An abbreviation for 'Professional Lighting and Sound Association'. Their website is: http://www.plasa.org. See also ISCE and AES.

Plate
A type of reverberation effect, so called because it may be produced by passing sound vibrations through a metal plate (though the effect is now usually produced digitally). See also Spring line and Hall.

Plate microphone
An alternative name for a boundary microphone.

Plexiglas
The trademarked name of a transparent plastic sheet material, sometimes used in the construction of on-stage items such as drum screens. Often incorrectly spelled 'plexiglass'. Generically known as PMMA (poly methyl methacrylate).

Plosive
The sound generated by a breath blast. This is a clever term, in that saying it generates the sound that it refers to − while at the same time being an abbreviation of 'explosion' (which, technically, is what happens at the lips during such a sound, when air under pressure is suddenly released). Compare Popping. See also Windshield.

Plot
A diagram, produced during the planning of an event, to indicate the audience area that is to be covered by each front-of-house speaker, cluster or array. It may also indicate the stage area that is to be covered by each monitor. Or, an abbreviation of 'lighting plot'.

Plug
A connector of the type that is attached to a cable. Note, however, that the term 'socket' may sometimes be applied to such a connector, if it is of the female gender.

Plug-in
An optional additional item of software that may be installed in order to enhance the capability of a computer application. See also DAW.

PM
An abbreviation for 'production manager'. See also SM.

PMA
An abbreviation for 'programme memory area', the area on a recordable audio compact disc (CD-R) or on a re-writable audio compact disc (CD-RW) that stores information about where on the disc the recorded tracks can be found, prior to the disc being finalised.

PME
An abbreviation for 'protective multiple earthing'. See TN−C−S.

PMP
An abbreviation for 'peak music power' − see PMPO.

PMPO
An abbreviation for 'peak music-power output'. A measurement of the power handling capacity of equipment (usually amplifiers or speakers) that takes into account the fluctuating level of actual programme material. PMPO figures are higher than 'continuous' or so-called 'RMS' power figures for the same equipment (particularly in the case of speakers), because the equipment is able provide (or accept) a fluctuating signal having a peak power level which is greater than the power level that could be provided (or accepted) if the signal being handled had a constant level. For more information see Power Ratings on the Amplifiers and Speakers page.

PMSE
An abbreviation for 'programme-making and special events'. A designation given to the allocation of radio-frequency spectrum for radio microphones, in-ear monitoring systems, etc. (whether licensed or licence-free) for entertainment and sports events purposes. This designation applies only to usage relating directly to the production of the programme or event − not to ancillary functions such as security etc. See also JFMG.

Pneumatic stand
A type of microphone stand which maintains its set height without using a manually tightened clamp. The height of this type is adjusted simply by pushing down or pulling up the top section, whereas types having a clamp require the clamp to be manually loosened before adjusting the height, and tightened again afterwards. Pneumatic stands usually have a heavy base (often of cast iron), to ensure that the lower section stays on the floor when pulling up the top section to raise the height of the stand.

Polar pattern
See Polar response.

Polar response
The pattern of sensitivity of a microphone or a speaker, showing the changes in sensitivity as one moves around the device from the front, to one side, to the back, to the other side and back to the front, whilst maintaining a constant distance from it. So, the polar response indicates the extent and nature of the microphone's directionality, and is an important factor to consider in microphone technique. The polar response of a particular microphone varies according to the frequency of the sound and may also be affected to some degree by the distance between the microphone and the sound source.

The three basic polar response patterns are omni-directional, uni-directional and bidirectional. Uni-directional types are available in several patterns, including subcardioid, cardioid, super-cardioid, hyper-cardioid and rifle. These responses are described in the Omni-directional or Uni-directional section on the Microphones page. See also Pick-up angle, Pressure gradient microphone, Distance factor and End-firing. For more information on microphones see the Microphones page.

View uni-directional polar responses illustration

Polarity
The "way-roundness" of something, usually of the wiring at an electrical connection or of the voltage carried on an interconnection. Correct polarity is often very important − for example in the connections of power supplies (mains and DC) and in the connection of speakers to power amplifiers (regarding the latter, see Phase (1)). For information on reversals of polarity, see the next definition.

Polarity reversal
The situation that exists when the polarity of an interconnection is reversed (whether intentionally or accidentally), for example by swapping over the 'hot' and 'cold' conductors of a balanced interconnection or the '+' and '−' connections of a speaker cable. Or, the situation that exists when an inversion is applied to a signal, so that positive instantaneous voltage changes become negative ones, and vice versa, for example by operation of a Phase switch on a mixer. The term 'phase reversal' is sometimes used to mean a polarity reversal.

To assist in the avoidance of accidental reversals of polarity, the insulation of cable conductors is usually colour coded, and connectors are physically polarised and have their poles identified with appropriate markings (see, for example, XLR and Speakon).

In the case of a signal interconnection such as a balanced line or the connections to a speaker, the effect of a polarity reversal is to cause the signal to be received in anti-phase relative to the original (or 'correct') polarity. In regard to audio signals, such a reversal would usually be unimportant if all related signals (if any) were also reversed, because the ear is largely insensitive to the polarity of sound waves. However, nearly all audio systems involve multiple related signals (e.g. multiple speaker feeds) and polarity reversal of a subset of such signals would usually cause problems, such as destructive interference. (Regarding speaker polarity, see Phase (1).)

In the case of a supply of power (whether mains or DC), the effect of incorrect polarity could be a serious risk of fire or electric shock, and/or serious damage to equipment, so the supply polarity should never be reversed unless you are certain that it is presently incorrect and that such a reversal would correct the original error at the same point in the supply at which that error occurred. (This is because a 'double reversal' may give rise to confusion and further danger.) A reversal of mains supply polarity is sometimes suggested as a means to reduce hum problems; this should only ever be performed by reversal of a non-polarised mains connector (however, such an approach is unlikely to effectively address the underlying source of the problem). See also Pair and Live (1).

Polarisation
In relation to radio microphones, a term used to describe the orientation of the radio waves. Vertically polarised radio waves have a vertically-oriented electric field, and use a vertical aerial for transmission and reception − this is the norm for radio microphones. Horizontally polarised radio waves have a horizontally-oriented electric field, and use a horizontal aerial for transmission and reception. In relation to connectors, see the next definition.

Polarised
Describes something whose polarity is significant. For example, a polarised connector is one that is designed in such a way that it can be mated in only one physical orientation, in order to avoid a reversal of polarity − i.e. a 'non-reversible' connector. A polarised electronic component such as a polarised electrolytic capacitor must be connected with the correct polarity, otherwise it may explode. See also Non-polarised connector and Non-polarised electrolytic capacitor.

Pole
The conducting part of a connector or of a switch. For example, a connector that has the facility for making connections for three independent conductors would be described as '3-pole'. Note that most multi-way connectors, e.g. XLRs and DIN connectors (but with the exception of the SCART), exclude the shell from the count of poles, whereas coaxial connectors such as phonos and BNCs include it. The jack connector is not classed as coaxial and does not have a shell; all of its conductors are counted as poles. In the case of some multi-way connectors (such as the DIN connector), the term 'pin' is sometimes informally used in place of 'pole', even for female connectors; e.g. '3-pin socket'. See also Hot, Cold, Signal earth, TRS and Core.

Polyphonic
Describes a sound source (usually an instrument) which is capable of producing more than one musical note at a time (e.g. a keyboard). Compare Monophonic.

Pop filter (or Pop shield)
Another name for a windshield, so-named because of its ability to reduce popping.

Popping
The sound resulting from plosives entering a microphone. It is particularly evident when the distance of the microphone from the mouth is about 3 inches (7.5 cm). It can be reduced by using a windshield (also called a 'pop filter'), or by placing the microphone such that the breath blast passes above it (or, less commonly, below it).

Port
An opening in a speaker enclosure, carefully designed to modify the bass resonances of the bass driver / enclosure combination so as to give the speaker an improved bass response. There may be several openings making up the porting system, and one or more of them may be fitted with 'tuning' tubes (usually internal to the enclosure). A ported enclosure is also called a 'reflex enclosure'. See also Thiele-Small parameters. Compare Sealed box.

Portamento
A gradual change (or 'slide') in pitch between two musical notes.

Positive feedback
The situation in which a signal that is fed back to an earlier point in a signal chain is in-phase with the signal that is already there, and so reinforces it. If the in-phase gain around the feedback loop is greater than unity at some frequency, then oscillation will result − usually at the frequency of greatest in-phase gain. See also Feedback and Oscillator. Compare Negative feedback.

Post Office jack
See Jack.

Post-EQ
Describes a signal after it has passed through the equaliser (EQ) − i.e. one which is affected by the position of the EQ controls. Compare Pre-EQ.

Post-fade
Describes a signal after it has passed through the fader − i.e. one whose level is affected by the position of the fader. See also Auxiliary. Compare Pre-fade.

Pot
A slang term for a potentiometer.

Potential
A capability, whether realised or not. Another word for voltage, because the existence of a voltage implies the capability for a flow of current.

Potential acoustic gain (PAG)
The theoretical (i.e. calculated) maximum amount of gain that a PA system can achieve before the onset of acoustic feedback. The calculation is a little complex, and only gives an approximate guide to the actual gain before feedback that the system will provide. In order for the system to be able to provide sufficient amplification, the potential acoustic gain must be at least as large as the needed acoustic gain (NAG) figure.

The simplified equation for the calculation, giving a result in decibels, is:

PAG = 20 x (log D0 + log D1 − log D2 − log Ds) − 10 x (log NOM) − 6

where:

• 'log' means take the logarithm (to the base 10) of the following value
• D0 = distance from sound source to listener
• D1 = distance from microphone to speaker
• D2 = distance from speaker to listener
• Ds = distance from sound source to microphone
• NOM = the number of open microphones
• '− 6' is a figure to give a 6 dB feedback stability margin

By way of example, consider a system with 6 open microphones in which the needed acoustic gain is 15 dB, and the distances are as follows:

• Sound source to listener, D0 = 10.2 m
• Microphone to speaker, D1 = 4 m
• Speaker to listener, D2 = 6 m
• Sound source to microphone, Ds = 0.2 m

Potential difference
Another name for voltage.

Potentiometer (Pot)
The device, inside an item of equipment, to which the knob of any control (other than a switch control) is attached − regardless of whether it is a rotary control or a slider control, and regardless of the function of the control. It consists of a moveable wiper in contact with a resistive track, usually arranged to 'tap off' a proportion of the applied signal (or voltage), the proportion being dependent upon the physical position of the control knob (or slider). A 0% setting of the control always gives 0% output and a 100% setting of the control always gives 100% output, but for intermediate settings it is more complicated.

Potentiometers are made with two basic 'laws', or 'tapers': linear (abbreviated 'lin' or 'LN'), in which the output is directly proportional to the physical setting of the control (a 50% setting gives 50% output, etc.) and logarithmic (abbreviated 'log' or 'LG'), in which the output varies approximately as the logarithm of the physical setting. The 'log' types are most suitable as level controls and faders. In practice, there are several variations on the log theme, which are designed to give a more satisfactory control of the level; these types are described as having an 'audio taper' (or 'A-taper'). Often called a "pot", for short. See also Travel, Noisy and Dirty (2).

Pouch
The usual name given to the flexible plastic bag used for the individual storage and transport of microphones, often supplied by the manufacturer with the microphone. They are usually equipped with a closure of some kind, often a zip. They are useful for protecting the mic from dust and other contaminants, but are not usually sufficiently padded to protect from drops and other mechanical shocks or pressure. Hard, foam-padded cases are much more suitable for regularly transported mics, and can hold several of them.

Power (1)
Strictly, the rate at which energy is being transferred or is being converted from one form to another. It is measured in watts, one watt being a transfer rate of one joule of energy per second. Although power can appear in many forms (motion, heat, light, etc.), in PA work we are generally only concerned with electrical power and acoustic power (i.e. sound). Electrical power that is converted into another form is said to be dissipated.

The amount of electrical power depends upon both the voltage and the current values. The power value (and direction) at any instant in time can be determined by multiplying together the instantaneous values of the voltage and current. This calculation always works in the case of DC, because these values are constant.

In the case of AC (e.g. for electrical supplies or for audio signals), then if both the voltage and the current are given as RMS values, multiplying these two figures together gives the average power value (often incorrectly referred to as the RMS power). As this calculation assumes that the voltage and the current are in phase with one another (unity power factor), the result it gives will be higher than a value which takes into account a phase difference.

For more information on speaker and amplifier power ratings, see Power Ratings on the Amplifiers and Speakers page.

Acoustic power is usually expressed in terms of the sound pressure level (SPL) − see the Decibels page for further information on this. See also Sound intensity level (SIL).

Power (2)
An informal name for the electrical supply that enables active equipment to operate. The term may refer either to the mains supply or to a low-voltage AC or DC supply, but it is always essential to ensure that the supply is of the appropriate voltage, type and polarity (for DC supplies), and is able to provide adequate current, before connecting it to the equipment. See also Power supply.

Power amplifier (Power amp)
An amplifier whose output(s) are intended for direct connection to one or more passive speakers. Normally refers to a separate item of equipment that converts a line-level input signal (e.g. from a mixer) into an output signal of sufficient voltage and current-supplying capability to drive one or more speakers at a specified power level. Such an item of equipment is often simply called an 'amplifier' (or 'amp'); the term 'power amplifier' is used to distinguish this type of amplifier (which is able to supply relatively large amount of power at its output(s)) from other kinds of amplifier (see Amplifier for a list of these). Alternatively the term 'PA amplifier' is sometimes used.

This equipment does not normally include any mixing facilities. However, the term 'power amplifier' may also be used to refer to a speaker-driving amplifier that is incorporated within a larger item of equipment that does include mixing facilities, such as a powered mixer or a mixer-amplifier.

Some power amplifiers are designed specifically for public address applications; these types generally have 100 volt line speaker outputs rather than the usual low impedance outputs, and are often provided with at least one microphone input and some basic mixing facilities.

Power amplifiers usually provide two separate paths of amplification (called 'channels') within a single unit (though they may share a common power supply). The two channels may be used either to enable stereo use, or may be used independently (e.g. to amplify two different monitor mixes). Some 4-channel types are also available. They often have very few controls (often just a level control for each channel), though some 2-channel types provide a facility to configure the two channels into single-channel bridged mode.

The more powerful types usually incorporate some type of overload protection (often thermally operated), short-circuit protection and speaker protection − but nevertheless attention must always be given to the value of connected load impedance, both to avoid damaging the amplifier and to achieve optimum power output. When switching on a PA system, it is important that the power amplifiers are switched on last of all (preferably with their level controls at minimum), so that the switch-on transients of other equipment are not passed to the speakers. Likewise, when the system is being powered-down they should be switched off first (preferably after setting their level controls to minimum). For further information on this type of amplifier see the Amplifiers and Speakers page. Compare Pre-amplifier.

Power Breaker
A trade-marked brand name of a range of mains power accessories. Most often used to refer to a plug-in RCD (whether of that brand or not) − to avoid possible confusion this usage is however best avoided.

Power compression
The dynamic compression effect that occurs in a driver when subjected to a sustained power level towards the upper limit of its power-handling capability. This effect is due to the temperature rise of the voice coil causing an increase in the coil's resistance, and therefore a rise in the impedance of the driver; this in turn reduces the power that is drawn from the amplifier by the speaker. The only solution is to use speakers with a higher power rating, or with improved voice-coil cooling. Note that the term has no direct connection with the naming of a compression driver. See also Ferrofluid.

Power conditioner
A general term for any item of equipment whose purpose is to improve the quality of the mains power that is fed to other items of equipment, with the intention of improving the performance of those items. Different types of power conditioner vary widely in their effectiveness in addressing the different aspects of mains power quality, and range from simple interference filters to full regeneration of the supply waveform. It is very important to ensure that any conditioner used is able to supply the maximum amount of supply current required in total by the items of equipment that it is feeding. See also Uninterruptible power supply, Distro, Line filter and VA.

Power factor
A numerical means of expressing the extent of phase difference between the current and the voltage of an AC mains supply, so as to directly indicate the proportion of the current drawn that is providing useful power input to the supplied equipment. For example, a power factor of 1 (often referred to as 'unity power factor') indicates that all of the current drawn provides useful power, whilst a power factor of 0.75 indicates that only three-quarters of the current drawn provides useful power. (Note: Take care not to confuse with efficiency.)

Or, conversely, it may be used to determine the actual current required to be drawn in order to provide a given amount of useful power (at a given voltage). For example, if (in a 230 V system) the power required is 1725 W and the power factor is 0.75, then 10 amps of current would be drawn.

In a large system powered from an AC mains supply, it is important to consider the amount of current required from the supply, in order to ensure that the supply is capable of providing it. For example, a maximum of 13 amps can be taken from a standard UK 3-pin plug. It might be thought that the total power requirement of several items of equipment could be determined by simply adding together the requirement of each item; then, since power equals voltage times current, the required current could be calculated by dividing the total power requirement by the supply voltage.

For example, if three items on a 230 V mains supply consume 100 W, 300 W and 520 W respectively, a total of 920 W, then one might expect that the total current drawn would be 920 divided by 230, i.e. 4 amps.

However, this calculation only works when, for each item of equipment, the current drawn is in-phase with the supplied voltage. For some kinds of equipment, such as heaters and undimmed stage lighting, this is approximately true, and these are said to have a 'good' power factor − a value only slightly less than 1. For other kinds of equipment, such as motors and some varieties of lighting dimmers, the voltage supplied and the current drawn are significantly out of phase, and these are said to have a 'poor' power factor − a value significantly less than 1.

When using equipment with a poor power factor, to find the total current requirement given the power requirement of each item, you also need to know the value of the power factor for each item. You must work out the current required by each different item separately, by dividing its power requirement (in watts) by the voltage and by the power factor for that item. Since the power factor for such items is always less than 1, this will give a higher current requirement for each item than if the power factor had not been taken into account. Then finally you must add up all the individual currents to get the total.

Thankfully, there is often an easier way to find the total current requirement: if the power requirement of each item is specified in VA (the 'apparent power' needed), then you can forget about power factors. Instead, just add up the VA value of each of the items to find the total VA value, and divide that total by the voltage.

Power filter
See Power conditioner.

Power on/off procedure
See Switch on/off procedure.

Power rails
See Rails.

Power supply
A device that converts mains electricity into the particular supplies at the voltage(s) (AC or DC) and current(s) required by an item of equipment, generally by use of a transformer. The power supply unit (PSU) may be internal or external to the item of equipment being powered. When external, it is essential that only the power supply and interconnecting cable that are intended for use with the equipment are used, as other types may not be electrically compatible (even though the connectors may be mechanically compatible) − for example the supplied voltage, the current rating or the polarity may be incorrect.

Increasingly, power supply units are of the switched-mode type, in which the conversion of mains voltage to the required voltage(s) is performed by high-frequency switching of the voltage applied to the transformer (or, occasionally, to an inductor). This technique enables the power supply to operate at a substantially improved efficiency as compared to a 'conventional' (or 'linear') power supply, in which the transformer operates at mains frequency. It also enables a substantial reduction in the size and weight of the unit, as a high-frequency transformer is much smaller and lighter than a mains-frequency one of equivalent power-handling capacity. The switching frequency is typically in the range of 25 to 40 kHz. The abbreviation 'SMPS' is sometimes used to refer to a switched-mode power supply.

Or, rarely, the term 'power supply' may refer to the mains electrical supply, or to the local outlet or point from which it is obtained (see Power (2)).

PowerCon
A trademarked name for the NAC3 range of locking mains power connectors manufactured by Neutrik, which are sometimes used on audio equipment. It looks similar to a Speakon connector, but will not inter-mate with it. Its current capacity is 20 amps for the original type and 32 amps for the -HC type, but note that these connectors must not be de-mated under load. The 20 amp type comes in two versions: one having a blue-coloured body for power inputs to equipment, and one having a grey-coloured body for power outputs from equipment. These two versions will not inter-mate. The 32 amp type has a black body, and will not inter-mate with the 20 amp types. See also IEC 320.

View PowerCon image

Powered mixer, Powered desk, Powered board
A mixer that incorporates power amplification facilities in the same unit. Usually refers to units having a 'desk-like' construction, like a conventional (i.e. unpowered) mixer. Compare Mixer-amplifier.

Powered monitor
A monitor that incorporates its own power amplifier(s) in the same enclosure. See also Slave monitor.

Powered multicore
A multicore equipped with cable conductors suitable for the connection of speakers to their associated amplifiers, enabling the amplifiers to be located at the mixer end of the multicore (as would be the case when using a powered mixer or a mixer-amplifier with a multicore).

Powered speaker
A speaker that incorporates its own power amplifier(s) within the speaker enclosure. The main advantages over passive speakers are the elimination of the speaker cable, which can be expensive and/or impair the quality of the sound, and the avoidance of concerns about the correct matching of speakers and amplifiers (see the Amplifiers and Speakers page). The main disadvantages are the need to supply mains power to the speaker, increased speaker weight, and often increased difficulty of access to the amplifier to make adjustments or for repair purposes (amplifiers develop faults more often than speakers).

Some powered speakers have a 'speaker output' facility which enables the connection of an unpowered speaker sharing the powered speaker's amplifier, however the user manual must be carefully consulted to ensure that the amplifier is not overloaded by use of this facility. Sometimes called a 'self-powered speaker' or an 'active speaker'.

Powerlock
A range of single-pole locking mains power connectors intended for applications where a higher current rating is required than that provided by CEE-form connectors. Available in 400 amp and 600 amp versions. In common with other types of single-pole power connectors, they must not be mated or de-mated whilst the circuit is live, and all poles of the circuit must be mated before the circuit is energised. The poles should mated in the sequence Earth first, then Neutral, finally the Phases − and be de-mated in the reverse sequence. The labelling and colour-coding of the connectors must conform to the up-to-date standards of the country concerned. Compare Snaplock and Camlock.

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PPM
An abbreviation for 'peak programme meter'. A type of level meter whose response is specially tailored to indicate the peak level of a signal. This type of indication is useful in any situation where the required peak level is close to the maximum permissible level, i.e. where there is limited headroom. (These would usually be situations where it is vital to make best use of the available dynamic range of equipment, so as to maximise the signal-to-noise ratio.)

Originally found mainly on mixers used in broadcast studios, where the peak level must be carefully monitored to avoid over-modulating the transmitter. Now also frequently found on mixers used in recording studios and in other situations where digital audio storage, processing or transmission is involved, as an aid to the avoidance of overs.

To satisfy its purpose, the PPM must have a fast response to increases in level and a slow response to decreases in level. The required behaviour is detailed in IEC standards 60268-10 and 60268-18, which specify an integration time of 5 ms and a decay time (to −20 dB) of 1.5 seconds. The specified integration time avoids the meter responding to very short-lived peaks, sometimes referred to as transients. However, PPM meters with a much faster integration time (0.5 ms or less) are required to ensure that recording equipment (especially digital types) is not overloaded.

The standard display style adopted for broadcast equipment in the UK is derived from BBC specifications, now referred to as IEC 60268-10 / IIa. This style has a scale marked with digits from 1 to 7, each division representing a 4 dB difference in level. A reading of '4' represents a steady-state signal level of 0 dBu. The normal broadcast peak programme level is +8 dBu (equivalent to 1.95 volts RMS), represented by a reading of '6' on the BBC-style meter. See also Bargraph meter. Compare VU meter.

Pre-amplifier (Pre-amp, Pre)
An amplifier that provides initial amplification of a signal; especially of very low-level signals obtained from certain sources such as microphones and instrument pick-ups. The gain provided by the pre-amplifier raises the signal level to a value suitable for processing (equalisation, mixing etc.), for passing through a substantial length of cable or for further amplification. Often abbreviated to just 'pre'.

All microphone-level signals require pre-amplification; in PA work this is usually provided by the mixer channels, but in studios very high quality separate units are often used in critical applications such as pre-amplification for vocal microphones.

The level of noise contributed by the pre-amplifier is of particular significance, especially when the level of signal applied to it is very low (for example, in distance miking applications), or when the final mix may be listened to in a low ambient noise environment (for example, most recording and broadcast applications). This noise contribution is usually specified as an A-weighted equivalent input noise (EIN) value in dBu. To take advantage of a low EIN value, it is necessary to use a source (e.g. microphone) with a correspondingly low noise level. (For conversion of a microphone noise level specified as an A-weighted equivalent noise level (or 'self-noise') figure in dB SPL to a value in dBu, see Microphone Noise Levels on the Microphones page.)

Instrument pre-amplifiers are usually equipped with a balanced DI output, to enable connection to the balanced inputs of a mixer − often via a multicore. See also Thermal noise and Gain structure. Compare Power amplifier.

Pre-delay
The feature of a digital reverberation unit that provides some initial delay to the signal before the start of the reverberated sound, so as to better simulate the acoustic behaviour of a real reverberating space. See also Hall.

Pre-EQ
Describes a signal before it has passed through the equaliser (EQ) − i.e. one which is unaffected by the position of the EQ controls. Compare Post-EQ.

Pre-fade
Describes a signal before it has passed through the fader − i.e. one whose level is unaffected by the position of the fader. See also Auxiliary. Compare Post-fade.

Precedence effect
See Haas effect.

Presence
Frequencies in the upper mid-range region (especially 1 to 2 kHz), or an equalisation control which affects such frequencies (most usually found on guitar combos and heads).

Presence peak
An intentional increase in the sensitivity of a microphone at frequencies in the presence region. Used mostly in vocal microphones, where it is intended to increase the degree to which the vocals are able to cut through instrumentation.

Pressure gradient microphone
A microphone in which the output signal is a function of the difference between the air pressure at the front and the rear of the diaphragm, so giving the microphone a uni-directional characteristic and a proximity effect. Compare Pressure microphone. See also Polar response.

Pressure microphone
A microphone in which the output signal is a function of the pressure of the air at the front of the diaphragm. At the rear of the diaphragm is a sealed cavity. So, the microphone has an omni-directional characteristic, and no proximity effect. Compare Pressure gradient microphone. See also Polar response.

Pressure zone microphone
See Boundary microphone.

Primary winding
See Transformer.

Principle of superposition
The physical law which states that, in a perfectly linear system, the output that is obtained when several summed inputs are applied is the sum of the outputs that would be obtained if each of the inputs were applied separately. For example, if inputting a signal 'A' gives an output 'X', and inputting a signal 'B' gives an output 'Y', then inputting 'A + B' gives 'X + Y'.

In the absence of unwanted non-linearities such as distortion and intentional ones such as compression, this vital principle essentially applies to a PA system and has three major impacts:

• Each signal, despite being mixed with other signals, passes through the system 'independently', i.e. unaffected by the presence of the other signals.
• No new signal components are introduced by interaction of the signals passing through the system.
• The frequency response of the system, as specified by the system's response to a single sine wave swept through the frequency range of interest, is sufficient to define the response of the system to any complex-waveform signal whose frequency components lie within that same frequency range. (See Fourier analysis.)

Print-through
In tape recording, an impression formed magnetically (usually over a long period of time) from one layer of tape onto the adjacent layer of tape on the reel. This causes an unwanted 'echo' effect on playback, either after or, most noticeably, before the wanted sound. Usually only of any significance in analogue recording.

Printed circuit board
See PCB.

Processor
See Signal processing.

Processor-controlled
Describes a speaker that is used with signal processing equipment (generally called a 'processor' or 'speaker management equipment') which is specifically designed to get the best performance from that model of speaker. Usually, the processor will be manufactured by the same company as the speaker and, in the case of a powered speaker, may be incorporated within the speaker enclosure. See also Speaker protection.

Processing
See Signal processing.

Production
As a noun, a show or other performed event. As a verb, the process of creating programme material for a live event or for recording or broadcast.

Programme
The audio or video material being handled by a system (or by part of a system) at a particular point in time, i.e. the information content of an audio or video signal. The term is most often applied to the mixed output signal from a studio, for broadcast, live relay or recording. See also PPM.

Programme level
The nominal signal level of an audio programme. It is usually quoted as an average level, measured using a VU meter, but the peak level, measured using a PPM, is also of great interest. See also SOL and Headroom.

Programme power
A power rating sometimes applied to speakers, which attempts to take into account the dynamic nature of real programme material. Also known as 'music power'. For further information see Power Ratings on the Amplifiers and Speakers page. See also RMS.

Progressive scan
In a video display system, a technique for the improvement of display definition, as compared to a system using interlace. It operates by arranging for every line of the display to be refreshed in sequence during each vertical scan of the screen, so avoiding the time-difference between adjacent lines that is inherent in interlaced systems. This improvement is especially evident in the rendition of fast-moving images. However, in order to avoid the flicker that was eliminated by interlacing, a high vertical scan rate is required. See also Raster, Frame, Field and HDTV.

Propagation time
The time taken for something to travel a particular distance. Usually refers to the time taken for a sound wave to travel a particular distance − especially from a speaker to a listening position. Most often, it is relative propagation times that are significant − for example the difference in time between the sound from two or more speakers being heard, at a given listening position. See also Speed of sound, Latency, Time alignment, Haas effect and Delay.

Proprietary
Describes the internal design techniques of equipment, or interfaces between equipment, that are specific to a single manufacturer (although they might be licensed to other manufacturers). Proprietary interfaces (such as MIDI sysex messages) are unlikely to be compatible between different makes of equipment.

Prosumer
Describes equipment which is semi-mass-produced for semi-professional use. Equipment which, in terms of performance, reliability, ruggedness and overall quality, lies somewhere between professional equipment and consumer equipment. The term is formed by a contraction of the words 'professional' and 'consumer'.

Protective bonding
See Bonding.

Protocol
An agreed set of rules used in the communication of digital information between items of equipment, usually published in a 'standard'. The communication will only work correctly if all items of connected equipment can use the same version of the protocol. See also Interface.

Proximity effect
The behaviour of uni-directional microphones whereby sound sources that are close to the microphone are picked up with a greater bass response than sounds that are further away. It can be noticable at distances up to 1 ft (30 cm), but is most obvious at distances less than 4 inches (10 cm). Microphones differ in the extent to which they exhibit this effect. The effect is caused by the difference in sound level between the front and rear ports of the microphone becoming more and more significant as the distance from the sound source decreases − this difference in level affects the proper operation of the microphone's directionality-producing mechanism at bass frequencies. See also Pressure gradient microphone and Microphone Technique on the Getting Started − for Performers page).

Pseudo-balanced
Describes an interconnection in which an unbalanced output is connected to an unbalanced input using a cable similar to that used in balanced interconnections. The flexible part of the cable consists of two signal-carrying conductors inside an overall screen (i.e. exactly as in a balanced cable), but at the connectors one of the signal-carrying cable conductors is connected to the signal earth (screen) terminal at both ends, and the cable screen is connected to that same terminal at just one end.

This type of cable is often considered by audiophiles to give superior performance as compared to a standard unbalanced cable. However, as the input to which it connects is an unbalanced input there is no rejection of common mode interference in the manner provided by a balanced interconnection. Furthermore, connection of the screen at only one end can result in increased pick-up of radio-frequency interference, due to it acting like an aerial. Nevertheless, such cables are sometimes marketed as 'balanced' cables, which is inappropriate terminology. The connectors are usually high-quality phono connectors, to suit domestic Hi-Fi equipment. Not to be confused with a semi-balanced (sometimes called quasi-balanced) interconnection, nor a quasi-floating interconnection. See also Ground-compensated. (A table comparing the most common types of balanced interconnections is provided under the 'Balanced' entry.)

PSU
An abbreviation for 'power supply unit'. See Power supply.

Psychoacoustics
The study of hearing (particularly human hearing) as regards how sound is perceived by the brain, as opposed to just the mechanics of how the ear converts sound into nerve impulses. Also known as the 'psychology of hearing'. See also Auditory filter.

Psychology of hearing
See Psychoacoustics.

PTT
An abbreviation for 'push to talk', a switch provided on some types of communications systems. This switch must be held depressed whilst speaking via the system, in order to keep the microphone open. See also Clearcom.

Public address
A system whose purpose is to enable a large number of gathered people to be addressed (i.e. spoken to), or which enables people scattered over a substantial but localised area (such as within a building or outdoor arena) to hear announcements. In the interest of economics, such systems have often provided only the limited audio bandwidth necessary for the reproduction of speech, and therefore gave poor results when mis-used for music applications. However, the term 'PA' (originally simply an abbreviation for 'public address') is now widely used to include high-quality sound reinforcement systems intended for music applications. See also 100 volt line, Installation speaker, Musac and CABA.

Pulse code modulation
The use of sequences of bits, that is, of binary codes, to represent analogue information − usually an audio or video programme signal. In general, this is the principle used by all digital programme signals, although in practice there are many standardised coding schemes in use. Usually abbreviated to 'PCM'.

In the simplest PCM scheme, the bit-stream consists (fundamentally) of consecutive fixed-length groups of bits. Each group of bits corresponds to a single sample of the analogue source programme, and the binary value of the bits in the group directly indicates the value of that sample. This is the original meaning of the term, and is the basis of the coding used for audio compact discs and in AES3 and S/PDIF bit-streams. However, the term is now also applied to more complex coding schemes such as floating point coding and compressed data (MP3 etc.). See also Analogue to digital conversion and Codec.

Pumping
An undesirable phenomenon that can occur during signal compression, whereby the resulting signal level noticeably moves up and down in correspondence with the signal content. Can usually be avoided to some degree by reducing the bass content of the side chain signal. See also Breathing.

Punter
A (UK) slang term for a member of the public attending an event.

Pure tone
A musical note, or other sound or signal, that consists only of a fundamental frequency and no harmonics, i.e. a sine wave. See also Tone (2).

PVC tape
Usually refers to self-adhesive electrical insulating tape, which is generally made from PVC and is available in a range of different colours. However, no tape should be used for electrical insulating purposes unless it is specifically approved for that purpose. PVC tape is frequently used for alternative purposes such as securing coils of cable in transit and (when white) as console tape. However, it is not suitable for use at high temperatures (e.g. on or close to lanterns). PVC stands for polyvinyl chloride, a type of plastic. See also Cable tie.

PVR
An abbreviation for 'personal video recorder'. See also DVR.

Pyro
An abbreviation for 'pyrotechnics' (fireworks). Refers to any explosive-type devices used during an event (usually located on-stage). These are normally triggered remotely, often by the lighting engineer. Special safety considerations always apply to the storage, handling and use of these devices. See the Safety page for general information on safety.

PZM
An abbreviation for 'pressure zone microphone'. See Boundary microphone.

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This page last updated 12-Nov-2009.