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The glossary pages provide definitions for over 2680 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.

In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.

F-hole * Fade down * Fade out * Fade up * Fader * Farad * Farads * Faston terminal * Fat * Fat channel * Fault protection * FCC * Feed * Feedback * Feedback destroyer * Female * Ferrite * Ferrofluid * Ferrous * Ferrule * FET * FFT * FHSS * Fibre-optic * Fidelity * Field * Field of view * Field sync * Fifth * Fifth-order * Figure-of-8 cable * Figure-of-8 microphone * Filament lamp * Fill * Fills * Filter * Filter classes * Filter order * Filter slope * Finalise * FIR * Fire muting * FireWire * Firmware * First generation * First-order * Fixture * FLAC * Flanger * Flanges * Flat * Fletcher-Munson curves * Flight case * Float * Float switch * Floating * Floor * Floor monitor * Floor tom * Fluff * Fly * Flyware * FM * FOH * FOH engineer * FOH mixer * FOH position * Foldback * Folded horn * Follow-spot * Foot drum * Forced cooling * Form factor * Formant * Format * Formulae * Fourier analysis * Fourth-order * Frame * Free field * Frequency * Frequency agile * Frequency modulation * Frequency range * Frequency response * Frequency shifter * Frequency spectrum * Fresnel * Fret buzz * Front end * Front-fill * Front-fills * Front-loaded * Front-of-house * Frontline * FS * FST * Full-duplex * Full normalling * Full range * Full scale * Full space * Full step * Fully normalled * Fully parametric equaliser * Functional earth * Fundamental * Fuse * Fuzz * FX * FXQ

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.

F-hole
The sound hole of a member of the violin family of instruments. Each hole (of which there are two) is in the shape of a lower-case letter 'f', hence the name. A similar-shaped pair of sound holes is found on some types of semi-acoustic guitar.

Fade down
To reduce the level of a signal, either by using a fader or by a remote or automatic means that controls a VCA. Often, but not always, the term implies a gradual action rather than a rapid one.

Fade out
To reduce the level of a signal until it can no longer be heard, either by using a fader or by a remote or automatic means that controls a VCA. Often, but not always, the term implies a gradual action rather than a rapid one.

Fade up
To increase the level of a signal, either by using a fader or by a remote or automatic means that controls a VCA. Often, but not always, the term implies a gradual action rather than a rapid one.

Fader
A control, of a mixer or amplifier, that allows the relative level of a channel, a group or an overall mix, to be adjusted. It is usually of the slider type, with markings in decibels. Or, a slider control on a lighting desk whose function is to set the brightness of the lanterns − these controls usually have markings of 0 to 10. See also Potentiometer, Master control, Fade up, Fade down, Fade out, Travel, Taper, Noisy, Dirty (2), Unity gain, 0 dB (2), Trim (2), Motorised fader, Active fader, Cross-fader and Console tape.

Farad, Farads
The unit of capacitance, usually abbreviated to 'F', named after the inventor Michael Faraday. In static DC terms, it is the amount of capacitance required to limit the voltage rise to just 1 volt when storing 1 coulomb of charge. In terms of varying currents, it is the amount of capacitance required to give 1 amp of current flow (instantaneous value) in response to an instantaneous rate of change of voltage of 1 volt per second. In the case of AC, a capacitor's reactance is given by 1 /  (2 π f C), where f is the frequency in hertz and C is the capacitance in farads. As a farad is an extremely large amount of capacitance, more usual units are the microfarad (F, one millionth or 10−6 of a farad), the nanofarad (nF, one thousand-millionth or 10−9 of a farad) and the picofarad (pF, one million-millionth or 10−12 of a farad). Compare Henry.

Faston terminal
A trademarked name for the type of terminal described at Spade terminal (1).

Fat
See Phat.

Fat channel
A facility of a digital mixer, in which extended signal processing features (such as compression, noise-gating and comprehensive EQ) are provided on an individual per-channel basis. The adjustments for these features are usually provided via a common set of display-screen based controls; access is obtained by activating a button on the relevant channel strip. A similar set of extended features is sometimes also made available on a per-Aux and/or per-Group basis, with the adjustments being accessed in a similar manner.

Fault protection
In electrical safety, the collective name given to measures intended to provide protection against electric shock caused by indirect contact, that is, electric shock occurring as a result of leakage (or short-circuit) of current to conductive parts that are not intended to be live (at a dangerous voltage) in normal use.

This protection is provided by the connection of accessible conductive parts to a safety earth ('earthing' or 'bonding'), or by the provision of additional insulation between such conductors and internal parts that are intended to be live ('double insulation'). Additional protection may be provided by a suitable RCD, but this must never be the sole means of fault protection. See also Earth fault. Compare Basic protection.

FCC
An abbreviation for 'Federal Communications Commission', the U.S. government agency responsible for the control of electronic communications and communications equipment in the U.S.A., including the placing of requirements upon other types of equipment to avoid problematical levels of radio-frequency interference (RFI). See also EMC.

Feed
A supply of a signal to or from a particular point, or the cable (or radio channel, etc.) carrying that signal. May also be used to refer to a supply of mains electrical power. See also Routing, Line and Drive (1).

Feedback
The situation in which some of the output of an amplification system, such as a PA system, is accidentally or deliberately supplied back into one or more of its inputs. It usually refers to either:

  • An undesirable sound resulting from too high an overall gain around a complete loop through the system, from the microphone(s), through the mixer and power amplifier(s), to the speaker(s) and back (through the air) to the microphones again. It usually occurs at just a single frequency. Such undesirable feedback is sometimes referred to as 'howl-round' or 'howl-back' (written with or without the hyphen, or as two separate words) and is often heard as either a short duration high-frequency 'squeal' or as a slowly building low-mid note. For practical guidance on avoiding undesirable feedback, see this question on the FAQ page. For a more detailed analysis see Potential acoustic gain.
  • The intentional continuous sound generated by an electric guitar when held close to its backline speaker or combo (provided that the controls of the guitar and of the backline amplifier or combo are adjusted to assist this occurring). This is caused by the vibration of the guitar body and strings being reinforced by the sound coming from the speaker.

The above cases are examples of acoustic feedback, where sound waves through the air are involved at some point in the feedback loop. However, there are also other kinds of feedback, including:

  • Electrical − where the output signal of an amplifier is fed back into the input through purely electrical connections. Although some types of effect unit use this as the basis of their internal operation, the feedback must be very carefully controlled. Do not attempt such a connection yourself, or equipment may be severely damaged.
  • Magnetic − where a magnetic field is involved at some point in the feedback loop. An example of this would be the undesirable pick-up of the field from an induction loop by a guitar pickup.

All the above refers to 'positive feedback', in which the signal is fed back to an earlier point in the signal path in such a way as to reinforce the original signal. The continuous sound referred to as 'feedback' only occurs if the amount of positive feedback is greater than a critical value − the value that gives an overall in-phase gain of unity around the complete feedback loop. The dominant feedback frequency will generally be the frequency at which this condition is first satisfied, as the loop gain is increased. (When the gain around the loop is less than unity, but is approaching that value, the effect will be an increased resonance or 'ringing' of the sound, which reduces clarity.)

Therefore, feedback may be suppressed by any means that reduces the overall loop gain at the frequencies prone to feedback. In the case of acoustic feedback, such means could include one or more of the following:

  • Reducing the amount of amplification that the PA system gives to signals picked-up by microphones. In order to avoid a corresponding reduction in the sound level experienced by the audience, it may be possible to reduce the distance between the microphone(s) and their respective sound sources. (Alternatively, or additionally, it may sometimes be possible to reduce the distance between the speaker(s) and the audience.)
  • Reducing the number of microphones that are open at any one time (see NOM). In any case, it is good practice to ensure that any microphones not in use are muted.
  • Changing the position or orientation of the microphone(s) or speaker(s), in order to reduce the sound level from the speakers that is picked up by the microphones. In this regard the polar response pattern of each microphone and the directivity of the speakers will be need to be taken into account.
  • Considering whether at least some, if not all, of the monitor speakers can be eliminated by using in-ear monitoring instead. (Monitor speakers are frequently a significant contributor to feedback problems, and using in-ear monitoring can also help improve the front-of-house sound quality.)
  • Introducing attenuation into the signal path at the feedback frequencies (by use of equalisation facilities, typically a graphic equaliser) − see Feedback destroyer and Ringing out.
  • Modification of the room acoustics, for example by the introduction of additional absorption at the feedback frequencies.

For more tips on the avoidance of acoustic and magnetic feedback, see the feedback question on the FAQ page. Note: Non-technical personnel are liable to confuse the term 'feedback' with 'foldback'. See also Negative feedback.

Feedback destroyer
An item of equipment whose purpose is to assist in the elimination of acoustic feedback by the introduction of attenuation into the signal path at the feedback frequencies. It is usually connected between the mixer and the power amplifiers. These devices vary in sophistication and in effectiveness. See also Ringing out.

Female
Describes a connector that makes all or most of its connections by means of hollow contacts, into which the pins of the mating connector fit. A female connector is informally known as a 'socket' − especially when it is of the type that is fixed to equipment. It may also be described as a receptacle. See also Gender. Compare Male.

Ferrite
A magnetic material from which the cores of high-frequency inductors are often made. See also Longitudinal choke.

Ferrofluid or Ferro-fluid
A liquid used to assist in the cooling of high-power driver voice coils, by conducting heat from the coil to the surrounding permanent magnet assembly. It consists of specially-coated magnetic nano-particles suspended in a carrier liquid, and is held in place by the permanent magnetic field. Usage is generally limited to tweeters / horns and mid-range drivers, because in low-frequency drivers (woofers) the coil excursion is too large for the fluid to be held reliably in place. Its viscosity contributes to the forces that restrain motion of the voice coil and as such is part of the driver's dynamic (as well as thermal) design, therefore it cannot be added to a 'dry' driver retrospectively. See also Power compression.

Ferrous
Describes an iron-based material. Such materials are noted for their magnetic properties, and are commonly used in the cores of inductors and transformers.

Ferrule
A short metal tube that is compressed around the stripped end of a stranded cable core or wire, in order to hold the strands of the conductor together. This has the benefit of easing termination of the conductor, and may also improve the quality of the connection made.

FET
An abbreviation for 'field-effect transistor', a type of transistor in which the main flow of current is controlled by a voltage. This mode of operation is very similar to that of valves − except that FETs can operate at very low power levels and do not require the high DC supply voltages needed by valves. Their voltage-controlled characteristic makes them ideal for use in amplification circuits requiring a very high input impedance, such as the input stages of pre-amplifiers for condenser microphones and instrument pick-ups. FET power transistors are used as the output transistors in the output stages of some power amplifiers; these are most usually MOSFETs. Another specific type of FET is the JFET. Compare Bipolar junction transistor.

FFT
An abbreviation for 'fast Fourier transform', a computationally efficient technique for conversion of a signal from the time domain to the frequency domain, suitable for real-time applications such as frequency-domain digital filters. Based on the principles of Fourier analysis.

FHSS
An abbreviation for 'frequency hopping spread spectrum', a radio-frequency transmission technology used by some wireless communications systems such as wireless data links. It provides improved security of communication and improved immunity to radio-frequency interference (RFI).

Fibre-optic
See Optical interface.

Fidelity
The degree to which a reproduced sound (or visual image) remains faithful to the original acoustic (or visual) source. That is, the degree to which the original source programme is accurately reproduced by equipment for recording, playback, amplification or signal processing, or by a speaker. See also Hi-Fi and Transparent.

Field (1)
An abbreviation for 'jack field'.

Field (2)
In a video display system, the set of raster lines that are scanned consecutively in one sweep from the top of the picture to its bottom. In a progressively scanned system a single field encompasses all the lines of the raster frame. In an interlaced system, however, a field consists of alternate raster lines and so two interleaved fields (an odd field and an even field) are required to complete the frame. Therefore, in the UK 625-line standard-definition broadcast video system having a frame rate of 25 Hz, the field frequency is 50 Hz. See also the next definition.

Field of view
The usable viewing angle of a display screen. See also Screen gain.

Field sync
In an analogue video signal, the synchronisation pulses that indicate the start of each field of the picture. In the UK 625-line standard-definition broadcast video system, the pulses occur at a frequency of 50 Hz, an interval of 20 ms. The field sync pulses are arranged as a group and are preceded and followed by a group of 'equalisation pulses' (no connection with audio equalisation). Both field and line sync pulses occupy the negative-most 0.3 volts of a standard 1 volt peak-to-peak video waveform. See also Composite video and Frame (2).

Fifth
An interval of musical pitch that corresponds to a frequency ratio very close to 1.5 (in practice the value is closer to 1.4983). So, two frequencies are said to be a fifth apart when one frequency is 1.5 times (or, of course, 2/3 of) the other.

For the musically minded, a fifth is so-named because this interval is reached at the 5th note of a tonic musical scale. Between the lowest and highest of these 5 notes are 4 intervals, made up of 3 tones and 1 semitone. Since a tone is a ratio of the sixth root of 2, and a semitone is a ratio of the twelfth root of 2, we can see that multiplying out these 4 intervals (i.e. 21/6 x 21/6 x 21/6 x 21/12) gives a resulting ratio very close to a value of 1.5, as initially stated. See also Octave. Compare Third.

Fifth-order
See Order.

Figure-of-8 cable
An un-screened two-conductor cable which does not have a sheath. The two insulated conductors are laid side-by-side (without a twist) and the touching edges of the insulation are bonded together along the length of the cable, giving the cross-section of the cable the appearance of a number 8. The bonding can easily be torn apart at the ends of the cable, to separate the conductors for stripping and terminating. As the insulation of the two conductors is usually the same colour (generally white or grey), the surface of one conductor's insulation is usually marked with a stripe or a raised rib, for identification purposes. The lack of a sheath means that this type of cable has inadequate mechanical protection for rough stage use, so it is generally used only for permanently installed interconnections. Also called 'zip cable'.

Figure-of-8 microphone
See Bidirectional microphone.

Filament lamp
See Incandescent lamp.

Fill
Supplementary sound, provided specifically to cover areas that are not adequately covered by the main front-of-house or monitor system(s)i.e. to 'fill in' the gaps in the coverage. Or, a term used to describe the speakers or other equipment associated with providing that sound (see the next definition). Specific types of fill include front-fill, side-fill and downfill.

Fills
The supplementary speakers that are provided specifically to cover areas that are not adequately covered by the main front-of-house or monitor system(s), i.e. to provide fill sound − see the previous definition.

Filter (1)
A device that applies a different amount of attenuation to the different frequencies contained in a signal, usually in order to remove (or to very substantially reduce in level) some unwanted part of the signal, or to select a specific part of the frequency range. Filters may be passive or active. See also Equaliser, Crossover, Low pass, High pass, Low cut, High cut, Roll-off, Slope, Bandpass, Bandstop, Cut-off frequency, Centre frequency, Passband, Stopband, Order, Brick wall filter, RC filter, LC filter, Chebyshev, Butterworth, Bessel, Linkwitz-Riley, FIR and IIR.

Filter (2)
A device, used within a lantern, which allows only certain wavelengths of light to pass and therefore gives the emitted light a specific colour. Also called a 'gel' (short for gelatine, a material from which filters may be made).

Filter (3)
A component of equipment that incorporates forced cooling, provided to reduce the amount of dust drawn into the equipment by the flow of air. It is important that filters are cleaned (or changed) sufficiently regularly to avoid the airflow being being significantly impeded by the build-up of dust in the filter, as failure to do so may result the equipment overheating.

Filter classes
See Chebyshev, Butterworth and Bessel. See also Linkwitz-Riley.

Filter order
See Order.

Filter slope
See Slope.

Finalise
The process of creating the table of contents (TOC) on a recordable audio compact disc (CD-R) or on a re-writable audio compact disc (CD-RW). This process is necessary in order to make the disc playable on an ordinary CD player, though CD recorders can usually play discs without a TOC. No further tracks can be added to a finalised CD, but a finalised CD-RW may be unfinalised in order to allow tracks to be added or erased.

Or, the process of creating the table of contents (TOC) on a recordable DVD (DVD-R or DVD+R) or on a re-writable DVD (DVD-RW or DVD+RW). This process is necessary in order to make the disc playable on an ordinary DVD player, though DVD recorders can usually play discs without a TOC. No further titles can be added to a finalised DVD. Compare Unfinalise. See also PMA.

FIR
An abbreviation for 'finite impulse response'. Describes a time-domain digital filter which incorporates only feed-forward paths. This avoids the stability and rounding error problems that can sometimes be encountered with IIR types and enables designs with a linear phase response to be implemented, giving constant delay across the frequency response. The name arises because the filter's response to an impulse input is time-limited.

Fire muting
An arrangement designed to interrupt one or more audio signal paths of a PA system in the event of an activation signal from a fire alarm system, so as to enable the alarm sounders to be clearly heard and to facilitate orderly evacuation of the building. Or, the facility of a power amplifier or of some other item of equipment that is involved in such an arrangement.

FireWire
See IEEE 1394.

Firmware
Software that is permanently or semi-permanently stored within an electronic component in an item of equipment. It usually provides one or more functions essential to the operation of the equipment. For example, a powered speaker that incorporates digital signal processing (DSP) will contain firmware that defines the DSP function(s). Firmware is typically stored in either read-only memory (ROM) or flash memory. In either case, this memory may be provided by a separate IC or may be contained within the processor IC. When stored in flash memory, the equipment may provide the facility for updates to be made to the firmware by downloading from an external source.

First generation
Describes an original recording − one that is not a copy. See also Generation.

First-order
See Order.

Fixture
See Lantern.

FLAC
A method for lossless compression of digital audio files, popular with some audiophiles. It stands for 'free lossless audio codec'. A very limited number of recordings in this format are commercially available on CD. See also SACD and HDCD.

Flanger
An effect unit which provides a sweeping kind of effect, often fairly extreme and harsh-sounding. Used mostly by guitarists.

Flanges
See 19 inch rack system.

Flat (1)
Describes the frequency response of an item of equipment, or of a part of an item of equipment (especially equalisation controls), when any change in signal level that it causes is applied equally (or approximately so) to all frequencies being handled, over the frequency range of interest. That is, is applies an equal gain or loss (if any), or (in the case of a microphone or speaker) has an equal sensitivity, at all frequencies of interest. For example, if some equalisation controls were set so as to have no effect on the signal they handle, they would be described as being 'set flat'. (This is most evident in the case of a graphic equaliser, where the position of the slider controls would physically form a flat line.)

Most types of audio equipment, notably most power amplifiers and speakers, strive in their design to reach the ideal of a flat response across their intended frequency range of operation, though technical and/or cost-related factors may mean that this is not entirely achievable in practice. However, some types of microphone are intentionally designed with specific regions of boost or cut in their frequency response − see for example Presence peak. See also Colouration, Tone (1), Low cut, Cut-off frequency and Proximity effect.

Flat (2)
A movable vertical surface on which stage scenery is painted. See also Scene (1), Set (2) and Cyclorama.

Flat (3)
Describes a musical note whose pitch is lower (usually, just a little lower) than it should be. See also Bum note. Compare Sharp.

Fletcher-Munson curves
A set of graphs that indicate how the human ear's frequency response typically varies with varying loudness. Strictly the term refers specifically to the set of graphs originally plotted by H. Fletcher and W. Munson in the 1930's. However, the term is commonly used to refer to other more recent sets of graphs that plot the variation in frequency response more accurately. Such sets of graphs are better referred to as 'equal loudness curves', or 'equal loudness contours', as they are presented as graphs of sound pressure levels (SPL), of differing frequency, that are subjectively judged to have the same loudness. Furthermore, a differently shaped curve is apparent depending upon the value of loudness in question. For full details see Weighting.

Flight case
A strong box, usually having handles, metal edging strips and a padded interior, for protecting equipment in transit. It often also has castors (wheels). See also Loom box. Compare SKB.

Float (1)
In digital audio, an abbreviation for 'floating point format'. A standardised word format in which the available bits are split into two groups − a group (called the mantissa) which specifies a value within a normalised range (typically between 0 and 1) and a group (called the exponent) which specifies the number of 0's to be appended to the mantissa to give the required overall value. This format is normally only used in interfaces between the software modules of digital audio packages (see DAW); it is not used over physical interconnections.

The float format allows very much larger values to be conveyed by a given number of bits, so hugely increasing the available dynamic range and thus avoiding overflow errors during digital signal processing. However, this advantage is at the expense of a less accurate representation of the sample values than would be obtained had all the bits been used in a linear manner, so the float format is not usually used with bit depths of less than 24 bits.

Float (2)
To arrange for the signal earth of an interconnection to be floating.

Float switch
See Earth lift.

Floating
Describes an interconnection, or an input or output, whose conductors have no low-impedance path to signal earth. For example, the output connection of a dynamic microphone, a MIDI input connection or an earth-free transformer-balanced input or output. See also Balanced, Earth lift and Quasi-floating.

Floor
The lowest level reached by a signal (compare Peak). Or, short for 'noise floor'.

Floor monitor
A monitor speaker that is designed to rest on the floor. Usually wedge-shaped, so often referred to as a 'wedge' (or sometimes as a 'slant').

Floor tom
Part of a drum kit; a large low-pitched drum with short legs that stand on the ground. See also Kick. Compare Rack tom.

Fluff
To make a mistake during a performance, especially in a vocal or spoken line or in an instrumental solo. See also Bum note.

Fly
The act of suspending something (usually speakers) overhead. Requires careful attention to safety. See also Rigging, SWL and the next definition.

Flyware
The hardware used for the flying of equipment, especially speakers. See also SWL and Rigging.

FM
An abbreviation for 'frequency modulation'. See Modulation.

FOH
An abbreviation for 'front-of-house'. Frequently used by sound engineers and related personnel as an abbreviation of 'FOH position'. Also, an abbreviated name for Front Of House magazine, an on-line and printed magazine for the professional sound reinforcement and production industry (not UK-specific). Their website is www.fohonline.com (external link, opens in a new window). In relation to the latter definition, see also PLSN and EPD.

FOH engineer
A sound engineer who is responsible for the front-of-house sound. Compare Monitor engineer.

FOH mixer
Short for 'front-of-house mixer' − the mixer used to mix the front-of-house sound (as distinct from the monitor mixer). The term may refer to a mixer that has been temporarily set up for a particular event, or to a permanently installed house system mixer. The latter may sometimes be distinguished by being referred to as the 'house mixer'.

FOH position
The location of the mixer (and related equipment) for front-of-house sound − typically a central or near-central position towards the rear of front-of-house. Often abbreviated by sound engineers and related personnel to just 'FOH'.

Foldback
An alternative term for a monitor mix. Note: Non-technical personnel are liable to confuse the term 'foldback' with 'feedback'.

Folded horn
A speaker in which the driver faces back into the enclosure, which contains a horn made up of surfaces that are angled or curved so as to direct the sound to the aperture at the front of the enclosure. An arrangement most usually found in bass bins and lower-mid-range speakers, where folding the horn is a useful way of accommodating its large size.

Follow-spot
A high-power spotlight that is readily moveable during a performance, to enable the movements of an on-stage performer to be tracked. Usually has its own operator, who physically points the spotlight as instructed via a communications link.

Foot drum
An alternative name for a kick drum.

Forced cooling
The use of one or more fans to assist in the cooling of equipment by increasing the air-flow through it. Usually mounted internal to the equipment, and used in conjunction with a heatsink. Some types of forced-cooled equipment, especially power amplifiers, incorporate fans that operate only when required, or which operate at a variable speed depending on the changing cooling requirements during use. The flow of air must not be obstructed, or the equipment may overheat and be seriously damaged. Where air filters are fitted to the inlet apertures, these filters must be cleaned at appropriate intervals. Compare Convection-cooled.

Form factor
The ratio of a waveform's RMS value to its average value. (Note that in determining this average value, negative parts of the waveform are considered to be positive.) Most often used of repetitive waveforms, for example the form factor of a sine wave is approximately 1.11 (0.707 divided by 0.636). See also Level. Compare Crest factor.

Formant
A region in a natural sound's frequency spectrum containing particularly high energy levels, caused by acoustic resonance of the space(s) in which the sound is created or through which it passes. The formants of the spoken word (or song) vary during vocalisation, and are particularly important in the identification of different vowel sounds. In the case of acoustic instruments, the formants are an important contributor to the timbre of the instrument. Significantly, the formant frequencies are essentially independent of the pitch being sounded by a particular person's voice or by a particular instrument.

Format
Literally, 'layout' or 'structure' − the agreed scheme or rules according to which information is arranged, coded or represented for conveying between points as a signal, or for storage, or according to which information is presented. Two examples of video formats are PAL and SVGA. Many formats in use are specified by a published standard, which assists manufacturers in ensuring that their equipment will be compatible when interconnected. See also Scaler. Compare Protocol.

Formulae
For general audio and electrical formulae see this entry on the FAQ page. For decibel formulae see the Decibels page.

Fourier analysis
The determination of the individual frequency components of a complex waveform. Named after the mathematician Fourier, who established that any continuous repetitive waveform may be considered to consist of a number (often large) of individual sine waves, in specific proportions, each of which is an exact multiple (from 1 upwards) of the repetition rate or 'fundamental frequency' of the waveform. The multiples from 2 upwards are called harmonics. This process is used in digital signal processing to enable the manipulation of waveforms in the frequency domain − see FFT.

Likewise, it is possible to synthesise (i.e. construct) any waveform by the addition of harmonics, in the appropriate proportions, to the fundamental. This approach to sound generation is used in some electronic keyboard instruments. See also Spectrum analyser, MLSSA and Principle of superposition.

Fourth-order
See Order.

Frame (1)
In a digital bit-stream or recording, a sequential collection of bits having a means to identify its start and having a defined internal format. Frames follow on one after the other ad-infinitum.

In digital audio tape-recording (DAT), frames of data are recorded at the rate of 100 frames every 3 seconds. Each of these frames consists of 1440 16-bit samples of the Left channel and 1440 16-bit samples of the Right channel, plus subcode data. (So, the maximum supported sample-rate is 48 kHz.) The frames are uniquely identified by absolute time code (ATC) stored in the subcode data.

In AES3 and S/PDIF bit-streams, a frame of data consists of two sub-frames, one for the Left channel and one for the Right channel. Each sub-frame contains 32 bits of data, of which between 16 and 24 (usually 16) constitute a single audio sample; the remainder contain subcode data and other control and synchronising information. The frame rate is the same as the sampling rate in use (32, 44.1 or 48 kHz). 192 of these frames make up a 'block', which constitutes a complete structure of the subcode information. (Note, however, that the subcode structure varies between AES3 and S/PDIF.)

Frame (2)
In a video display system, one complete scan of the picture, encompassing all the lines of the raster. In an interlaced system, a frame consists of two interleaved fields of lines. Therefore, in the UK 625-line standard-definition broadcast video system having a field frequency of 50 Hz, the frame rate is 25 Hz. See also Field sync.

Free field
Describes a location at which direct sound energy from a particular sound source predominates over the indirect sound energy from that source. Within the free field, the inverse square law applies. Compare Diffuse field and Near field. See also Radius of reverberation and Critical distance.

Frequency
A measure of how rapidly something is repeating. In relation to an audible sound or signal that has a repetitive waveform, its frequency is a measure of how rapidly each cycle of the waveform is repeated − this determines its musical pitch. The unit of measurement is the hertz (usually abbreviated to 'Hz'), which is a rate of repetition of one cycle per second.

In the case of a sine wave at a constant level, just a single frequency is present. In the case of other repetitive waveforms at a constant level, several (possibly many) frequencies are present simultaneously, each at a constant level. The lowest frequency present, called the fundamental, determines the repetition rate of the cycles while the other frequencies determine the shape of the waveform. These other frequencies, called harmonics, are all whole multiples of the fundamental frequency and in the case of an audible sound their relative levels determine the timbre of the sound.

In the case of non-repetitive waveform such as an analogue audio programme signal, many frequencies, possibly covering a wide range, are likely to be present simultaneously at constantly varying levels.

The effect of linear frequency-dependent processing, such as equalisation, on a signal in which multiple frequencies are present simultaneously is to act on each frequency present independently, i.e. regardless of the presence of the other frequencies − see Principle of superposition.

The frequency of a repetitive waveform is inversely proportional to its period: the frequency in Hz is equal to 1 divided by the period in seconds. Frequency is also inversely proportional to wavelength: the frequency in Hz is equal to the velocity (speed) of the wave in metres per second divided by the wavelength in metres − for sound waves in air see Speed of sound.

The human ear can detect sounds in the approximate frequency range of 20 Hz to 20 kHz, though this varies from person to person and depends on the level of the sound. Put another way, the ear's sensitivity varies with frequency in a level-dependent manner − see Fletcher-Munson curves. In general, ability to hear frequencies towards the upper end of the range decreases with age − see Presbycusis.

Middle C on a piano ('C4') has a fundamental frequency of approximately 262 Hz, and the 'A' above middle C ('A4') has a standardised frequency (at 'concert pitch') of exactly 440 Hz. See also Cycle, Period, Wavelength, Bass (1), Treble, Mid-range, Band (1), Audio-frequency, Radio-frequency, Crossover frequency, Cut-off frequency, Centre frequency, Resonant frequency, Equaliser, Graphic equaliser, Octave, Fundamental, Harmonic, Pitch (1), Filter (1), Bandwidth, Weighting and Fourier analysis. Compare Rate.

Frequency agile
Describes a radio system (such as is used within radio microphone and in-ear monitoring equipment) that is user-configurable to operate on any of a number of different radio frequencies, or 'channels'.

Frequency modulation
See Modulation.

Frequency range
Frequencies that lie between specified (or implied) minimum and maximum values. Also called a 'frequency band'. In PA work, the term usually refers to the common audio-frequency ranges − see Sub-bass, Bass (1), Mid-range and Treble.

Frequency response
An indication of the way in which the output level of an item of equipment is dependent upon the frequency of the input signal, given that the input level remains constant as its frequency is varied through the entire range of interest. Most often, the term is used as an indication of the range of frequencies which are adequately handled by the equipment, and in this case it is usually quoted as the upper and lower frequencies at which the output level of the equipment has reduced by 3 dB, relative to its output level at some mid-band reference frequency (often 1 kHz, for audio equipment). These are referred to as the '−3 dB' frequencies, and are the frequencies at which the output would give half the power (not half the voltage) of that obtained at the reference frequency, given the same level of input. (See the Decibels page for more information on decibels.)

Because such a substantial drop in output would usually be unacceptable even at the limits of the audio range, and because the drops occurring in several items of equipment will accumulate when they are connected together in the signal chain, it is usual to find frequency response figures quoted that are outside the actual frequency range of interest. For example, an amplifier with a quoted −3 dB frequency response of 10 Hz to 50 kHz is likely to have a response between, say, 20 Hz and 20 kHz that dips only marginally below the output obtained at 1 kHz. See also Curve, Cut-off frequency, Flat, Tone (1), Slope and Slew rate.

Frequency shifter
A device used to increase the amount of amplification possible before the onset of acoustic feedback. It achieves this by causing the sound from the speakers to be offset in frequency relative to the sound source − typically by around 5 Hz. Good for speech (e.g. presentations and theatre), but of very limited use for music because the all-important harmonic relationship between the frequencies is disturbed. For example, after frequency shifting a note at 400 Hz upwards by 5 Hz, it is no longer an octave above a note at 200 Hz that has been shifted by the same amount, because although 400 is exactly 2 x 200, 405 is only approximately 2 x 205.

Frequency spectrum
See Spectrum.

Fresnel
A type of lantern that uses a specific kind of lens, having a 'stepped' surface. This lantern gives a diffuse beam of light with soft edges. Named after its inventor, and pronounced 'fren-ell'. See also Wash light.

Fret buzz
An undesirable sound caused by a vibrating guitar string contacting one or more frets that it should be clear of, or improperly contacting a fret that it should be in hard contact with. Sometimes this sound can be confused with unwanted distortion of the guitar signal.

Front end
The input stage of a radio receiver, handling the radio-frequency signal direct from the reception antenna or antenna distribution unit.

Front-fill, Front-fills
The additional sound that is required in order to provide fill at the very front of the audience. Or, describes the supplementary, lower-power front-of-house speakers placed so as to provide that coverage. Such speakers may be flown, placed on the front edge of the stage apron, mounted on stands or wall-mounted, as appropriate, and are generally referred to as the front-fills. See also Downfill and Side-fill.

Front-loaded
Describes a speaker enclosure that has its driver(s) mounted into it from the front. This is the most common method of mounting drivers in modern enclosures.

Front-of-house
The area forward of the stage − i.e. the area occupied by the audience. Commonly abbreviated to FOH. In PA work, the term is most often used in reference to producing the sound for that area (contrasted with sound for the stage area), as in 'the FOH mix' or 'the FOH speakers'. Technical personnel often use the term to refer to the specific location of the FOH mixing position, as in "I left my SLM at FOH". However, the term may also be used in non-PA contexts, such as to refer to the lighting serving the audience area. See also the next definition and Auditorium, House (and following terms such as House mixer), Booth, Monitor mixer and Backline.

Frontline
The speakers directed at the audience, more frequently described as 'front-of-house' (FOH). Compare Backline.

FS
An abbreviation for 'full scale'; see, for example, dB FS on the Decibels page. See also Over.

FST
An abbreviation for 'foil-screened twin'; a type of balanced signal cable intended for fixed installation use. It has a foil screen (usually aluminium-based), surrounding the signal pair and a drain wire. Some versions have solid-cored conductors, suitable for use with insulation displacement terminations, while some have stranded-cored conductors. Very small diameter versions of this cable are available, useful to conserve space in locations where many cables are routed together. See also STP.

Full-duplex
See Duplex.

Full normalling
See Normalling.

Full range
Describes something which adequately covers the majority of the audible frequency spectrum of 20 Hz to 20 kHz, and is therefore suitable for use without the need to provide additional equipment covering specific parts of the spectrum. Usually used as a description of speakers.

In practice, equipment such as speakers rarely covers the whole of this frequency range − see Frequency response. However, a speaker with an effective frequency response of, say, 50 Hz to 18 kHz would still be described as 'full range' because, in appropriate circumstances, it could be used without additional speakers to cover the missing parts of the spectrum at the extreme top and bottom ends.

As a single type of driver is unable to satisfactorily reproduce the full audio frequency spectrum, a full-range speaker usually contains at least two (sometimes three) different types of driver. The bass drivers are called woofers and the treble drivers are called horns or tweeters. (Mid-range drivers have no particular name.) It is essential that each type of driver is supplied only with the relevant band of frequencies. This is achieved either by use of a passive crossover to split the output of a single amplifier, or by use of separate amplifiers for each frequency band, in which case the amplifiers are fed from an active crossover − see Bi-amping.

Full scale
The highest possible value of some quantity, or the highest value that may be registered, often abbreviated to 'FS'. For example, see dB FS on the Decibels page. See also Over.

Full space
A measurement environment in which the whole of the space around the item under test is considered. Most often used in anechoic measurements of speakers, where it simulates the speaker being used away from any reflective wall and so gives lower sensitivity figures at bass frequencies. Also referred to as 4-pi space. Compare Half space.

Full step
An alternative term for a Tone (3). Compare Half step.

Fully normalled
See Normalling.

Fully parametric equaliser
See Parametric equaliser.

Functional earth
See Technical earth.

Fundamental
The frequency of which all other members of a set of frequencies are harmonics, i.e. are whole multiples. (The fundamental frequency may or may not itself be present in the set.) When such a set of frequencies, present simultaneously at constant levels, consitute a sound wave, the wave will have a repetitive waveform. The fundamental frequency will determine the perceived pitch of the sound, while the relative levels of the harmonics will determine its timbre.

Mathematically speaking, the highest possible fundamental of a set of harmonics is the highest common factor of the set. For example, the set of harmonics at 600 Hz, kHz and 1.4 kHz could have 200 Hz as their fundamental (in which case those harmonics would be the 3rd, 5th and 7th). For further information see Harmonic.

Fuse
A circuit protection device whose function is to automatically interrupt the flow of current if its value becomes excessive, for example in the case of an overload or short circuit. Such an automatic interruption is commonly referred to as 'blowing' of the fuse, or more formally as 'operation' or 'rupturing' of the fuse, and necessitates its replacement. To adequately protect against the risk of fire, it is essential to ensure that fuse ratings are properly co-ordinated with the maximum current rating of the associated cabling and connectors, for example in accordance with BS 7671. A blown fuse must always be replaced with one of the correct type and rating.

Typical fuse ratings used in mains distribution circuits in the UK are 5, 15, 20, 30, 45 and 60 A. Fuses are also used in BS 1363A mains plugs and within items of equipment − sometimes fuse-holders are mounted on a panel of the equipment, enabling convenient fuse replacement. Fuse ratings are the values of current that can be carried continuously by the fuse without operating (blowing) it; the current required to operate it will be substantially higher than this figure. (Note that is in contrast to the nominal residual current rating of RCDs.)

Fuses have other important parameters besides their current rating. These include their physical dimensions, voltage rating, breaking capacity and speed of operation. The breaking capacity is the maximum fault current that the fuse is capable of interrupting, and is usually classified as low ('LBC') or high ('HBC'). The speed of operation relates to the fuse's ability to tolerate very short-duration current levels in excess of its rating, and is usually classified as fast or slow − see Temporised fuse.

It should be remembered that operation of a fuse may indicate a serious fault, and therefore the reason for operation should always be investigated.

In many mains distribution applications MCBs are now used in preference to fuses, because of their ease of resetting and their very rapid tripping under short circuit conditions. In some equipment, especially those with high inrush currents such as power amplifiers, a thermal circuit breaker is used in place of a mains input fuse. See also Distro, Distribution board, RCD and Ring main.

Fuzz
A now little-used term for intentional distortion introduced as an effect, usually for an electric guitar. The unit to produced this effect is called a 'fuzz box' (now more usually called a 'distortion pedal').

FX
In PA work, an abbreviation for 'effects'. However, other technical staff may use this abbreviation to refer to special effects.

FXQ
An abbreviation for 'effects cue'. This most commonly refers to special effects, rather than to sound effects associated with the PA system. Compare SXQ. See also LXQ.

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