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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.

Haas effect * Half-duplex * Half normalling * Half-octave * Half-power bandwidth * Half space * Half stack * Half step * Half track * Hall * Handling noise * Handover * Handshake * Hard-wired * Hard clipping * Hard knee * Hard panning * Hardware * Harmonic * Harmoniser * Harmony * Harness * Harp * Hats * Hazard * Hazard tape * Hazardous area * HBC * HD * HD video * HD15 * HDCD * HDCP * HDMI * HDTV * Head * Head amplifier * Headphone amplifier * Headphones * Headroom * Headset * Hearing enhancement * Hearing loop * Heater * Heatsink * Helical antenna * Henry * Henrys * Hertz * Hex * Hexadecimal * HF * HF driver * Hi-Fi * Hi-Z * High cut * High end * High frequency * High hats * High impedance * High pass * High resistance connection * High-Z * Higher mid-range * Highs * Hiss * Hitting the rails * HM * HMI * HMID * Hold time * Hook clamp * Horn * Horn protection * Hot * Hot condition * Hot standby * Hours * House * House lights * House mixer * House system * House tabs * HoW * Howl-back, Howl-round * HPF * HRC * HRTF * HSE * HT * HTP * Hum * Humbucker * Humidity * Hummer * Hundred-volt line * HVAC * HX * Hybrid * Hyper-cardioid * Hysteresis * Hz

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.

Haas effect

Named after the person that first documented it, this effect explains why sound seems to be coming only from the nearest speaker, when others not very much further away are reproducing the same (or a similar) sound, at the same level − or even up to about 10 dB louder. The reason is that this situation is similar to the natural-life situation in which we hear sounds not just from their point of origin, but also echoes from nearby surfaces − which always arrive at our ears slightly later than the sound that travelled in a straight line direct from the source.

So that we are able to readily identify the true source of sounds (which could be a matter of life and death), our ears and brains are designed to "not hear" such close-following echoes (those less than around 40 to 50 milliseconds after the original sound, depending on the relative levels), and so to disregard their apparent location. This reasoning does not apply to later echoes, because of the increased probability that they are actually not echoes at all, but rather are new sounds of the same kind, that may need to be separately located.

This effect partly explains why a stereo sound is never as good when the distance between the listener and the Left speaker differs by more than a few metres from the distance between the listener and the Right speaker − even if the relative level of the two speakers is adjusted to compensate.

It is sometimes utilised in PA applications in which, for most of the audience, the amplified sound is no more than 10 dB louder than the original sound source (such as some theatrical and operatic productions). In such cases, introducing a delay of about 30 milliseconds into the signal has the effect of helping the audience to focus on the original sound source rather than on the sound coming from the speakers.

Also called the 'precedence effect'. See also Speed of sound, Delay (2) and Psychoacoustics.


See Duplex.

Half normalling

See Normalling.


Describes a graphic equaliser whose controls provide adjustment of frequency bands that are spaced half an octave apart. For further details see Graphic equaliser.

Half-power bandwidth

The bandwidth measured between the two frequencies (upper and lower) at which the output power level of an item of equipment would fall to a half of its highest value, if a constant-level sine wave were applied to the input and swept across the whole frequency range of interest. Or, the bandwidth between the two frequencies at which the response is reduced by 3 dB, as compared to the highest (or average) response at mid-band frequencies. This is the method conventionally adopted when specifying the frequency response of equipment or systems. See also Q (1).

Half space

A measurement environment in which only the space in front of the item under test is considered. Most often used in anechoic measurements of speakers, where it simulates the speaker being used against a reflective wall and so gives higher sensitivity figures at bass frequencies. Also referred to as 2-pi space. Compare Full space.

Half stack

See Stack.

Half step

An alternative term for a Semitone. Compare Full step.

Half track

See Reel-to-reel.


A setting provided on most digital reverberation and delay-based effects units. This setting selects simulation of the natural reverberation of a performance hall. Several variants are often provided, usually to allow different sizes of hall to be simulated (e.g. 'small hall', 'medium hall' and 'large hall'). The simulation may include some pre-delay. See also Plate and Spring line.

Handling noise

The unwanted signal produced by a microphone as a result of it being handled or knocked during use.


The process of passing responsibility for a system from one person (or team, or organisation) to another; this is usually a formal process. For example, at the completion of a newly installed or assembled system, after safety testing and operational verification, responsibility for the system may be handed-over from its installer(s) or assembler(s) to its operator(s). The term may be used of both temporary and permanent systems. See also Briefcase engineer.


An intitial brief interchange of data between items of equipment, that occurs shortly after communication is established between them by means of a cabled interconnection or a wireless link. If an interconnection is already in place, then the handshake occurs when the items are switched on or are (re-)initialised. Its purpose is to enable the items to determine each other's identity and to negotiate the protocol (or its version) that will be used for subsequent communication between them. The term is most usually used in respect of just two communicating items.


Describes a direct, physical, galvanic connection between conductors, usually one that is intended to be permanent or semi-permanent. So, such a connection would usually be made without the use of a connector, and typically would be soldered, screw-clamped or crimped. See also Terminal, Binding post and Barrier strip.

Hard clipping

Clipping in which the 'flattening' of the waveform commences abruptly at a certain point in the waveform, and no further increase in instantaneous voltage is possible beyond that point. For further information see Overload (1). Compare Soft clipping.

Hard knee

See Knee.

Hard panning

The practice of setting a pan control fully, or nearly fully, to its extreme Left or Right position. See also Stereo image.

Hardware (1)

In computers and computer-controlled equipment, hardware refers to the physical parts of the equipment, as compared to the software and firmware that runs on it. For example, the CPU, memory, disk storage, display, etc.

Hardware (2)

In installation work, this term usually refers to items such as fixing screws, bolts, nuts, washers, brackets, racking, chains, etc, as compared with the main items of equipment or cabling being installed. However, the term may also be used much more generally, with a similar meaning to 'equipment'; in this sense there is some overlap with the meaning given in Hardware (1).


A frequency that is an exact whole multiple of some other frequency, called the 'fundamental'. A single musical note usually contains many harmonic frequencies simultaneously, all of which are multiples of the same fundamental; the term is therefore most often used in the plural. Strictly, the term 'harmonics' includes the first harmonic, i.e. the fundamental frequency, but is most frequently used in reference to the higher harmonics.

The term 'overtone', more commonly used by musicians, has an equivalent meaning except that it refers only to whole multiples greater than one, i.e. it excludes the fundamental. Thus, the first overtone is the second harmonic. For the same reason as 'harmonics', this term too is most often used in the plural.

An appropriate amount of the right harmonics adds a 'richness' to what would, in the absence of any harmonics, be a very 'plain' sounding pure note − a sine wave. However, an excess of odd-numbered harmonics (i.e. 3 times, 5 times, etc.), will introduce a discordant harshness to the sound. So, the level of each of the harmonics, relative to the level of the fundamental, determines the tonal quality, or 'timbre' of the note, while the frequency of the fundamental establishes the note's basic musical pitch. The human ear is able to perceive a fundamental that is completely missing from a sound, based solely on the harmonics that are present. See also Distortion, Fourier analysis and Psychoacoustics.


An effects unit intended to add one or more musical harmony parts to a signal, so as to simulate the effect of one or more additional harmonising notes being played or sung. Sometimes such artificial harmonies are most effective in conjunction with a little delay and/or chorus effect, which may be provided by the same unit. Most commonly used on lead guitar and on vocals.


Usually refers to an additional vocal part in a song, that sings the same words as the melody part, at the same time, but at different notes. Often sung by backing vocals. Harmony parts are frequently a third or a fifth above or below the melody part. Harmony parts may sometimes be added artificially by use of a harmoniser. See also SATB and Unison. Compare Melody.


See Loom.

Harp (1)

A large many-stringed instrument that is plucked with the fingers of both hands. There are two common types:

Harp (2)

A slang term for a harmonica, or 'mouth-organ'. The term can be considered to be an abbreviation of 'blues harp', and usually refers to the diatonic instrument, not the chromatic type.


Short for high hats.


A source of potential danger. For example, a person working at height, or the presence on the floor of objects over which persons may trip. N.B. This definition of the term may differ from officially recognised definitions. See also Risk. For further information on safety see the Safety page.

Hazard tape

A brightly coloured self-adhesive tape (usually striped black and yellow) intended to clearly mark hazardous areas such as trip hazards, low ceilings, etc. For example, cables might be secured to a floor using gaffer tape and then their presence highlighted by means of hazard tape stuck to the floor on either side of the cable run. Sometimes referred to as 'caution tape'. For general information on safety see the Safety page. See also Zebra tape. Compare Barrier tape.

Hazardous area

A location in which the risk from expected hazards is unusually high, or in which unexpected or unusual hazards are present. Such areas may require the provision of specific signage and/or barriers to warn against or prevent unauthorised access. When considering such a designation, consideration must be given to the level of risk likely to be perceived by each kind of person who may enter the area, and the competence of each kind of person to avoid consequent danger to themselves and others. See also the previous definition and Risk assessment.


An abbreviation for 'high breaking capacity', which describes a fuse that is able to interrupt large fault currents. HBC fuses usually have a ceramic body, and may be marked with an 'H'. Also known as an HRC (high rupturing capacity) fuse, though that term is generally used only with much larger fuses. Compare LBC.

HD (1)

An abbreviation for 'hard disk' (or 'hard drive'). This term is used to describe equipment that employs a hard disk drive for data storage, such as the many multi-track recorders that now use this technology.

HD (2)

An abbreviation for 'high density' − a description of a large quantity of something packed into a small space. For example, it is applied to the small 15-pin D-sub connector used for VGA interconnections.

HD (3)

An abbreviation for 'high definition' − a description of improved quality, especially in the sense of an improved resolution. Applied to both video (e.g. see HDTV) and audio (e.g. see HDCD). In the case of video, see also UHD.

HD video

Usually refers to video at a resolution of either 720p or 1080i, though there are other variants − see HDTV for further details. See also UHD.


A name commonly used for a DE-15 15-pin high-density D-sub connector, also referred to as a 'VGA connector'.


An abbreviation for 'high definition compatible digital' (not 'high definition compact disc', as commonly supposed). A complex encoding scheme for enhancing the characteristics of compact disc recordings, especially in regard to distortion and transient response. Although HDCD encoded discs can be played on standard CD players, the sound will not be correct and the full benefit of the improvements can only be obtained by using an HDCD player. HDCD discs do not comply with the Red Book standard (see CD standards). See also SACD.


An abbreviation for 'high definition content protection' or for 'high-bandwidth digital content protection'. A copyright protection system used with DVI and HDMI interfaces. See also DRM, SCMS and SDMI.


An abbreviation for 'high-definition multimedia interface'. An interconnection standard for digital video and multi-channel digital audio signals (although analogue signals can be carried). It replaces the SCART interface for connections between audio-visual equipment in consumer and semi-professional systems.

Electrically, the Type A HDMI standard uses the same 19 connections as the DVI interface (but in a more compact connector), and so an HDMI device can be connected to a DVI device by using an adaptor cable; the HDMI device will then operate in DVI mode. However, note that HDMI incorporates audio channels, whereas DVI carries video only. The Type B HDMI connector is larger, with 29 connections, Type C is a miniature version of Type A (19 connections) and Type D is a 'micro' version of Type A (19 connections). For pin allocations see www.hardwarebook.info/HDMI. The maximum HDMI cable length depends upon the quality of the cable, but is unlikely to be more than 15 metres without special arrangements such as repeaters or optical adaptors. See also HDCP, EDID and DisplayPort. Compare SDI.

View HDMI (Type A) image


An abbreviation for 'high definition television'. Any one of a number of formats for television systems providing an improved video resolution, i.e. sharper pictures. The most common European formats are listed below. Note, however, that these are interface designations, and the signals carried may be displayed using equipment that has a higher or lower resolution, in terms of their physical number of pixels horizontally and vertically − provided that the equipment is capable of performing the required format conversion. The frame rate value (the number after the 'p' or 'i') is often omitted.

See also Blu-ray. Compare UHD. For information on the most common video resolutions see VGA.

An amplifier specifically designed to accept an input signal from one or more instruments (usually guitar or bass) and to provide an output signal suitable for connection to one or more backline speaker cabinets (cabs). So-named because it is usually placed on top of the relevant cab(s) − see Stack. However, for an alternative meaning see also Head amplifier. Compare Combo.

Head (2)

The pick-up, recording or erasing device that acts upon the recording medium of a tape or disk-based magnetic playback or recording system (such as DAT, reel-to-reel, VCRs, a hard disk drive, etc). In systems where the head is in mechanical contact with the medium, heads are prone to wear and contamination, resulting in impaired performance. In all systems, correct alignment of the heads is critical to achieve optimum performance − see Azimuth and Zenith. See also Bias (1).

Head amplifier

Any amplifier at or near the source of a signal. The term is most commonly used to refer to microphone pre-amplifiers that are provided as units remotely located from the mixer, e.g. close to the stage. Many such units handle multiple signal channels and additionally provide analogue to digital conversion and multiplexing of the resulting digital signals. This enables many audio channels to be carried to the mixer location by a single lightweight and low-cost UTP cable (typically a CAT 5e cable). However, for an alternative meaning see also Head (1).

Headphone amplifier

An amplifier that is specifically designed to provide one or more outputs suitable for the connection of headphones, for example for use by performers on stage or in a studio. Suitable earphones may alternatively be connected, to provide wired in-ear monitoring. Typically the amplifier input(s) are designed to accept balanced line level signals. The units may be single channel types, may provide several identical (or level-adjustable) outputs from a single input for several performers requiring the same mix, or may be true multi-channel units accepting several different input signals for supplying different performers' headphones or earphones with different mixes. See also Distribution amplifier.


A device in which miniature drivers are held in direct contact with the ears by means of a sprung headband. Most headphones have a driver for each ear to enable stereo listening, but some types have only one to allow continued listening to the room sound. The slang term 'cans' is sometimes used. There are many different designs, aimed at meeting differing requirements, see for example Open back, Closed back, Circumaural, Supraaural and Intraaural. Headphones which include a microphone are usually referred to as a headset.

Stereo headphones intended for professional use are usually fitted with a 3-pole (TRS) ¼″ (6.35 mm) jack plug, though some are fitted with a 3.5 mm jack plug, often requiring use of a 3.5 mm to ¼″ adaptor. Stereo headphones intended for use with mobile phones and similar devices sometimes incorporate a microphone, and may be fitted with a 4-pole (TRRS) 3.5 mm jack plug, wired in accordance with the CTIA standard. Where a signalling (e.g. 'answer') button is also included, this is typically arranged to short out the microphone.

Note that the connection of headphones to the headphone outputs of equipment is similar to the connection of speakers to power amplifier outputs, in that the total connected load impedance should not be less than the minimum value specified for the headphone output(s) of the particular equipment concerned. Many professional mixers and headphone amplifiers have a minimum headphone load impedance of 100 ohms or more, therefore the lower impedance types common for 'Hi-Fi' use (which are commonly less than 50 ohms) are often unsuitable for professional applications. When multiple sets of headphones are connected to a single headphone output channel (e.g. of a headphone amplifier), the combined parallel impedance of all the connected sets must be higher than the minimum load impedance specified for the channel.

Compare Earphones.


The margin in signal level that is available between the normal working level (usually quoted as an average value − see VU) and the level at which unacceptable distortion ('overload') would occur. Usually, such distortion will be in the form of clipping. This margin is essential in order to accommodate the normal peaks in the signal, which can momentarily raise its level very much higher than the average value. Note, however, that if the working level is quoted as a peak level, then much less headroom is required relative to that level − just a small safety margin to ensure that clipping will not occur during exceptional peaks. Headroom is usually measured in decibels (no reference value being necessary as the figure is describing a difference in levels).

The maximum level before clipping in any item of equipment is normally determined by its design − particularly by the voltage of its power rails. Therefore the only way to increase headroom is to use a lower average signal level, which will worsten the signal-to-noise ratio. So although a high value of headroom is desirable in terms of allowing for large signal peaks, a compromise must be reached between headroom and signal-to-noise ratio. (The sum of these two values equals the dynamic range of the equipment, which is a fixed value.) The levels through professional systems are normally aligned to provide a headroom of between 16 and 22 dB.

As an example, if we assume that a particular signal has an average level of +4 dBu and a peak level of +18 dBu, then an item of equipment which clips at a level of +24 dBu provides a headroom normally quoted as 20 dB. (That same equipment would provide a headroom of 6 dB above that signal's peak level.)

In order to preserve the required headroom throughout the signal chain, whilst maintaining the optimum signal-to-noise ratio, it is important to ensure that the level controls of each item of equipment are adjusted such that the margin between the normal working level through that item and the level at which that item would clip the signal (or produce an unacceptable degree of distortion) is approximately the same for each and every item. This process is referred to as setting the gain structure of the system, and may be achieved by use of a test tone.

The amount of headroom required from particular items of equipment can be reduced by applying compression or limiting to the signal prior to those items. This reduces the headroom required because the difference between the average level of the signal and its peak level is reduced. See also PPM and Standard operating level.


Audio equipment worn on the head, usually secured by means of a band over the top of or around the back of the head (sometimes with an additional securing part around one or both ears). The term is frequently used to refer to a miniature head-worn microphone. As the microphone is secured close to the mouth and at a fixed distance from it, the pick-up of sound is usually superior to and more consistent than that obtained from a lavalier microphone.

The term is also commonly used to refer to headphones that have a microphone attached (as used in communications between technical staff), or occasionally to a standard set of headphones. See also Earhook, Comms and Talkback.

Hearing enhancement

An alternative term for assistive listening.

Hearing loop

An term often used to refer to an audio-frequency induction loop system, installed for assistive listening purposes.


Apart from its general meaning, this term is used specifically to refer to the part of a valve that heats the cathode of the valve to its operating temperature, or to refer to the electrical supply for such valve heaters. This is usually a low voltage AC supply, derived from the mains supply by the internal power supply of the equipment, and may be provided with one or more fuses that are separate from the equipment's mains fuse. See also Standby (1) and HT.


A block of metal (usually aluminium) used to assist the transfer of the heat produced by electronic components (especially the power transistors of power amplifiers) to the surrounding air. To do this effectively, they are often made with 'fins', as this increases their surface area. The passage of air around a heatsink must not be obstructed, or the flow of heat away from the components will be impaired and the equipment may overheat and be seriously damaged. A secondary purpose of a heatsink is to absorb short-duration peaks in the amount of heat produced by some components, resulting in a smaller rise in temperature during those peaks. See also Dissipation, Convection-cooled and Forced cooling.

Helical antenna

An antenna that consists of a wire or a conductive tape wound so as to form a helix. This type of antenna is sometimes used with receivers for radio microphones or with transmitters for in-ear monitoring (IEM) systems, because its directional behaviour provides a substantial gain in the forward direction and substantial rejection of interference in the opposite direction. This type of antenna is sometimes housed within a plastic dome, and these are sometimes referred to as dome antennae. Antenna gain can be particularly useful with IEM systems, because their receivers are usually non-diversity types. See also Antenna distribution unit. Compare LPDA.

Henry, Henrys

The unit of inductance, usually abbreviated to 'H'. In terms of varying currents, it is the amount of inductance required to give 1 volt of back-emf (instantaneous value) in response to an instantaneous rate of change of current of 1 amp per second. In the case of AC, an inductor's reactance is given by π f L, where f is the frequency in hertz and L is the inductance in henrys. As the henry is a fairly large amount of inductance, often the units of millihenrys (mH, one thousandth or 10−3 of a henry) are used. Compare Farad.


See Hz.

Hex, Hexadecimal

Describes a number that is expressed in base 16. In such a number, each successive place, moving leftwards, has a significance 16 times as great. (Compare this with a normal decimal number, in which each place to the left is ten times as significant.) So, the right-most place indicates the number of 1's, and successive places to the left indicate the number of 16's, 256's, 4096's, 655366's, etc.. This means that each place needs to be able to represent the values 0 to 15; this is achieved by use of the numbers 0 to 9 in the normal way and additionally by the letters A, B, C, D, E and F, representing the values 10, 11, 12, 13, 14 and 15 respectively. For example, the decimal number 379 would be written as 17B (= 1x256 + 7x16 + 11x1). Despite its use of letters, a hexadecimal number may be said to consist of 'digits' (perhaps more accurately, 'hexadecimal digits'). 'Hexadecimal' is frequently abbreviated to 'hex'.

This scheme is useful because it enables values to be written compactly while at the same time providing the ability to very readily convert the number to binary form; likewise, a binary number may readily be converted to hexadecimal form. This is because each hexadecimal digit represents a group of four binary digits, enabling each hexadecimal digit to be converted separately. For example, the three digits of the hexadecimal value 17B convert to 0001, 0111 and 1011 respectively, so its complete binary equivalent is 000101111011.


An abbreviation for 'high frequency'. See Treble.

HF driver

An abbreviation for 'high-frequency driver' − an alternative name for a horn or tweeter. Compare LF driver.


Strictly, an abbreviation for 'high fidelity', describing equipment that is claimed to offer a superior quality of sound reproduction. Now more usually used to refer to any 'installed' (i.e. non-portable) domestic audio system, or to distinguish equipment that is more suited to such consumer applications than for use in a PA system. See also Audiophile.


See High impedance.

High cut

The process of reducing the treble content of an audio signal. Or, in relation to filters, see Low pass. Compare Low cut.

High end

The treble end of the audio-frequency spectrum, also called the 'top end'. Compare Low end.

High frequency

For high audio frequencies see Treble. See also Ultrasonic and Radio-frequency.

High hats

A part of a drum kit, consisting of two horizontal cymbals with a foot-operated mechanism allowing them to be separated and re-contacted. Sometimes just called 'hats'.

High impedance

Usually describes equipment or interconnections in which the source impedance is significantly greater than 600 ohms, or in which the load impedance is significantly greater than kilohms. However, the term is also used to describe a fault condition in which an electrical connection is not making proper contact (see High resistance connection) or is open-circuit. Sometimes written 'High-Z' or 'Hi-Z', because Z is the symbol for impedance. For information on the impedance of microphones, see the Microphones page. See also Matching. Compare Low impedance.

High pass

Describes a filter that attenuates ('cuts') frequencies lower than a particular value, but allows frequencies higher than that value to pass through relatively unaffected. The boundary between the range of frequencies cut and those allowed to pass is called the cut-off frequency, however in practice the transition between 'cutting' and 'passing' is not abrupt, but takes place over a range of frequencies. The cut-off frequency of a high pass filter is usually considered to be the frequency at which the attenuation is dB greater than the average attenuation at frequencies that are high enough to be clear of the transition region.

Or, the name of a switch or control(s) that provides this filtering function (e.g. on a mixer, where it may be considered to be a part of the channel EQ section). On a mixer, a variety of forms are found, depending on the mixer sophistication − for details see Equalisation on the Mixing Facilities page.

May be abbreviated to 'HP', or may alternatively be referred to as 'low cut' (LC). For cross-references to more filtering-related terms see Filter. Compare Low pass.

High resistance connection

A defective electrical connection that allows current to pass but that presents an excessive amount of opposition (resistance) to its flow. Such defective connections most commonly occur at screw-terminals of connectors, and are typically caused by an inadequately tight terminal screw or by oxidation of the relevant cable conductor at the point of connection, or of the terminal itself. A high resistance connection can also occur between the contact surfaces of mated connectors, e.g. due to surface contamination or to a loss of contact pressure or contact area. In the case of a soldered joint, a high resistance connection can be caused by a dry joint.

High resistance connections can give rise to a variety of symptoms, depending upon whether it is a power connection or a signal connection, and in the latter case depending upon the type of signal being carried.

Compare Open-circuit (2).


See High impedance.

Higher mid-range

Describes an audio frequency at the upper end of the mid-range frequencies, typically between 800 Hz and kHz. Often abbreviated to 'HMID' or 'HM'. Also called 'upper mid-range'. Compare Lower mid-range.


Another name for treble frequencies. Compare Lows.


A specific kind of noise; an unwanted background 'hissing' sound. Its cause is often incorrect settings of level controls (especially gain controls). It can never be eliminated completely, but can usually be reduced to an acceptable amount in comparison with the wanted sound by ensuring that each item of equipment is handling a sufficient level of signal. See also Signal-to-noise ratio and Thermal noise.

Hitting the rails

A slang term describing an amplifier that is clipping. See also Overload.


An abbreviation for 'higher mid-range'.


A trademark of Osram lighting, but also a term that is sometimes used more generally to identify lighting that uses a specific type of mercury arc lamp employing a medium length arc. The abbreviation stands for 'hydrargyrum medium-arc iodide lamp', hydrargyrum being the original name for the element mercury (from which the chemical symbol 'Hg' is derived). Lanterns using HMI lamps are able to efficiently produce continuous high intensity light with a similar colour spectrum to daylight, which is useful in applications such as filming. HMI lamps also find application in projectors and in moving-head lanterns, but in many cases are gradually being superseded by LED-based designs.


An abbreviation for 'higher mid-range'.

Hold time

A control which adjusts the time for which a compressor extends the period of compression, after the signal level falls below the threshold level. The release commences after the expiry of the hold time. See also Attack.

Hook clamp

A clamp used to attach a lantern to a lighting bar. It usually constructed from flat steel strip, bent into a 'U' shape at one (or both) ends and equipped with a bolt which secures it in place on the bar. This design enables the clamp to be hooked over the bar before the securing bolt is tightened. As these clamps generally have a limited SWL, they are not suitable for attaching heavy items such as speakers. The US equivalent is called a 'C clamp'.

Horn (1)

Strictly, a carefully shaped 'funnel' which may be used to match the output of any type of speaker driver to free air, so as to improve efficiency and achieve the required dispersion angles. It can in theory be used at any frequency, through in practice horns for use at anything below upper mid-range need to be very large, and so some are folded. Mid-range horns are often of the exponential type, whilst those for treble frequencies are usually constant directivity types.

However, in general PA system usage the term 'horn' is mostly used to refer to a horn-loaded high-frequency driver − a driver which is designed specifically to handle treble frequencies, also called an HF driver or a tweeter. Such drivers are commonly used in full-range speakers. See also Throat, Compression driver, Bullet, Ferrofluid, Crossover and Bi-amping. See the Amps and Speakers page for further information. Compare Woofer.

Horn (2)

A musical instrument in the wind (brass) section, alternatively referred to as the French horn, having a long length of tubing that is circularly coiled.

Horn protection

See Speaker protection.


Warning: In the context of mains supply conductors or other power conductors or terminals, particularly in American terminology, 'hot' may refer to a conductor at a dangerously high voltage with respect to the general mass of the Earth (equivalent to the UK term 'live'). In interconnections between power amplifiers and speakers, it should be assumed that both (or all) of the conductors or terminals are at a significant (and potentially dangerous) voltage with respect to earth potential.

In the context of the individual conductors of other signal interconnections, the meaning of 'hot' is different depending on whether an unbalanced or a balanced interconnection is being referred to.

In the context of signal levels, 'hot' refers to a level that is in the upper region of the range of levels that might be expected in the circumstances, or to one that is somewhat higher than what might be expected.

See also Pair. Compare Cold.

Hot condition (connector)

In general, describes any type of connector that is designed to operate at elevated temperatures. However, the term most frequently refers to a specific type of IEC connector, more properly designated C15 (female) and C16 (male), as used to connect to equipment that becomes hot in use, such as some types of lantern. For further information see IEC 320.

Hot standby

Describes redundant equipment that is switched on and ready for use in the case of a failure, but is not currently being used. For further information see Redundancy and Standby (1). Compare Cold standby.


Apart from a way of measuring time, a way of specifying or recording the position of a rotary control. In this method, the pointer on the control knob is considered to be like the hour hand of a clock. So, in the usual case of controls that rotate clockwise from bottom left around to bottom right, a fully anti-clockwise setting is referred to as "7 o'clock", a central setting as "12 o'clock" and a fully clockwise setting as "5 o'clock". Sometimes referred to as the "o'clock" method, for obvious reasons.


The part of the venue that accommodates the audience. More properly known as the auditorium. Or, as an adjective, the term indicates a connection with that area, as in the examples below. See also Front-of-house.

House lights

Lighting whose purpose is to illuminate the auditorium. See also Front-of-house.

House mixer

Short for 'front-of-house mixer' − the mixer used to mix the front-of-house sound (as distinct from the monitor mixer). However, where a venue hosts events for which a specific PA system is temporarily set up, the term may be used to refer to the permanent house system mixer in order to distinguish it from the temporary mixer.

House system

The PA (or lighting rig, etc.) that is permanently installed in a venue, as distinct from that which is temporarily set up for a particular event or performer. See also House mixer.

House tabs

The curtains which close to separate the stage area from the front-of-house area. See also Apron.


An abbreviation for 'House of Worship', a generic term for any venue that is used exclusively, or mostly, for religious worship gatherings. The term is most commonly used in reference to a Christian church, but may also refer to venues used by other faiths. It may also describe a religious event taking place in a venue used mostly for other purposes. Some houses of worship have quite large and complex PA system installations.

Howl-back, Howl-round

Alternative names for acoustic feedback ('howl-round' is the more common of the two). These terms may be written with or without the hyphen, or as two separate words.


An abbreviation for 'high pass filter'.


An abbreviation for 'high rupturing capacity', which describes a fuse that is able to interrupt large fault currents. It usually has a ceramic body. Also known as an HBC (high breaking capacity) fuse, though that term is generally used only with smaller fuses. Compare LBC.


An abbreviation for 'head-related transfer function', an approximation to the frequency-dependent characteristics of human hearing that takes into account the effects of the shape of the head, ear-lobes, etc.. The effect of the HRTF is included in K-weighting − see Weighting.


An abbreviation for 'Health and Safety Executive', the UK government body which provides safety advice and guidance, sets safety standards and investigates incidents such as accidents and breaches of safety regulations. Their website is www.hse.gov.uk (opens in a new window). See also OSHA.


An abbreviation for 'high tension', meaning high voltage. This abbreviation is most commonly used to refer to the high voltage DC supply needed by valves, in order to distinguish it from low voltage supplies that are needed within the same equipment (e.g. for valve heaters). HT supplies are usually in the range of 150 to 500 volts, and in the case of some US equipment are sometimes referred to as the 'B', 'B+' or 'plate' supply. This is a dangerous and potentially lethal voltage.

The HT supply is usually derived from the mains supply by the internal power supply of the equipment, and may be provided with a fuse and/or a switch (sometimes labeled 'Standby') that is separate from the equipment's mains fuse or switch. Warning: A 'Standby' switch, if present, does not in every case switch the HT supplies. Further, HT supplies may remain at lethal voltages within equipment long after the mains power has been disconnected, due to the charge stored in internal capacitors. See also Standby (1) and Anode.


In stage lighting, an abbreviation for 'highest takes precedence'. This refers to the method used to determine the DMX control values sent by lighting desks to destinations such as dimmer packs for traditional lanterns using incandescent lamps. This method is contrasted with the 'latest takes precedence' (LTP) method, which is more applicable to fixtures such as moving-head lanterns. In the HTP method, the most recent value determined by the desk is ignored if it is lower than any other values that might be concurrently relevant for the same DMX channel; the highest concurrently relevant value is always output. Compare LTP.


A specific kind of noise introduced into analogue audio signal paths by equipment or by nearby mains cables, usually due to inductive coupling from mains transformers, due to earth loops, or due to a lack of appropriate earthing. Warning: The lack of a suitable safety earth where one is required constitutes a possibly serious safety hazard, so the source of a hum should always be investigated. A hum signal has a frequency of 50 or 100 Hz (60 or 120 Hz in the USA), often with added harmonics. When the harmonics are at a relatively low level, the sound produced by a hum has a substantial bass content, but when the harmonics are more dominant the sound becomes 'thinner' and more 'edgy' in nature, and is then often referred to as a 'buzz'. A hum or buzz may be caused by faulty or poor quality equipment or audio cables, but these are by no means the only causes. For guidance on resolving such problems see the FAQ. See also Common mode interference.


A type of guitar pickup that is designed to have a reduced sensitivity to stray magnetic fields, and therefore a reduced hum output (for a given level of stray field) as compared to its wanted signal output. Such stray fields typically originate from nearby amplification equipment such as combos or heads. The humbucker works by using a pair of coils instead of just one − the pair are connected in such a way that the magnetically induced hum voltage produced by one coil is largely cancelled by the hum voltage produced by the other.

This behaviour of humbucking pickups is also useful in avoiding problems caused by inductive coupling into the pickup from induction loops. In some pickups of this type, a side effect of the humbucking design is that a more mellow tone is produced, as compared to a single coil type − this may or may not be considered desirable. Compare Single-coil.


See RH.


A test device, used to test audio equipment for the presence of a "pin 1 problem".

Hundred-volt line

See 100 volt line.


An abbreviation for 'heating, ventilation and air-conditioning', often a significant contributor to the ambient noise SPL in auditoria (along with equipment cooling fans and, of course, the audience). In situations of unavoidably high auditorium background noise, there is little point in maximising the signal-to-noise ratio of the PA system at the expense of compromised headroom.


A trademarked abbreviation for 'headroom extension', a system for increasing the high frequency headroom of an analogue tape recording system. It operates by making use of the self-biasing effect of the signal to be recorded, enabling the tape bias to be decreased when the signal contains a sufficiently high level of high frequencies and so increasing the threshold of tape saturation. This is not a noise reduction system, and differs from such systems in that no 'decoding' operation is required on playback of the recording.

Hybrid (1)

A general term which describes anything that is a combination of two (or more) different components or aspects, or which makes use of two (or more) different technologies or methodologies. For example, a combo might be described as a hybrid design if it uses a mixture of valve and solid state technology.

Hybrid (2)

Originally, a passive transformer-like device for combining two impedance-matched signals in such a way that the maximum power transfer is achieved from each source to the load, minimising the amount of power fed from each source back into the other. Or, conversely, the same device used 'in reverse' to achieve the most efficient split of power from a single source to two loads. Or, a similar device used in the interface with a 2-wire analogue telephone line in order to convert between the bidirectional signals on the line and separate local 'send' and 'receive' signals. In all of these cases, the minimum theoretical loss is dB (generally nearer 3.5 dB in practice). Now also sometimes used to refer to active devices with a similar function. See also Microphone splitter, Mix-minus and Bridge (2).


Describes the polar response of a uni-directional microphone which provides even less sensitivity to sounds from the sides than does a super-cardioid type, but at the expense of providing a wider and stronger pick-up pattern at the back of the microphone. Its useful angle of acceptance (measured from side to side) is around 105. Its minimum sensitivity to sounds is at an angle of around 110, measured from the front axis (i.e. 70 from the rear axis). Sometimes referred to as a rifle or shotgun microphone, but properly speaking these are names for a polar response that is even narrower than a hyper-cardioid. See the Microphones page for more detail.

View uni-directional polar responses illustration


The behaviour of a system, or of a component of a system, in which its output value for a given input value depends on whether that input value was approached in a rising or a falling direction. In general, this is usually an undesirable phenomenon.


An abbreviation for 'hertz' − the unit of frequency. The number of Hz is the number of complete cycles of change taking place per second. The unit is named after Heinrich Rudolf Hertz, researcher and inventor, and is pronounced like "hurts". See also kHz and Audio-frequency.

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This page last updated 26-Nov-2019.