| Glossary of PA Terms - H
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The glossary pages provide definitions for over 2680 PA-related
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In the list below, the most commonly looked-up terms
are in bold, lighting-specific terms are in
and video-specific terms are in
Haas effect *
Half normalling *
Half-power bandwidth *
Half space *
Half step *
Half track *
Handling noise *
Hard clipping *
Hard knee *
Hard panning *
Hazard tape *
Hazardous area *
HD video *
Head amplifier *
Headphone amplifier *
Hearing loop *
Helical antenna *
High cut *
High end *
High frequency *
High hats *
High impedance *
High pass *
High resistance connection *
Higher mid-range *
Hitting the rails *
Hold time *
Hook clamp *
Horn protection *
Hot condition *
Hot standby *
House lights *
House mixer *
House system *
House tabs *
Howl-back, Howl-round *
Hundred-volt line *
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.
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
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
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
Also called the 'precedence effect'.
Speed of sound,
Delay (2) and
Describes a graphic
equaliser whose controls provide adjustment of
bands that are spaced
half an octave apart.
For further details see
The bandwidth measured
between the two frequencies
(upper and lower) at which
level of an item of equipment
would fall to a half of its highest value, if a
sine wave were
applied to the input
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
This is the method conventionally adopted when specifying
response of equipment or systems.
See also Q (1).
A measurement environment in which only the space
in front of the item under test is considered.
Most often used in
of speakers, where it
simulates the speaker being used against a reflective wall
and so gives higher
figures at bass
Also referred to as 2-pi space.
Compare Full space.
An alternative term for a
Compare Full step.
A setting provided on most
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
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)
to its operator(s). The term may be
used of both temporary and permanent systems. See also
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,
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,
and Barrier strip.
in which the 'flattening' of the
abruptly at a certain point in the waveform,
and no further increase in
is possible beyond that point. For further information see
Compare Soft clipping.
The practice of setting a pan
control fully, or nearly fully, to its extreme Left or
Right position. See also
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
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
An effects unit
intended to add one or more musical
harmony parts to a
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
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
See also SATB and
A large many-stringed instrument that is plucked with
the fingers of both hands. There are two common types:
The concert harp, a large instrument which rests on the
floor in an upright position. Effective
is not easy − the difficulties are similar to those
encountered with a piano.
The folk harp, a smaller cousin of the concert harp.
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
A brightly coloured self-adhesive tape (usually striped
black and yellow) intended to clearly mark
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
Compare Barrier tape.
A location in which the risk
from expected hazards
is unusually high, or in which unexpected or unusual
hazards are present. 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
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.
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
connector used for
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
audio (e.g. see
HDCD). In the case of video,
see also UHD.
Usually refers to video
at a resolution of either 720p or
1080i, though there are
other variants − see HDTV
for further details. See also
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
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
See also DRM,
An abbreviation for 'high-definition multimedia
An interconnection standard for
digital video and
signals can be carried). It replaces
the SCART interface
for connections between audio-visual equipment in
Electrically, the Type A HDMI standard uses the same 19
connections as the DVI
interface (but in a more compact
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,
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
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.
720p50 − 50 Hz frame
rate; 720 lines per frame,
scanned; 1280 horizontal samples per line;
16:9 aspect ratio.
1080i25 − 25 Hz frame rate; 1080 lines per frame,
horizontal samples per line;
16:9 aspect ratio.
1080p25 − 25 Hz frame rate; 1080 lines per frame,
progressively scanned; 1920 horizontal samples
per line; 16:9 aspect ratio.
1080p50 − 50 Hz frame rate; 1080 lines per frame,
progressively scanned; 1920 horizontal samples
per line; 16:9 aspect ratio.
See also Blu-ray.
For information on the most common video resolutions
specifically designed to accept
signal from one or more
instruments (usually guitar or electric bass)
and to provide an
output signal suitable
for connection to one or more
So-named because it is usually placed on top
of the relevant speaker(s).
However, see also
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,
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
at or near the source of a
signal. The term is most
commonly used to refer to
that are provided as units remotely located from the
mixer, e.g. close
to the stage. Many such units handle multiple signal
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, see also Head (1).
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
Typically the amplifier input(s)
are designed to accept balanced
The units may be single channel
types, may provide several identical (or
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
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, but some
types have only one. The slang term 'cans' is sometimes
used. There are many different designs, aimed at meeting
differing requirements, see for example
Headphones which include a
microphone are usually
referred to as a headset.
Note that the connection of headphones to the headphone
outputs of equipment is similar to
the connection of speakers to
outputs, in that the total connected
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),
parallel impedance of all the
connected sets must be higher than the minimum load
impedance specified for the channel.
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
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
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
gain structure of the
system, and may be achieved by use of a test
The amount of headroom required from particular items
of equipment can be reduced by applying
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.
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
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
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
See also Earhook,
An informal term for an
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
AC supply, derived from the
mains supply by the
internal power supply
of the equipment, and may be provided with a
fuse that is separate
from the equipment's mains fuse. See also
Standby (1) and
A block of metal (usually aluminium) used to
assist the transfer of the heat produced by electronic
components (especially the
power transistors of
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
See also Dissipation,
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
or with transmitters for
(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
distribution unit. Compare
The unit of
abbreviated to 'H'. In terms of varying
it is the amount of inductance required to give
1 volt of
value) in response to an instantaneous
rate of change of current of
1 amp per second.
In the case of AC, an
reactance is given by
2 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.
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 +
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
An abbreviation for
Strictly, an abbreviation for 'high
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
applications than for use in a PA
See also Audiophile.
The process of reducing the
treble content of an
signal. Or, in relation
to filters, see
Compare Low cut.
The treble end of the
spectrum, also called the 'top end'.
Compare Low end.
See also Ultrasonic and
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
Usually describes equipment or interconnections in which the
is significantly greater than
or in which the
is significantly greater than
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
Sometimes written 'High-Z'
or 'Hi-Z', because Z is the
symbol for impedance.
For information on the impedance of
microphones, see the
See also Matching.
Describes a filter that
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
however in practice the transition between 'cutting'
and 'passing' is not abrupt, but takes place over a range
The cut-off frequency of a high pass filter is usually
considered to be the frequency at which the
is 3 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
On a mixer, a variety of forms are found, depending
on the mixer sophistication − for details see
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.
A defective electrical connection that allows
current to pass but
that presents an excessive amount of opposition
to its flow.
Such defective connections most commonly occur at
are typically caused by an inadequately tight
terminal screw or by
oxidation of the
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
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.
In the case of power connections (e.g.
a significant temperature rise can
occur at high resistance connections, and this can
cause accelerated oxidation of conductors
and damage to the cable insulation and/or the
connector, with a possible risk of fire.
There may also be a noticable reduction in the
or voltage available
through the defective connection.
Similar effects can occur in the case of
interconnections, because of the high
In the case of
interconnections, symptoms of high resistance
connection problems (which in this case are
sometimes referred to as
connections) may include
at the upper end of the mid-range
frequencies, typically between
and 6 kHz.
Often abbreviated to 'HMID' or 'HM'.
Also called 'upper mid-range'.
Compare Lower mid-range.
Another name for treble
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
A slang term describing an
See also Overload.
An abbreviation for
An abbreviation for
A control which adjusts the time for which a
compressor extends the
period of compression,
after the signal
level falls below the
The release commences
after the expiry of the hold time.
See also Attack.
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'.
Strictly, a carefully shaped 'funnel' which may be
used to match the output of any type of
free air, so as to improve efficiency and achieve the
angles. It can in theory be used at any
through in practice horns for use at anything below
need to be very large, and so some are
Mid-range horns are
often of the
type, whilst those for
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
also called an HF driver or a
Such drivers are commonly used in
See also Throat,
Amps and Speakers page for
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.
Warning: In the context of
conductors or other
power conductors or
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
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
In the context of the individual conductors of other
the meaning of 'hot' is different depending on whether an
or a balanced interconnection
is being referred to.
In an unbalanced signal interconnection, 'hot' describes a
conductor which, in normal use, is at a significant (but not
necessarily high) voltage with respect to the relevant
signal earth potential;
that is, a non-earthy conductor. In
interconnections it is formed by the inner conductor;
in other cases it may be marked '+' or coloured red.
It connects to the tip of a 2-pole
A hot signal conductor is sometimes referred to as a
In a balanced signal interconnection, the 'hot'
leg operates in conjunction
with the 'cold' leg to form a
balanced pair. The hot leg
of the pair is the one that carries a signal whose
i.e. is in-phase with,
the reference signal polarity at that point.
In the case of an analogue
audio signal, this is
often considered to be a voltage that
becomes more positive as the original (unbalanced)
signal or sound pressure becomes more positive.
(Note that in some balanced interconnections, the hot
leg is the only one to carry a signal, the cold
leg being used purely for balancing purposes −
see for example Semi-balanced.)
It connects to pin 2 of an
XLR or to the tip of a
3-pole (TRS) jack plug.
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.
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
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
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 which accommodates the audience.
More properly known as the
auditorium. See also
Lighting whose purpose is to illuminate the
auditorium. See also
Short for 'front-of-house mixer' − the
mixer used to mix the
(as distinct from the
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.
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.
The curtains which close to separate the stage area from
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.
Alternative names for acoustic
('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
An abbreviation for 'head-related transfer
function', an approximation to the
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
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
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' supply or the 'plate' supply.
The HT supply is usually derived from the
mains supply by the
internal power supply
of the equipment, and may be provided with a
and/or a switch (sometimes
labeled 'Standby') that is separate from the equipment's
mains fuse or switch. Warning: HT supplies may
remain at lethal voltages within equipment long after
the mains power has been disconnected, due to the
charge stored in internal
See also Standby (1).
In stage lighting, an abbreviation for 'highest takes
precedence'. This refers to the method used to determine
the DMX control values sent by
to destinations such as
for traditional lanterns using
This method is contrasted with the
'latest takes precedence' (LTP)
method, which is more applicable to
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.
A specific kind of noise
introduced into analogue
signal paths by
equipment or by nearby
mains cables, usually due to
from mains transformers,
due to earth loops,
or due to a lack of appropriate
Warning: The lack of a
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 significant level,
the sound becomes more 'edgy' in nature and is then
often referred to as a 'buzz'.
A hum 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 hum problems see the
FAQ. See also
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
equipment such as combos or
The humbucker works by using a pair of coils instead of
just one − the pair are connected in such a
way that the magnetically
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
into the pickup from
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.
A test device, used to test
audio equipment for the
presence of a
"pin 1 problem".
See 100 volt line.
An abbreviation for 'heating, ventilation and
air-conditioning', often a significant contributor to the
(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
ratio of the PA system at the
expense of compromised
A trademarked abbreviation for 'headroom extension',
a system for increasing the
headroom of an
analogue tape recording
system. It operates by making use of the
effect of the signal
to be recorded, enabling the tape bias to be decreased
when the signal contains a sufficiently high
high frequencies and so increasing the threshold of
tape saturation. This is not a
system, and differs from such systems in that no
'decoding' operation is required on playback of the
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
be described as a hybrid design if it uses a mixture of
Originally, a passive
for combining two
signals in such a way that
the maximum power transfer
is achieved from each source to the
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
(generally nearer 3.5 dB in practice).
Now also sometimes used to refer to
active devices with a similar
function. See also
polar response of a
microphone which provides
even less sensitivity
to sounds from the sides than does a
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 105º.
Its minimum sensitivity to sounds is at an angle
of around 110º, measured from the front
axis (i.e. 70º
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.
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
An abbreviation for 'hertz' − the unit of
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
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This page last updated 19-Mar-2018.