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Glossary of PA Terms - 0-9 |
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The glossary pages provide alphabetically-listed definitions for over 1900 PA-related terms. If you're not sure what term you are looking for, then the structured list of topics on the Training page may be a useful place to start, as there are links from there to many of the Glossary entries.
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0 dB * 19 inch rack system * 100 volt line * 1080i * 1080p * 2-pi space * 2-TK, 2-TRK * 2-track * 2-way * 3 dB point * 3-phase * 3-to-1 rule * 3-way * 4 mm connector * 4-pi space * 5.1 * 57 * 58 * 5th * 5th-order * 7.1 * 70 volt line * 720p * 75 ohm interconnection
0 dB (1)
Applied to the level of a
signal, it
refers to the agreed
reference level
of that signal; the signal's 'zero level'.
When 'dB' is followed by other characters, those
characters usually indicate the applicable reference
level. Common examples of such reference levels are:
- 0 dB SPL − A sound pressure level of 20 µPa (= 200 pbar).
- 0 dBu − A voltage of 0.775 V RMS.
- 0 dBV − A voltage of 1 V RMS.
- 0 dB FS − A full-scale digital signal level.
- 0 dBm − A power of 1 milliwatt.
0 dB (2)
Applied to the setting of a
level control, it
refers to the reference setting of that control.
On some level controls of some equipment, this is the
setting at which the relevant section of the equipment
provides unity gain.
For further information see the
Decibels page.
0 dB (3)
Applied to the setting of an
EQ control (including a
band of a
graphic equaliser), it
refers to the setting that has no effect on the
signal
being equalised (usually the mid-position of the control).
See also Flat.
For further information on decibels see the
Decibels page.
0 dB (4)
Indicated on a level
meter, it indicates a
signal
level that is equal to the
relevant reference level. For meters that indicate
the level at
line-level
outputs, a list of the
most common reference levels used is
given below. Consult the equipment handbook to check
which 0 dB output reference level is applicable
to your equipment.
- +4 dBu − Most professional equipment
- 0 dBu − Some professional equipment (e.g. some Mackie mixers)
- −10 dBV − Most amateur and some semi-pro equipment
- 0 dB FS − Digital equipment
19 inch
rack system
A metal (usually steel) mounting system for equipment.
Each item of equipment is secured to the rack by bolts
through flanges (or 'ears') at the left and right edges
of its front panel. The bolts screw into removable
caged nuts that are
attached to vertical strips, one at each side of the rack.
Plastic washers under the bolt-heads are usually used to
protect the flanges from damage.
The nuts may be installed, as required, at fixed intervals
in order to accommodate equipment of differing front panel
heights. All front panel heights are a multiple of
one vertical unit, or U.
The total width of the equipment front panels (including the
flanges) is 19 inches (482 mm), hence the name.
100 volt line
An arrangement for the connection of (passive)
speakers
to an amplifier,
which accommodates a much greater distance between the
speakers and the amplifier than would normally be possible
without a significant reduction in
power due to
loss in the interconnecting
cable.
It works by using a much
larger voltage than normal,
so that the current flow
(and therefore the power loss in the cables) is much reduced
− i.e. it operates at a much higher
impedance than normal.
In the UK the
nominal voltage used
is 100 volts RMS.
In the USA the value is 70.7 volts RMS,
and the system there is known as a '70 volt line'
or as a 'constant voltage' system.
Special speakers must be used with these systems. These speakers (which usually have a low power rating) are equipped with a transformer which steps the voltage down to a value suitable for the driver(s). The transformer often has several taps, which are usually marked with power ratings such as 5 W, 10 W, 15 W and 20 W. The effect of selecting a higher power tap is that a lower impedance is presented to the line by the speaker, and so a larger current is drawn from it. The speakers are always connected in parallel to the line, which is often looped from speaker to speaker. The total connected load (usually specified in watts) must not exceed the drive capability of the amplifier, or damage to the amplifier may result.
This arrangement is generally used only in public address installations (whether fixed within buildings or for outdoor sports events and the like), where a large number of low-power speakers, located over a wide area, are to be connected to a common amplifier. Historically, these speakers were often designed for cost-effective reproduction of voice frequencies only (rather than music), with maximum efficiency being given higher priority than sound quality. Therefore, in the past 100 V line systems have gained a reputation for poor audio quality. However, systems providing much improved audio quality are now available.
The term 'constant voltage' derives not from the output voltage of a particular amplifier of this type being constant in use, but from the fact that, in contrast to 'normal' (i.e. low impedance) amplifiers, two amplifiers of this type that differ in their power output capability do not differ in their maximum voltage output − their difference in power output is accounted for solely by a difference in the amount of current that they are able to supply. This means that, when additional speakers need to be added to a system, the amplifier can be upgraded to a higher-powered model without affecting the power level supplied to the existing speakers. See also Installation speaker.
2-pi space
See Half space.
2-TK, 2-TRK
An abbreviation for
2-track.
2-track
An alternative term for
stereo.
A name often given to a stereo
input on a
mixer, intended for
a playback connection from a stereo audio player
(e.g. a CD player); often labelled '2-TK' or
'2-TRK'. Such inputs are often
equipped with a reduced set of facilities (especially
as regards equalisation
and routing) as compared
to a channel strip.
2-way
Describes a speaker
equipped with two different types of
driver, each handling
a different frequency
range. Typically, the two types of drivers will be
woofers to handle the
low (bass) frequencies
and horns to handle the
high (treble) frequencies.
Such speakers
usually include a passive
crossover to
direct the bass and treble frequencies to the appropriate
drivers, but may also be equipped with separate inputs
to allow bi-amping.
Similarly, a speaker that incorporates three different types of driver, handling three different frequency ranges (low, mid-range and high) is described as a 3-way speaker.
3 dB point
See Cut-off
frequency.
3-to-1 rule
The rule-of-thumb that recommends that when two or
more microphones are
used to pick up different
sound sources,
the distance between the microphones should be
at least three times the distance between each microphone
and its respective sound source. Or, to put it another
way, the microphones' distances from their respective
sound sources should be no more than one third
the distance they are from each other.
This is to ensure that each sound source is picked up predominantly by just one microphone, and so minimise the extent of undesirable comb filtering effects which can occur when a sound source is picked up equally (or approximately so) by two or more microphones whose outputs are subsequently mixed. See also Microphone technique.
4 mm connector
A single-pole
connector sometimes used
(in pairs) for speaker
connections, so-named because the
nominal diameter of the
male prong is 4 mm.
Usually the female
connector can be unscrewed by hand for alternative use
as a binding post.
The male connector is commonly referred to as a
banana plug.
See also Speakon.
4-pi space
See Full space.
5.1
The multichannel digital audio
scheme developed by
MPEG to provide a
surround sound
capability for films in many formats,
including
DVDs, tape and celluloid.
Also used in some high definition
television (HDTV) broadcasts. It incorporates five
full-range
(20 Hz to
20 kHz)
channels, for the
left front, right front, centre front, left rear
and right rear speakers,
and one sub-bass
(20-120 Hz) channel, hence the designation '5.1'.
A 4-pole 3.5 mm
jack
connector is
sometimes used for 5.1 digital connections. See also
7.1 and
AC-3.
57
A slang term for a Shure SM57
microphone.
58
A slang term for a Shure SM58
microphone.
7.1
A multichannel digital audio
scheme similar to the 5.1
scheme but with the addition of 'Left Side' and 'Right Side'
channels.
70 volt line
See 100 volt line.
75 ohm
interconnection
See Impedance-matched.
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