AC
|
Alternating
Current. Electricity is characterised by the movement of electrons
through a conductor. In a DC circuit (see DC, below) the electrons always move in the same
direction (from negative to positive). In AC circuits the electron movement
alternates in direction. In some (e.g. mains) circuits, the direction of
electron movement alternates regularly at a fixed frequency (50 Hz, in the case
of UK mains). In other circuits (e.g. audio) the electron flow follows a
complicated pattern, corresponding with the frequency and amplitude of the
combined signal.
|
Active
|
Generally
the word active is used to describe equipment that requires
a power source (as opposed to passive equipment, which does not). Not all
active equipment is mains powered (active D.I. boxes and effects or volume
pedals are usually battery or phantom powered).
In PA systems,
crossovers are commonly described
as active or passive, and historically these terms were also used to distinguish
between (active) loudspeakers that required an external crossover with a
separate amplifier for each driver and (passive) loudspeakers that did not.
In
recent years, however, loudspeakers that have an inbuilt amplifier have become
very common, and these are also - quite reasonably - described as active.
However, this can give rise to confusion, as many loudspeaker manufacturers and audio
professionals still use these terms to refer to their earlier (crossover-based)
meanings.
Also, some modern "passive" loudspeakers (e.g. the Martin Audio S15 & S18, which we use), have an
active/passive switch. Here, the active setting is for use with an external
active crossover, while the passive setting routes the input signal
through the speaker's internal passive crossover. According to the more recent
idiom,
however, whichever setting is used the loudspeaker is nevertheless passive in the sense that it does not contain an
amplifier.
|
AFL
|
After
Fade Listen. A switch routing the post-fade signal (from a channel,
group or auxiliary) to the mixer's meters and headphones. See the Mixer
page for more details. |
Aux
|
Auxiliary.
The fader output from a mixer channel can be routed to the main left and
right outputs (and, if the mixer has them, to groups). Extra channel
outputs (individually controlled by knobs in the channel strip) are
usually know as Auxiliary Outputs. |
Balanced
|
A
signal transmission protocol in which two copies of the signal are
transmitted on separate conductors. The two signal copies are opposite in
phase but otherwise identical. The receiving device responds to the
difference between the signals, so that any cable-borne interference
(which will be in the same phase on both conductors) is cancelled. Also
see Unbalanced,
below. |
Bandwidth
|
Frequency
range (in Octaves). See the Parametric
EQ page for a little more detail. |
Bass
|
A
general term for the lowest audible frequencies. There is no universal
definition, but Bass generally describes frequencies below about 200Hz.
On most mixers, Bass EQ controls
operate at 80 or 100Hz. |
Board
|
Another
word for Mixer. |
Cardioid
|
Literally,
Heart-Shaped. A cardioid microphone has a heart-shaped sensitivity
pattern, and is more sensitive to sounds arriving from the front than from
the sides or rear of its capsule. |
Compressor
|
Device
that reduces the Dynamic
Range of an audio signal. See the Compressors
page for more details. |
Condenser
|
A
type of microphone. See the Microphones
page for further details. |
Console
|
Another
word for Mixer. |
Critical
Distance
|
The
point at which reflected sound is equal in level to direct sound. See the
section on Speaker
Position for more details. |
dB
|
See
the Decibel page. |
DC
|
Direct
Current. Electricity is characterised by the movement of electrons
through a conductor. In a DC circuit the electrons always move in the same
direction (from negative to positive). In A.C. circuits (see AC,
above) the electron movement
alternates in direction. |
DI
|
Direct
Input, or Direct Injection. See
the DI Box page. |
Decibel
|
See
the Decibel page. |
Desk
|
Another
word for Mixer. |
Distortion
|
A
non-linear difference between input and output signals in any audio device.
Ideally, any audio device (e.g. pickup, D.I. box, mixer channel, amplifier or
speaker) will change a signal in magnitude, but not in any other way. Any
non-linear change in the waveform (even if - as in the case of compression or EQ - the
change is intentional) represents distortion. Proportional changes in magnitude between input
and output are linear changes. Distortion describes any non-linear
changes.
Some
types of distortion are more objectionable than others: the ear can and does
adjust to moderate shifts in frequency balance (tone), so that mild tonal
colouration that is initially noticeable may become unnoticeable after a short
time. Also, some tonal changes (particularly modest reduction in the upper
midrange at high volume) may sound "better" than the undistorted
signal.
The
sort of distortion that is produced when circuits are saturated (resulting in
squaring of the signal peaks, also known as clipping) is generally noticeable
when the total distortion exceeds about 1%, and can become objectionable - and
remain objectionable over time - at higher levels. Here too, however, mild
distortion - e.g. saturation of analogue recording devices or
"overdriven" guitars - can sometimes sound "better" than the
undistorted signal.
|
Dry Hire
|
Hiring equipment without crew. |
Dynamic
|
A
type of microphone. See the Microphones
page for further details. |
Dynamic
Range
|
Expressed
in Decibels: the
difference between the quietest and loudest parts of a sound. |
Effects
|
Devices
that change a signal by adding altered versions of the original signal to
it (see the Effects
Units page). |
Enhancer
|
Effect
that adds synthesised harmonics to the original sound. See Enhancers
and Exciters on the Other
Effects page for more details. |
EQ
|
Equalisation or
Equaliser
(see Equaliser, below). |
Equaliser
|
A form of tone
control. Early audio equipment tended to reproduce audible frequencies very unevenly,
accentuating some frequencies and reducing others. Filter circuits, acting on parts of the audio
frequency range, were initially designed to correct (equalise) this. Although
equalisers are now more often used for other purposes (either to combat feedback
or "creatively" to tailor the sound for
individual tastes), the original description is still used universally. For details of
modern EQ, see the Graphic EQ and
Parametric EQ pages, or the EQ section on
the Mixers page. |
Exciter
|
See
Enhancer
(above). |
Expander
|
Device
that increases the Dynamic
Range of an audio signal. See the Expanders
page for more details. |
Feedback
|
Feedback
is what happens when some or all of the output of a device or system is
fed to its input. There are two kinds: positive
feedback (where the output reinforces the input) and negative
feedback (where the output opposes the input). Many electronic
circuits (as well as other devices) use negative feedback intentionally as
a means of regulating output. Feedback in PA systems, however, is
undesirable, and most commonly occurs when a microphone or other input
device picks up the sound coming from loudspeakers. |
Foldback
|
Monitor
systems provide foldback. |
Gain
|
Usually
stated in Decibels: the
ratio between input voltage and output voltage. Note that the ratio does
not have to be positive (the output voltage may be a fraction of the input
voltage, rather than a multiple of it): although in common vocabulary
"gain" has connotations of "increase", in electronic
circuits gain can be negative as well as positive, so that a PAD switch or
attenuator is applying negative gain (-dB). |
Hertz
|
Unit
of frequency (abbreviation Hz): the number of cycles per second. |
HF
|
High-Frequency.
In audio frequencies (nominally from 20Hz to 20kHz), there is no universal
definition of what counts as high-frequency, but depending on who is
speaking it can refer to anywhere from about 1kHz upwards (and in many
loudspeaker systems the HF driver operates from a crossover point of
1kHz). Most people would describe sounds above about 3kHz as
high-frequency. On most mixers, HF EQ
controls operate at around 10kHz. |
High-Pass
|
A
filter that allows only frequencies higher than its cut-off point to pass. |
Hypercardioid
|
Literally,
"excessively heart-shaped". A Hypercardioid microphone
has a narrower sensitivity pattern than a Cardioid
or Supercardioid
microphone. |
Hz
|
Abbreviation
of Hertz. |
Impedance
|
Impedance
(unit of measurement the Ohm, symbol Ω)
is the A.C. equivalent of resistance, and represents opposition to the
flow of current.
|
Insert
|
On
a mixer, an insert is a point at which an external device (e.g.
compressor) can be inserted in the signal path. See the Mixers
page for further details. |
Intelligibility
|
The
clarity of speech in an auditorium (whether or not a PA system is in use). In
rooms where reverberation times exceed about 1.5 seconds (-60dB),
intelligibility is often poor. Regardless of reverberation times,
intelligibility can also be poor in any audience position beyond the critical
distance.
A
standard measure of intelligibility is derived from the proportion of random spoken words a
listener can correctly identify. This is a more difficult task than listening to
a speech or announcement, where in the context of a sentence a listener may be
able to guess any less coherent words.
|
Lavaliere
|
Lavaliere
Microphone is often used to refer to a microphone that is hung around the
neck or clipped onto a tie or lapel. A lavaliere is a type of cravat, which is
how the term came to be used for neck-worn microphones.
|
LF
|
Low
Frequency. In audio frequencies (nominally from 20Hz to 20kHz), there is
no universal definition of what counts as low frequency, but the term is
often used interchangeably with Bass. |
Limiter
|
A
device that reduces the gain on large signals, limiting its output voltage
(see the Limiters
page). |
Linear
|
A
linear relationship is one that can be
represented by a straight line on a graph. In PA equipment, if input
voltage is plotted on one axis and output voltage plotted on another, the
effect of the equipment is linear if
the result is a straight line. Linearity represents freedom from
distortion. A linear power supply is one in which
output voltage is proportional to input voltage. |
Line-Level
|
In
audio circuits, line-level refers to a
nominal operating voltage of 315mV (-10 dBV) in domestic equipment, or
1.23V (+4 dBu) in professional equipment. |
Logarithm
|
A device used to describe a number by
referring to it as the power of another number. Commonly (and where no
other information is given you can assume) logarithms - aka logs - use
powers of ten to describe other numbers. For example, 100 = 10^2, so the
logarithm (log) of 100 is 2. Similarly, the log of 1,000 is 3, and the log
of 10,000 is 4. Thus we can describe very large numbers using much smaller
ones.
Logarithms become marginally more complicated
when the number we are describing is not a whole power of ten (for
example, 2 = 10^0.30103: the log of 2 is approximately 0.3), but most
modern calculators or spreadsheets do the detailed calculation for you,
using the log function.
Logarithms are used for decibel
(dB) calculations. |
Low-Pass
|
A
filter that allows only frequencies lower than its cut-off point to pass. |
Mid
or Midrange
|
"Middle"
audio frequencies. There is no universal definition of these, and because Mid
can refer to such a wide range of frequencies, the further qualification
of Low-Mid or High-Mid
(or Upper-Mid) is often applied. Alas, there is no universal definition of
these terms either. In
general discussion (or when someone asks for "more mid" or
"less mid") it could mean any - wide or narrow - range of
frequencies anywhere between about 250Hz and 3kHz. Midrange loudspeakers
commonly cover a range from around 120Hz to as high as 5kHz. For
this reason, other than for generalities Mid isn't a very useful
word, although the Mid driver in a loudspeaker
system will cover the range between the Bass
and HF drivers
(so "mid" does tell us something useful about it),
and if there are Low-Mid and High-Mid drivers it is also obvious which one is covering the lower and which the upper
part of this range. On
mixer channels with a single fixed Mid EQ, the EQ is typically
centred somewhere between 1kHz and 3kHz. Single Swept Mid controls
can sometimes cover a very wide frequency band (100Hz to 8kHz is not
unheard of, although something in the order of 250Hz to 5kHz is more
typical). Where there are two swept mids these can cover most of the audio
frequency range between them, with the Low-Mid covering from as low
as 40Hz up to 1kHz or 2kHz, and the High-Mid covering an
overlapping range from 400Hz or so up to 10kHz and beyond. |
Mixer
|
A
device that combines separate audio signals. See the Mixers
page for details of common mixer functions. |
Monitor
|
Loudspeaker
provided for musicians or engineers (rather than for the audience). See the Monitors
page for more information. |
Mute
|
Most
mixers have Mute switches enabling a
sound engineer to silence individual channels, groups, or other signal
busses. Radio microphones also commonly have a Mute switch. |
Octave
|
Musically
an Octave is an interval of eight whole notes (twelve semitones).
However, it also represents a doubling (or halving) of frequency, so that
there is an interval of one octave between 500Hz and 1,000Hz, and an
interval of two octaves between 500Hz and 2,000Hz. |
Ohm
|
Unit of
resistance (and, in A.C. circuits, of impedance). |
Omni
|
Omnidirectional.
In microphones, equally sensitive to sounds in any direction. |
Pan
|
Control
for varying a signal's position in the stereo soundstage. |
Passive
|
See
Active, above. |
Parallel
|
Speakers (or anything else) connected in parallel
are connected as the resistors (R1 and R2) are in the diagram below:
|
Parametric
|
Literally,
of or pertaining to parameters. A Parametric
EQ is one that has controls for all the parameters of EQ. |
Peak
|
Maximum.
In an AC sine wave, the Peak voltage is 1.414 (√2)
times the RMS
voltage. |
PFL
|
Pre-Fade Listen. A switch routing the
pre-fade signal (from a channel,
group or auxiliary) to the mixer's meters and headphones. See the Mixer
page for more details. |
Phantom
|
Phantom power. DC voltage (typically 48V, but it can range from 9V
to 56V) supplied through the microphone cable from a mixer's XLR inputs.
This can be used to power external input devices (e.g. condenser microphones
or DI boxes). Most
condenser and electret microphones require phantom power to operate, and
many other devices (e.g. preamplifiers and DI Boxes) that would
otherwise run on batteries can run on phantom power. If
the phantom supply is working and everything is wired correctly, a meter
should show: Pin 1 = 0V (ground) Pin
2 = +48V DC Pin 3 = +48V DC. |
PMPO
|
Peak Music Power Output. My dad is four hundred, and our car does
a zillion. |
Post
|
After.
Post-fade or
Post-EQ
happens after the fader or EQ section respectively. |
Pre
|
Before.
Pre-fade or
Pre-EQ
happens before the fader or EQ section respectively. |
Processor
|
A
device that alters (processes) an audio signal. See the Processors page
for further details. |
Q
|
Q refers to Quality
of Resonance, and in audio filter circuits - primarily parametric
equalisers and crossovers - describes the narrowness of a filter (the higher the Q value, the narrower
the filter). In audio filter circuits, Q can range from less than 0.26 (about
4 octaves) to 90 (about 1/60 octave) or more. A filter with a Q of 1.41
has a
bandwidth of one octave.
See
the Parametric
EQ page for more details. |
Reverb
|
Reverb
is an effect that mimics natural reverberation. See the Reverb
page for more details. |
RMS
|
Root
Mean Squared.
In an A.C. circuit, half of the voltage* waveform is
positive, and
half negative. The average (mean) of identical positive and negative values
is zero, regardless of how big or small the waveform is.
If the average is always zero, "average
voltage" doesn't tell us anything useful about the size of an A.C. waveform.
However, if the instantaneous values are squared, each value yields a positive
number (e.g. −2 x −2 = +4). Finding the square root of the average squared
values gives us a positive number - the RMS value - that is representative of
the average voltage, and allows us to compare one A.C. waveform with another.
*In A.C. circuits, RMS only
applies to voltage. There is no such thing as "RMS power" (power can't
have a negative value). If someone uses RMS to describe power, the best you can
hope for is that they are talking about average power calculated from
RMS voltage. More likely, however, they are using a technical-sounding term
without knowing what they are talking about. The reason they usually get away
with it is that most of the people they talk to know even less.
|
Series
|
Speakers (or anything else) connected in series
are connected as the resistors (R1 and R2) are in the diagram below:
|
SPL
|
Sound
Pressure Level. |
Supercardioid
|
Literally,
"beyond heart-shaped". A Supercardioid microphone has a
narrower sensitivity pattern than a Cardioid
microphone. |
Transducer
|
A
device that converts energy from one form to another. In PA systems this
includes microphones (converting sound to electricity) and loudspeakers
(converting electricity to sound), as well as contact pickups and other
less common equipment. |
TRS
|
In
jack plugs, TRS stands for Tip-Ring-Sleeve. Normally a TRS plug should be
wired in one of the following ways: |
|
Tip
= Hot
Ring
= Cold
Sleeve
= Ground |
Tip
= Left
Ring
= Right
Sleeve
= Ground |
Tip
= Send
Ring
= Return
Sleeve
= Ground |
|
In
Cables, TRS stands for Tough Rubber Sheath. TRS mains cables for touring
systems are commonplace. |
Unbalanced
|
A
signal transmission protocol in which the signal is carried on a single
core conductor, and the signal's voltage varies with respect to the (0V)
screen conductor. See Balanced,
above. |
VCA
|
Voltage
Controlled Amplifier. Many large-scale professional mixers include a VCA section.
See the paragraph on VCAs
on the Mixer page for
more details. |
