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Key terms and concepts
Audio desk
See Mixer and Mixing
Desks
Audio equipment and accessories
This includes audio desks (mixers), amplifiers, CD players, input
source equipment, loudspeakers (both front of house and monitor/foldback), microphones,
signal processing equipment and audio accessories such as MiniDisc, cassette,
direct injection units, cables, multi-cores, power supplies and RF units.
Audio installation plans
These are usually 'ground plans' or 'bird's eye views' of
the equipment and its position in a venue. They include representations of
all installed audio equipment, signal flow and possibly some peripherals or
accessories.
Audio operations
Those tasks which involve operating audio equipment. These include operating
a PA system, input sources, microphones, mixer, amplifiers and loudspeakers.
Audio technician
A person who has the skill, expertise and knowledge to undertake audio operations.
Cables
All electrical components in a sound system are
interconnected with cables - wiring which has been temporarily rigged to
carry electrical current.
Decibel levels (dB)
A relative measurement for the volume (loudness) of sound. One dB is the smallest
variation in loudness that the human ear can detect. It is also used to measure
the difference between two voltages, or two currents. Zero dB (0dB) is the
common reference point when discussing sound levels. Levels above 0dB are expressed
as positive (+5dB) and those below as negative (-20dB). 0 dB represents the
threshold of normal human hearing; 130 dB represents the threshold for pain;
140 dB causes irreparable hearing damage; 150 dB can cause instant deafness.
Anything greater than about 192 dB can cause death.
Electricity
Conductive objects are always full of movable electric charges
(electrons), and electric currents are motions of these charges. Voltage pushes
the conductors' own
charges along. A conductor has a certain amount of electrical resistance or
'friction' and friction with the flowing charges heats up the resistive object
(the cable or filament). The flow rate of the moving charges is measured in Amperes,
while the transfer of electrical energy (as well as the rate of heat output)
is measured in Watts. The electrical resistance is measured in Ohms. Electrical
power is used to drive all components in an audio system - even the potential
energy in the magnetic field inside a microphone results in electrical energy.
Hertz (Hz)
A measurement of the frequency of sound vibration. (replaced the old measurement:
cycles per second) One hertz is equal to one cycle per second. Frequency is
heard as pitch (high or low in sound) and the extremes of human hearing are
lowest at around 20 Hz and highest at around 20 kHz (20, 000 Hz). 10 Hz is
the cyclic rate of a typical car engine at idle (equivalent to 600 rpm). 100
Hz is the cyclic rate of a typical car engine at redline (equivalent to 6000
rpm
Mixer (mixing desk, sound desk, sound board)
A desk
comprising a number of input channels, where each sound source is provided
with its own control channel through which sound signals are routed into two
or more outputs. Many mixing desks can also change the quality of the sound
through equalisation. A powered mixer has an amplifier built into it. Sound
sources of varying levels are accepted which can be amplified if necessary.
Occupational health and safety (OHS)
There are many OHS issues with regard to audio operations but the most important
are actually related to excessive volume (SPL) in the workplace.
Phase and phase cancellation
Two identical waves are said to be 'in phase' whereas two identical sound
waves which are slightly apart in time are said to be 'out of phase'.. Two
identical waves in opposite phase (one is up while the other is equally down)
cancel each other out acoustically. This has significant repercussions for
an audio operator. If one speaker is out of phase with the other in a PA (the
negative cable is connected to the positive input on one speaker) then several
sounds will not actually be reproduced. This can be a problem in live sound
and a blessing in recording where a copy of an unwanted signal can be put out
of phase and removed completely.
Positioning and equalising techniques
Positioning of microphones and loudspeakers with regard to performers and
the walls/floor/surfaces of the venue will have a significant effect on the
overall sound of a PA. Particular frequencies may be increased or reduced and
standing waves and feedback may be created. Equalisation (EQ) is the process
of decreasing or increasing particular frequencies that are altered by the
acoustic properties of a room or theatre.
Problems and faults
Audio is one of the hardest features of live theatre and events to control.
If something is not working properly (a problem) or not working at all (a fault)
the audio technician is expected to find the offending device, cable or connection
and rectify the situation.
Rigging and positioning
Rigging and positioning of audio equipment is essential to its optimal operation.
Amplifiers must be positioned in cool, dry positions; loudspeakers must be
directed to the ear-height of the audience; microphones must be directed at
(or near) the sound source; and cables should be rigged above the audience
or placed carefully so as to avoid trip hazards or disconnection.
Signal processing equipment
Those devices in a PA/sound system designed to modify or alter the electronic
signal from a microphone, instrument or any other signal input source like
a CD player. Signal processing equipment includes reverb units, equalisation
units, distortion pedals, compressors, limiters, expanders and effects units.
Sound in a circuit
A circuit is a complete path that allows electrical
current from one terminal of a voltage source to pass to the other terminal,
creating a complete electrical 'loop' around
which current can flow. If 'sound' is in a circuit, it is an electrical 'analogue'
of that sound which is being passed through a conductive material from one
device to another.
Sound Pressure Level measurement
Sound Pressure Level (SPL) is the loudness
of an acoustic wave stated in dB that is proportional to the logarithm of its
intensity. It is a measurement of the loudness or 'volume' of a sound.
Sound system signal flow chart
A signal flow chart of a sound system outlines the direction of signal flow
between the devices in a sound system. This usually includes (at least) sources
(mics, CD players instruments etc.), mixer, amplifiers and loudspeakers.
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