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Please note that this chapter is in outline form and will be fleshed out later. Use it to guide you through your own research.
Low Impedance Systems
4, 8 or 16 ohms
To expand, speakers must be connected in combination series-parallel arrangements to maintain the correct impedance.
Constant-voltage Speaker Systems
Eliminates impedance matching issues with multiple speakers in parallel
A constant voltage system places all speakers in parallel. As speakers are added, the total impedance decreases (see resistors in parallel). The amplifier compensates by lowering its output impedance to match the speaker network.
High voltage
Reduced current and, therefore, wire size required to distribute the signal to distant speakers
25-volt, 70-volt, 70.7-volt, 100-volt and higher
70-volt
Based on 1200 watts into 4 ohms.
A step-up 70-volt transformer is needed for less-powerful systems
Transformers connect the 70-volt line to speakers
Some PA speakers come with built-in transformers
Transformers may have taps for different-powered speakers
Large Venue Systems
Often have two systems, one for the audience (the front of house) and another for the performers (the monitor system [speakers are called wedges]).
Individual performers may have their own speakers so they can hear themselves at a different level than the audience (a backup singer may need to hear himself/herself at a higher level).
Modern systems may have in-ear monitors
Each performer can choose the channel and volume he/she hears.
Feedback
Feedback is the audio tone generated when the output of the speakers returns to the microphone under certain conditions, creating an acoustically-coupled oscillator.
Reduction
directional microphones
reducing the volume from speakers near the microphones
angling speakers away from the person speaking
lowering gain at frequencies where feedback occurs
When setting up a PA system, audio engineers will test the venue acoustics and reduce the volume of frequencies that tend to feedback. This is called "equalizing the room". Some modern systems do this automatically.
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