Public Address Systems

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. This variable impedance results in a constant voltage (assuming a constant volume).

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.