You would use PA Systems in situations such as making speeches, playing pre-recorded music, or to distribute messages around a building for people to hear.
This electronic system reinforces any given sounds by amplifying it through a mixer unit and then broadcasting it through a specified or dedicated set of speakers.
Small Venue PA Systems
Small PA systems are simple amplification devices that consist of a microphone, a low powered mixer unit (which amplifies and then disperses the amplified sound signals) and a set of loud speakers (one or more).
This particular type of system provides as much as 50 to 200 watts of power and is often used in small venues such as school auditoriums, churches and small bars.
The input sources for these devices can be from either a microphone or even a CD player, which in turn provides the mixer unit with a sound source that is then amplified into audio signals of 50v, 70v or 100v (which are speaker line levels). Once the mixer unit amplifies the sound to the correct voltage it then distributes the signal to the speakers.
Large Venue PA Systems
These setups are for instances such as a live music concerts where a far more powerful pa system is required, these systems are far more complicated, but are ideal for live sound reproduction.
Usually in a concert scenario there would be two complete PA systems, one would be the main system and the other would be the monitor PA system.
Each system would include its own microphones, a mixing board, sound processing equipment, amplifiers, and speakers as explained below.
FOH PA System
Known as Front of House or simply FOH PA System the main system provides the amplified sounds that the audience hears at a concert. There are usually a number of powerful amplifiers which distribute sound signals to a series of powerful, heavy duty speakers and also a set of low frequency speaker cabinets called subwoofers.
A FOH PA system can provide as much as 1000 to 10,000 watts of power to the main speakers, depending on what scenario it is being used for (eg nightclubs or outdoor concerts).
Monitor PA System
The monitor PA system reproduces the sounds from the performers and then directs it directly back towards them to assist them in their performance by helping the performers to hear their instruments or vocals. This setup is referred to as the "fold back" system.
During live performances at concerts or nightclubs, there would be technical staff named sound engineers that would control the mixing boards for complete PA systems including the FOH PA system and monitor PA system.