The Basic PA System
What is a PA system used for?
PA systems have two common names that describe their basic use, “public address” and “sound reinforcement.” Anytime someone wants to make something louder, a PA system is the sum of all the equipment to get the job done. There is one in just about every business, performance hall, stadium, arena, restaurant,
A PA can be as basic as a couple, battery-powered speakers strapped to a person’s vest, or as complicated as hundreds of thousands of watts being pumped into hundreds of speaker cabinets. No matter the form factor, the basic parts are the same.
Signal Flow in a Basic PA System
I’ve been talking
Audio Sources: Audio signals are picked up by some sort of transducer, which turns them into electrical currents. These are microphones, instrument pickups, and the outputs of keyboards, audio players, and other equipment and instruments.
The Mixer: Audio sources are sent via cables (or wirelessly) to a mixing console, either analog or digital. This console acts as a master routing device for the entire system. It accepts inputs, alters them in many ways (gain, EQ, panning, mute, solo, and other onboard effects), groups them as needed, then sends them out to hardware effects units, recorders, perhaps a computer, and eventually to some amplifiers. In a portable PA, the mixer might be attached to the back of a speaker cabinet. If a system only has a single input, there may not even be a mixer (since there’s nothing to mix).
Amplifiers: In a large venue, there is usually one or more racks of amplifiers, each of which feeds a speaker cabinet or cluster of cabinets. These “passive” speakers are lighter and therefore easier to install in large quanities—this is especially important when hanging them from the ceiling, as it makes the entire system safer. In smaller venues, speakers may all have built-in amplifiers. These are usually more expensive than passive speakers, are heavier, and you don’t have the luxury of going to an accessible rack if an amplifier fails.
Monitors and Mains: Speakers that are meant for the performers and engineers to hear are called “monitors.” Speakers for the audience are called “mains.” The mixer is capable of sending different aural content to the monitors, and usually different content to each (since each musician usually wants to hear a different mix of inputs/effects). In the first image on this page, you can see two large line arrays and many auxiliary main cabinets, plus
It’s easy to see what a system is like when it’s properly documented, and any prior planning saves time, money, and trouble. When a large show is planned, the performing group’s management usually sends the venue and the production company a “rider,” detailing the needs of the show. Some basic parts of a rider that the audio engineer needs to be aware of are:
The Stage Plot: This is a graphical representation of the layout of the stage. It should include an approximation of where everything will be placed.
The Input List: This one shows all the inputs and where they should plug into the mixer. It may also include any breakout boxes, mic types, possible effects on each input channel, and some basic routing.
Gear List: This is an itemized list of all gear required to make the show run. It’s especially important if two or more parties are providing equipment, so everyone gets back what’s theirs at the end of the show.
Monitor Mix Sheet: If available, this tells the engineer what inputs to send to each monitor on stage. For mall gigs, this is often set up during the sound check without documentation.
The Lighting/Rigging Plot: This one isn’t of high concern to the sound engineer during the show, but it’s good to make sure everyone’s needs are met. If the lighting isn’t set up during the sound check, for example, the engineer may have to move things when show time comes.
The Show Schedule: Punctuality is definitely a key trait for everyone involved in live shows. It’s also great to know exactly how much of a hurry you’re in. Running out of time can create catastrophic mistakes, and can cost you a lot of work in the future. It’s best to show up early to everything and good practice to get as long of a sound check as possible.