Understanding How a Basic PA System Works
Updated: Aug 17, 2021
Many people in need of renting a PA system for their event may have little to no idea of how a typical PA works, thus leading to the possibility of renting a system either too large or too small for the event’s requirements. One unique feature of PA systems is that no two are alike. While many manufacturers might make every component that a system needs, it is common to mix and match components from different manufacturers, usually stemming from an engineer’s personal preferences. This guide will explain the basic parts and functions of the most common type of PA systems, basic setups with ground-supported speakers. We won’t be getting into large-scale concert PA systems with line arrays in this post, those types of systems deserve their own post Let’s dive in!
What is a PA System?
A PA system stands for “Public Address System.” The origins of the PA system dates back to around 1910, when the Automatic Electric Company of Chicago, Illinois, announced it had developed a loudspeaker, which it marketed under the name of the Automatic Enunciator. By 1913, multiple units were installed throughout the Comiskey Park baseball stadium in Chicago, both to make announcements and to provide musical interludes. Charles A. Comiskey was quoted as saying: "The day of the megaphone man has passed.” PA systems don’t just refer to the speaker cabinets prevalent at music venues and festivals. Any system of one or more speakers designed to replicate audio or speech to a group of people qualifies as a PA system. However, for the purpose of this post, we’re going to talk about the typical concert and event PA systems.
The first component that comes to mind when thinking about PA systems is the most important one, the speakers. PA speakers come in countless different shapes and sizes. There are three main types of PA speakers: mains sometimes referred to as “tops,” subwoofers, sometimes referred to as “bottoms,” and stage monitors. Each type of speaker serves a different function within the system, but each depends on the other.
Main Speakers create the bulk of the PA’s sound. In basic PA systems, the main speakers are either placed on speaker stands or mounted on top of the subwoofers. Main speakers in your basic PA system are normally sized between 10"-15" with a smaller tweeter speaker above the woofer.
Subwoofers are larger than the main speakers and produce lower frequencies than the mains. This has the effect of “filling out” the sound that the ear will hear. Subwoofers are typically 15"-20" speakers, although dual 12" subs have become more common. To separate the sound of the subwoofers and mains, a crossover unit will almost always be employed. The crossover is usually rack-mounted and separates the signal going through it by frequency, sending lower frequencies to the subwoofers and higher ones to the mains. An important part of tuning a PA system is selecting the correct crossover frequency for the room in which the PA is situated.
Stage Monitors are usually positioned near the performer or speaker to help them hear themselves. They are on a separate mix than the mains and subs, also known as the front-of-house speakers. Many main speakers are purpose-designed to also act as stage monitors if so required. Stage monitors are usually on the ground tilted at an angle towards the performer.
PA speakers can either be passive or active. Active speakers have an internal amplifier of their own while passive speakers have no internal amplifier and require an external amplifier to convert the line level signal of the mixer to a level where it can drive the speaker to the necessary volume. Amplifiers can be an expensive item, but deservedly so. In a passive PA system, you are trusting the entirety of the system’s sound to one component.
A mixing console is one of the most important parts of a PA system and like other PA components, the options on the market are endless. A mixing board will have a set number of channels and is responsible for combining sounds, routing, and changing the volume level, timbre (tone color), or dynamics of many different audio signals. Inputs on a mixer are commonly XLR and TRS (¼"). A mixer can provide phantom power for capacitor microphones, pan control on each channel, and monitoring mixes, for the stage monitors. Most mixing consoles will have left and right main outputs and individual outputs known as auxiliary sends, most commonly used for stage monitors or effects.
To connect the components of a PA system and transmit an audio signal, various cabling is required. PA speakers most commonly take one of three forms of cable: XLR, TRS, or Speakon. Mixers and amplifiers usually have main outputs and inputs of both XLR and TRS. Some amplifiers can have a form of RCA outputs called banana cabling. Using the correct cabling when setting up a PA is vitally important. If wrong cables and/or connectors are used, equipment may not operate correctly. In the worst-case, using the wrong cables or connectors can be dangerous.
An optional, yet common component of a typical PA system is effects. Many modern mixers will have onboard effects, however, effects paired with a PA system are usually outboard, meaning stand-alone units. Common effects paired with a PA system are reverb, compression, delay, gates, and equalizers.
PA systems have various applications, thus there are a variety of common sound sources for PA systems. The most common source is the sound from a microphone. Microphones also have a variety of uses and placements, ranging from vocal mics, instruments mics, and room mics. PA systems are also great at reproducing already recorded music. Music can be played through a PA system by feeding the sound through one or more channels on the mixer.
The signal flow of audio within a PA system is inherently simple, however, there are many points at which a novice might become confused. Every PA system is different, however, a typical signal flow is depicted in the figure below.
Operating even a simple PA system can be frustrating, although rewarding. For many small-scale events such as speeches and conferences, little to no tweaking of the settings on the mixer is required after soundcheck. However, for large-scale productions such as concerts, it is imperative that an engineer is present to mix the sound for the duration of the event. Due to the complex nature of music, vital changes to the PA systems sound are often required constantly. Those renting a PA system can often regret opting to forego hiring an engineer, as they find out quickly that their event’s sound required much more attention to detail.
Interested in learning more about PA systems? Check out our free ebook How Does a PA System Work?
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