AUDIO SYSTEMS - PART 1

Basic Principles of a PA System

PART 1 - Basic PA System Configurations

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PA or Public Address Systems can trace their origins back to the early 1900’s when scientists

first experimented with a dynamic moving coil loudspeaker, a microphone
and an amplifier.

These three basic components are what shaped the future for the
modern day PA system as we know it today.
So, today the basic fundamentals of a PA system consist of these basic elements, but to better
understand how it works, the following is key to understand before setting up a system.

1. Source

The most common source is a microphone, but a sound source can also come from a singer's voice before being captured by a microphone. Other sound sources are pre-recorded music playback devices, such as a Laptop/ MP3 player.

2. Modifier

This is commonly where a sound source is modified or manipulated after it is received from the microphone or music playback device. The common modifier is called a mixer/ sound console. This device is typically between the source before it reaches the destination: speakers. Amplifiers are therefore modifiers.

3. Destination

Speakers connected to the Amplifier, is the final destination, before the amplified sound

reaches our ears. Here is a simple diagram to show the signal flow of a PA system,

illustrating the Source, Modifier and Destination devices.

The final destination after the speakers produce sound is your ears.

So, what is the basic principle of a PA System?

It’s used to amplify a sound source (voice into microphone) by means of a modifier/s

(mixer into amplifier) to reach its destination (speakers to the human ear)This is ultimately to convey a message that is loud enough to be heard by a group of people

that could otherwise not be heard without the use of a PA System.

A PA System is used for sound reinforcement to enable groups of people to hear a

conversation, speech or music within an environment, either indoor or outdoors.

PA System Types

PA systems are used in many different types of configurations and each serve a very different

purpose. Let’s look at the four most common types of PA System configurations:

Full Range Point Source PA Systems

This is the most basic form of PA system, with an amplifier and two full range speakers(a cabinet containing a high frequency and a mid-low frequency speaker driver) as an example. 

Two-Way PA System

This system will have a full range speaker and a separate bass or subwoofer speaker driven

by separate amplifiers and a two-way active crossover.

Three-Way PA System

This system will have separate high frequency, mid frequency and low frequency speakers in

separate cabinets, driven by separate amplifiers and a 3-way active crossover.

Distributed PA System

This system is found in office, boardrooms, restaurants, etc. primarily with downward firing

ceiling speakers, and a high voltage 75 volt/ 100 volt  Amplifier.

This system is most commonly referred to as a100 volt line or high impedance system

Line Array PA System

This type of system is found in either indoor or outdoor venues, and is most commonly used

for larger venues for music concerts, public gatherings and sporting events.

The same principle of source, modifier and destination applies to all PA System types.

Full Range Point Source PA Systems

This type of system is the most basic type of PA System. It would commonly comprise a microphone,

a mixer, an amplifier and two full range speakers or two active speakers, all interconnected to

form a system. If a decent mixer is used with a good number of inputs, multiple microphones,

instruments and music sources, such as a laptop or MP3 player can be connected to the mixer.

A PA System that makes use of a mixer and amplifier to power the speakers, the speakers are referred

to as Passive Speakers. If a PA System consists of a mixer and speakers that have their own

built-in amplifiers, these speakers are referred to as Active Speakers.

There are also mixers available with built-in amplifiers, and connect to Passive Speakers.

Below follow examples of basic PA Systems:

Two-Way PA Systems

A two-way PA System consists of separate bass and mid-high speakers.

These systems will always consist of separate amplifiers for bass and mid-high frequencies,

but the most important part of a two-way PA System is an Active Crossover.

This separates bass and mid-high frequencies before it is sent to the individual bass and mid-high

amplifiers. A two-way system can also make use of either passive or active speakers. 

The difference between a full range point source PA system and a two-way PA system is that a two-way PA System makes use of separate speakers and separate amplifiers for low and mid-high frequencies. An active audio crossover forms an integral part of making a two-way system work, to split frequencies.

So, what is an Active Crossover and what is its purpose?

An active crossover is a device that modifies an audio signal before the amplifier.

It works on line-level or preamp signals only. 

Active crossovers combine op-amps, resistors and capacitors to alter different frequency ranges

of the signal. An active crossover has selectable frequencies for bass and mid-high speakers. 

A crossover point can be manually adjusted on the active crossover, meaning it is at the crossover point where frequencies lower than this point will go to the bass speakers and frequencies higher than the crossover point will go to the mid-high speakers. Most modern active crossovers are either usable as two-way stereo or three-way mono active crossovers, meaning that two active crossovers in three-way mode would be needed for a three-way system.

If the crossover point is too low, the frequencies above the crossover point or high pass frequencies, may not be correctly produced in the mid-high speakers and the same will go for the bass speaker, as it may not be able to produce the low frequencies from the crossover point, if the bass speaker is not designed for low frequencies below its designed specifications. 

For instance, if the crossover point is set to 60Hz, the bass and mid-high speakers that are used, may not be able to effectively produce a good enough quality sound, meaning the mid-high speakers may not be able to produce bass frequencies as low as 60Hz, but the mid-high speakers might be able to work better at a higher crossover point of 120Hz. 

Let’s say we choose a crossover point of 120Hz, this means frequencies from 120Hz and below will be sent to the bass speakers and frequencies from 120Hz above will be sent to the mid-high speakers. 

Below is a diagram illustrating a crossover point of approximately 400Hz

Active Crossover

Front

Rear

Here INPUT levels can be adjusted, as well as the crossover frequency point, and also separate low and high output levels.

Below follows a series of illustrations, demonstrating the wiring interconnectivity between an Active Crossover, Bass and High Amplifiers and Bass and Mid-High Speakers.

Two-Way Active Crossover Inputs and Outputs

Interconnectivity between the Crossover and Amplifiers

Interconnectivity between Amplifiers and Speakers

Quick Recap of what we've learned so far

1. Source, Modifier and Destinations are terms used to describe the basic principle of how

a PA System works, as it needs a sound source, which is then modified and fine tuned before it

reaches its destination.

2. A basic PA System consists of a microphone, mixer, amplifier, two speakers and the required

speaker and signal cables to connect it all together.
3. A two-way PA System consists of an active crossover to send dedicated frequencies to the

correct speaker.
4. Two-way PA Systems also consist of separate amplifiers and separate bass and mid-high speakers.

Next Up...

We discuss Three-Way and Distributed PA Systems. Line Array PA Systems will follow in

the third in an in-depth and separate article.

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