AUDIO SYSTEMS - PART 4

Audio Systems - Basic Principles of a PA System

PART 4 - Line Array PA Systems

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So far we’ve discussed the following:

• How a PA System works, and its intended function.
• Basic Elements on how to connect a PA System and splitting this up as:

Sources, Modifiers and Destinations.

• Types of PA Systems,
• Full Range Point Source PA Systems
• Two-Way PA System
• Three-way PA Systems
• Distributed PA Systems

• Explaining the fundamental differences between a Two-Way and Three-Way PA System
• The advantages and disadvantages of Distributed PA Systems

This article will cover where and how Line Array PA Systems are used, its origins,

basic configuration and why they are so important to make use of in modern times.

Quick History Lesson...

The line array effect of the narrowing of the beam with increasing frequency was first demonstrated by

acoustical pioneer Harry Olson.He published his findings in his 1957 text, Acoustical Engineering.

Olson used line array concepts to develop the column speaker in which vertically aligned drivers in a single enclosure produced mid-range output in a wide horizontal and narrow vertical pattern. 

Line arrays have been around for over half a century but until recently most were voice range only.

The application for these was for highly reverberant spaces where a narrow vertical design kept

from exciting the reverberant field. A multi-band line array element in a horizontally oriented enclosure

was suggested by Joseph D'Appolito in 1983. 

This led to the discovery that a more level and smoother frequency response can come from fewer

boxes in a line array. The industry also soon found that there was no destructive interference in the

horizontal plane and waves combined mostly in phase in the vertical plane, causing many loudspeaker

manufacturers to quickly develop their own similar product.

So, what is a Line Array PA System?

The term “line-array” is used to describe a speaker system having multiple speaker units with the same

characteristics arranged vertically and fed in phase, to emulate a linear acoustical source. 

It is a series of loudspeakers that covers the same frequency range and is stacked above one another.

The key to a line array is that the speakers face slightly different vertical angles, allowing them to

consistently cover a greater depth of field than a single PA speaker can. 

Essentially, Line Array PA Systems provide a very narrow vertical output pattern

useful for focusing sound at audiences without wasting output energy on ceilings or empty air

above the audience. They have become the go-to choice for large concerts and arena shows due

to their excellent projection of sound over long distances.

What is the difference between line array and point source audio systems?

Line arrays can be adjusted in the field for different coverage patterns, but with limitations.

Line Array PA Systems are more expensive compared to a Point Source PA System.

Another benefit of line arrays is that they provide good control of the vertical coverage, but provide

a predetermined horizontal coverage.

Line arrays can get loud, and historically this is the major benefit over point source systems.

So, what is the benefit of a Line Array PA System?

The line-array speaker offers the following three advantages over standard point-source

speakers. 

Sound reinforcement with more consistent sound pressure 

With this type of speaker, sound pressure is affected less by distance, allowing for clearer,

more consistent sound reinforcement. As a result, ample sound pressure can be delivered

to the back-of-house without making front-of-stage excessively loud. 

Point Source Speaker

More attenuation over distance—i.e., sound pressure attenuates by 6 dB for each doubling. 

Sound pressure is thus affected to a greater degree by distance, resulting in excessive
in-front-of-stage volume.

Line-array speaker (line source) 

Less attenuation over distance—i.e., sound pressure attenuates by 3 dB for each doubling. Sound pressure is not affected as much by distance, so in-front-of-stage volume does not need to be excessive. 

Clear sound reinforcement 

Undesirable reflection can be reduced, thereby ensuring clearer reinforcement of sound

even in highly reverberant spaces. 

Standard speaker (point source) 

Because the sound also disperses widely in the vertical plane, a significant amount of

reflection occurs at the roof and floor surfaces.

These reflections are heard as reverberation and make the reinforced sound less clear.

Line-array speaker (line source)

Sound waves disperse very little in the vertical plane, significantly reducing the amount of reflection at the roof and floor surfaces.

Traveling directly from array to audience and free of reflections, the reinforced sound is clearer overall.

Less prone to microphone feedback

As we have seen, the total output from line-array speakers is smaller and undesirable reflections are fewer, making feedback less likely to occur.

Another important factor, however, makes the line array less prone to this type of problem,

and this is described below. 

Whenever a certain sound pressure must be achieved at a specific distance, the line-array
speaker, which adds the sound energy output by multiple speaker units, will require less
output energy per unit than a standard point-source speaker system.
A microphone in close proximity to the array will only pick up the relatively small amount of
energy from the closest units.
The total incoming energy will thus be considerably smaller than with the standard speakers,
meaning that feedback is less likely to occur. 

Standard Speaker (Point Source)

The microphone picks up a larger amount of incident energy.
The reinforced sound comes from a single source, so the energy is not scattered.

Line-array speaker (line source) 

The microphone picks up a smaller amount of incident energy.

The reinforced sound comes from multiple speaker units, and the total energy is distributed among them.
The energy from only some of these speakers is picked up by the microphone.

How to predict the behaviour of a Line Array System?

The best way to do this is with a software program.
Most manufacturers have their own software available to users, to accurately determine the

behaviour of a specific model system, in a specific environment.

This will help audio engineers and designers to:

• Plan an entire portable or fixed loudspeaker system and determine delay settings for fill loudspeakers.
• See interactions among loudspeakers and minimise destructive interference.
• Place microphones anywhere in the sound field and predict the frequency response,
• impulse response and sound pressure at the microphone position.
• Refine system design to provide the best coverage of the intended audience area.
• Use a virtual equaliser to pre–determine the correct settings for best system response.Gain load information about the array, to determine rigging capacities.

A basic line array PA System Setup

Depending on the brand of Line Array PA System and the orientation required for the use of it,

the setup of a line array system can vary greatly, with most applications opting to fly/ rig

the system overhead. Most manufacturers will provide rigging tutorials as well as setup guides,

to effectively set up systems.

In this instance, let’s look at a very simple indoor hall setup, with the line array system flown/ rigged from the roof support structure, and the subs being stacked at the front of the stage.

Amplifiers are placed to the left and right side of the stage, to keep cabling lengths to a minimum. 

It is very important to note that if you are unsure how to rig/ fly the line array, always consult with a

professional rigger/ rigging company, to help you with your project.

When rigging a Line Array system, always follow the manufacturer's guidelines and make sure that

the person/s installing the system are qualified to do so (professional riggers).

This is very important, as safety is of absolute paramount importance when rigging a line array system.

Before deciding to fly a system, best to check with the venue with regards to rigging your system,

and if it is alloyed or not. 

Most venues will specify load capacities, and in some cases, will provide a rigger/ rigging contractor

at cost, to install your system safely.

Below is an illustration of the system:

Basic indoor 1.8m high stage, roof beams, line array support frame, trussing, etc.

And Remember: Always follow safe rigging practices!

Line Array speakers flown left and right, with subs at the front of the stage

Quick Recap

So, we’ve learned the following with Line Array PA systems:

* Line Array PA Systems deliver sound reinforcement with more consistent sound pressure

and clear sound reinforcement.

* They are less prone to microphone feedback compared to Point Source PA Systems.

* Manufactures provide software to calculate sound pressure within a specified venue, to help
audio engineers and designers to determine the best possible audio coverage.

* When rigging a Line Array System, always follow the manufacturers guidelines, and always
make use of safe rigging practices.

Next Up…

We start exploring Video Systems and look into projection, projection screens, LED screens,

commercial LED screens/ panels, touch screens, video camera systems,

broadcast video systems and so forth.

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