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.

© Intellectual Property Rights Reserved - Marius van Vuuren, Dubai, UAE

AUDIO SYSTEMS - PART 3

Audio Systems - Basic Principles of a PA System

PART 3 - Distributed PA Systems

Please Follow me on Instagram and subscribe to my Youtube channel

So far we’ve discussed the following:

1. How a PA System works, and its intended function

2. Basic Elements on how to connect a PA System and splitting this up as sources, modifiers and destinations.

3. Types of PA Systems,

1. Full Range Point Source PA System

2. Two-Way PA System

3. Three-way PA Systems

4. The fundamental differences between a Two-Way and Three-Way PA System

This article will cover basic Distributed PA Systems, and look where this type of PA System is found, its functionality and versatility.

So, what is a Distributed PA System?

A distributed audio system simply delivers audio to many speakers rather than to just a few. 

Distributed PA Systems can be found in fully integrated meeting rooms or boardrooms, hotel’s, schools, supermarkets, restaurants, health clubs, shopping malls, etc.

These systems are commonly referred to as “constant power systems” or “70 volt systems” or even “100V line systems”. For this article, we will look at 100V line systems.

Why will a normal 8Ω speaker not work in a Distributed PA System?

The typical 8-ohm speakers you might use in a home hi-fi system or a basic PA system wouldn't work well as for a distributed audio system. The purpose of a distributed PA System is to amplify a lot more speakers and, critically, they're much, much farther away from the amplifier than they are in your living room.

To get around this, distributed audio systems make use of a typical 8-ohm speaker, but with the addition of a transformer. The transformer essentially steps down voltages coming to the speaker. 

Also important to note is that amplifiers in a distributed system send 100V to the speakers, hence why it's important to step down the voltage to enable the use of multiple speakers with transformers connected to them.

To get around this, distributed audio systems ordinarily use 8-ohm speakers with an important modification: a transformer that takes a 100V input and steps it down to the lower voltages needed by the 8-ohm speaker. To understand why this would make sense, we need to remember Ohm's Law, which states:

P=VI (power = voltage x current)

The transformer isolates the speaker's impedance load from the amplifier. The transformer can be set to a wattage rating, allowing for independent volume level for each speaker. The net result is a 100V line system allows for long ‘strings’ of speakers to hang off a single amplifier output, each speaker is connected to the next one in the circuit by daisy chaining the wiring.

Advantages and Disadvantages of 100V Line Systems

Advantages

• Being able to connect a number of speakers to one amplifier.

• As needs change, additional speakers can easily be added or subtracted from a circuit.

• Simple load calculation. Simply add together the wattage of each speaker in the string, and ensure it is below the rated wattage of the amplifier.

• Individual transformer “tap” settings allow tailored volume levels for each speaker.

• Suitable for use with long runs, making use of relatively small gauge speaker cabling.

Disadvantages

• Sound quality. Most 100V line speakers do not perform brilliantly at low frequencies, however for general announcing this is not generally a concern.

• Additional cost per speaker due to fitment of transformer, line monitoring capacitor and regulatory requirements (such as speaker, transformer enclosures and cable restraint plates).

• Usage is limited to paging, voice and low level background music applications.

How does a 100V Line System Work?

A 100V line speaker is operated by applying 100V RMS from the amplifier to the speaker step down transformer via suitable speaker cabling. 

The step down transformer then converts the 100 Volts down to a standard speaker level to output audio to the speaker. 

100V line amplifiers are a bit different from conventional amplifiers, but there are certain professional amplifiers that will produce 100V RMS or more when the amplifier is run in bridge mode. Always check the amplifier manufacturer specifications first before attempting to use the amplifier in bridge mode for a 100V line system.

Above is a typical distributed PA System speaker with a transformer. Always check the manufacturer specifications, speaker rating and transformer rating to make sure it will work for your planned installation.

Above is an example of a 20W transformer with tappings at 20W, 15W and 10W

The illustration above shows connections on

How many Speakers can I connect to my 100V Line System?

Each speaker should be wired in parallel and the wattage of each speaker tap is added together to find the net result of wattage load on the amplifier. So for instance, a 100W amplifier is capable of powering 100 speakers when each speaker is set to a 1W tap, or 50 speakers when each speaker is tapped at 2W, and so on.

You may turn up at an installation that was done years ago, and you have no idea how many speakers are installed or what power taps they have been set to. In this case you will need to resort to measuring the AC impedance of the speaker system.

Importantly, this differs from DC resistance that most multimeters offer as a standard range setting. To measure the AC impedance you will need a dedicated Audio Impedance meter. 

Below is a table to work out 100v system loads (Impedance measured in Ω)

LOAD	100V SYSTEM
0.5W	20kΩ
1W	10kΩ
2W	5kΩ
3W	20kΩ
5W	3.3kΩ
10W	1kΩ
15W	666Ω
20W	500Ω
30W	333Ω
40W	250Ω
60W	166Ω
100W	100Ω

As an example, the above schematic shows how four speakers are connected to a 100V line amplifier and the speakers with transformers, all wired together in parallel.

Here’s a few tips for 100V Line PA System Installations.

• Keep all the speakers in the same polarity. Amplifiers are marked with (+) and (-) on the 100V output terminals and speakers are marked with red and black on the terminal blocks. Maintain the same polarity throughout your speaker circuit.

• Be conservative with your ratings. Always design a system with 20% spare capacity, so if you have 80W of speakers, buy a 100W amplifier. This allows for expansion of the circuit later or changes to individual power taps for areas that might need extra volume.

• Take advantage of higher wattage speakers where possible. If it means you need to buy and install less speakers overall the system will work out considerably cheaper.

• Cater for local attenuation (or volume level) control. More often than not people like to adjust the level of sound in their work environment, especially with background music. Allowing for local level control makes it easy to set a comfortable volume.

• Fixing or expanding an existing install? Use an impedance meter! It’s an essential tool for a PA installer to ensure the speaker load stays below the amplifier rating.

• Do not EVER mix 100V line speakers with traditional low impedance speakers within the same circuit. 100V Line amplifiers are not designed for powering low impedance speakers and 100V line speakers at the same time. If this is required, I suggest using two completely separate amplifiers.

Quick Recap

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

• Distributed PA systems are commonly referred to as 100V line systems.

• 100V line systems have a multitude of speakers connected in parallel to one amplifier.

• The speakers in a 100V line system all have transformers to step down voltages coming to the speaker.

• A 100V line system is limited to the amplifier output power and number of speakers tapped at the appropriate wattage.

• Calculate your systems overall wattage and calculate at least 20% additional amplifier power for future speaker additions.

Next Up…

Line Array PA Systems: These systems are primarily used for indoor or outdoor public gatherings and concerts.

© Intellectual Property Rights Reserved - Marius van Vuuren, Dubai, UAE