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Live Sound (Part 1)

Introduction To PA

This comprehensive series starts by looking at the basic components of a mobile PA system.


Many studio musicians would love to get out and expose their music to an audience, but are daunted by equipment and techniques they don't understand. In this major new series, Paul White demystifies live sound, starting with the components of the PA system.

Music is about more than just recording, though since Recording Musician is directed primarily towards the musician who — um — records, it's all too easy to lose sight of the fact that many also perform live from time to time. Indeed, we've already had letters from readers wanting more information on live sound systems, and as many of the components and techniques are common to the recording studio anyway, we're going to show you what's what in the world of live sound with a comprehensive series starting right from square one.

The PA



For larger venues and tours, it's usual practice to hire in a Public Address (PA) system and engineer, but for pubs, clubs, smaller college venues and the like, it may be more cost effective to buy your own. A stereo system capable of outputting several hundreds of watts of power can be small enough to fit into the back of an estate car or small van, but it's important to choose the right type of system for your application, especially when it comes to loudspeakers and amplifiers. For example, if you're a solo folk singer working with voice and guitar, there's no point in getting a system with powerful bass bins — you won't be generating much in the way of low-frequency sound in the first place. Conversely, if you want to amplify an entire rock band or a synthesiser setup that is generating deep bass and drum sounds, then you need to have a system that can reproduce those frequencies.

The role of a concert PA system is fairly easily understood, in that it has to amplify the whole band and project the sound to fill a large auditorium, but in a club environment, the PA might be more accurately considered as a sound reinforcement system augmenting the sound of the on-stage amplification, or back-line (guitar and keyboard amplification and so on), as it is called. In this case, the PA system still needs to be able to handle the full audio spectrum, but its main job will be to lift the vocals over the level of the back-line and drums. If you want the engineer to have control over a mix in a small venue, it is helpful to take modestly-powered back-line amplification and then mic it up. Not only does this give the mix engineer more control, it also reduces the amount of spill from the back-line amplifiers into the vocal mics, resulting in a clearer sound all round.

The System



A PA system is more than just a big hi-fi system, because it often has to work in a less than ideal acoustic environment and must be designed to cope with the highly reverberant rooms that are so often encountered on the gig circuit. If the room is very reverberant, obtaining an intelligible vocal sound can be very difficult; a basic understanding of the relationship between the room acoustics and acoustical properties of the loudspeakers is vital. In any room, the direct sound from the loudspeakers will become quieter as you move further away from them; that relationship is governed by the inverse square law, as you may remember from school physics. However, the reverberant or reflected sound does not follow the inverse square law, as it does not emanate from a single point source but bounces off every hard surface in the room. Consequently, the reverberant sound might be equally loud wherever you are in the room. As you move further from the loudspeaker, the direct sound gets weaker compared to the reverberant sound, until you reach a point when both direct and reverberant sound are equal in intensity. This is known as the critical distance, and beyond this point, the reverberant sound predominates, making it progressively harder to define detail in the direct sound. The critical distance varies depending on the room characteristics and on the directional characteristics of the loudspeaker system; as a rule, the more directional the system, the further the critical distance from the loudspeaker. As we want the audience to hear as much direct sound as possible, it follows that the critical distance should be as large as possible. However, the directional characteristics of the speaker must also be chosen so as to provide acceptable coverage of the audience. The choice and positioning of loudspeakers will form a significant part of this series, but for this first part at least, I'll stick to generalities.

Your mixer need not be complex; indeed, basic stage mixers are usually simpler than multitrack studio mixers, but they do need good, basic tone controls and should have provision to connect external effects units. Digital effects units are commonly used to add reverb and echo to live performances, and the use of more sophisticated effects is by no means unusual — if it can be done in the studio, it is only natural to want to be able to recreate it live. Again, stage mixers and their facilities will be covered later in the series.

Stage Monitoring



A portable PA system must also be very rugged, both electrically and mechanically, and it must be easy to set up and take down. Correct positioning of the speakers is critical if acoustic feedback is to be avoided, and for all but the smallest systems, there is the business of on-stage monitoring, or foldback, to worry about. This may be considered as an independent sound system, comprising power amplifiers and speakers running from the foldback output on the mixing console, their purpose being to provide the performers with a mix of the performance to help them keep in time and in tune (after all, the performers on stage are behind the main PA and so usually unable to hear it). The most important job of a small foldback system is to ensure that the singers can hear themselves properly, but in a larger system, it is commonplace also to add the instruments into the foldback mix.

Touring PA companies have very elaborate foldback systems, usually controlled by a separate engineer from a position close to the stage or even on it. These utilise special monitor mixing consoles and can provide several different versions of a mix over several different monitors so that each performer hears precisely what he or she needs to. For example, the singers may need to hear each other more than anything else, whereas the bass player might need to hear the drummer and the drummer the bass player.

Monitor systems are often neglected or chosen on the basis of low price, but as we shall see later in the series, poor quality monitor loudspeakers greatly increase the chance of acoustic feedback and can seriously compromise a performance. Stage monitors often take the form of floor-standing, wedge-shaped speaker enclosures, but there are alternatives, such as small stand-mounted loudspeakers, full-range speaker systems suspended above the performers, or even miniature earphones.

Power Amplifiers



Power amplifiers should be chosen with enough power to deliver a clean signal at the full rated power of the loudspeakers. You might think that choosing smaller amplifiers will protect your speakers from damage in the case of overload, but the truth is the exact opposite. Once an amplifier is driven into distortion, it produces a lot of high frequency harmonics, which can fry a tweeter in seconds!

Because power amplifiers handle a lot of power, they also generate a lot of heat when they are working hard, so adequate ventilation is essential. A rack system with an integral fan is ideal for systems rated above a kilowatt or so, and when mounting several amplifiers in one rack, unless they are individually fan cooled, it is wise to leave a space between the amplifiers to allow some air flow.

To achieve the maximum power from an amplifier, it should be run into the lowest impedance load that it can safely handle, which is usually four ohms, though some larger professional models will drive loads of only two ohms. If an amplifier rated at 200 watts into four ohms is used with an eight ohm speaker system, you may not be able to get more than 100 watts out of it.

To get the right amplifier loading, it is necessary to understand series and parallel loudspeaker connection, which will be covered next month.

Cabling



For very small PA systems, a combined mixer and amplifier can be very convenient, the main disadvantage being that long leads are often required to connect the loudspeakers. Separate power amplifiers can be placed close to the loudspeakers, enabling shorter cable runs to be used. This is important, as all cable has a finite resistance, and in loudspeaker applications where the speakers themselves have a low electrical resistance, you can end up in a situation where a significant part of your amplifier power is being wasted warming up the cables! The rule is to use the heaviest gauge cable you can lay your hands on, and the higher the power of your system, the more carefully you need to consider the type of cable. Guitar leads or other coaxial screened cables are simply not on for use with any loudspeaker system, as their electrical resistance is too high.

If the mixing console is remote from the stage, as it generally is if you're not mixing and playing at the same time, the microphone cables have to be routed to the mixer, and that is normally accomplished by a multicore cable with a stage box housing mic sockets at one end and connectors for the mixing desk at the other. Essentially, a multicore cable comprises several individually screened cable pairs in one flexible outer sheath, and this is usually rolled onto a drum or reel when not in use. Good multicore cable is, unfortunately, expensive, but there's no easy or cheap way around this.

Figure 1: A typical small PA system.


Microphones



Dynamic cardioid mics tend to be the mainstay of live sound because they are tough, they produce a hard-hitting sound and their cardioid response helps prevent sound leakage and feedback. In addition to being used extensively for stage vocals, dynamic mics produce a hard-hitting drum sound and are the preferred choice for miking basses and guitars. Capacitor microphones are more sensitive and may be used in some critical applications, such as miking acoustic instruments, but in pop applications, the potential feedback problems associated with miking quiet instruments have led to the widespread use of transducer pickup systems, especially in the case of acoustic guitars and other stringed instruments. Brass instruments, on the other hand, tend to produce high levels of sound and can be miked with little difficulty. (See our feature on Microphone Basics in this issue for a full explanation of mic types and their characteristics.) The microphone techniques employed on stage often differ from those used in the studio because sound separation rather than absolute accuracy takes priority. For this reason, it may be necessary to use equalisation to improve a fundamentally imperfect sound whereas in the studio, the first recourse would normally be to move or change the microphone. Figure 1 shows a typical small PA system; later in the series, I'll be dealing with all the elements in turn, as they all affect the final performance of the system in some way. Next month I'll be looking at loudspeaker systems.

Acoustic Feedback

When a microphone is used with a PA system, it not only picks up the sound of the instrument or singer at which it is directed, but also other sound from around the room, including the sound of the PA system itself — either directly or reflected from the room's surfaces. If too much of the sound from the PA speakers leaks back into the microphone, it will increase in level as it recirculates, quickly building up into a continuous whine or whistle known as feedback.

There is always one frequency that has slightly more gain than the others, either because of the EQ settings, the loudspeaker characteristics, the microphone characteristics or the room acoustics, which is why feedback occurs at a single pitch. If a system has unwanted peaks in its frequency response, then it will be especially susceptible to feedback at those frequencies, making it more vulnerable than a system that has a flat response across the whole audio spectrum.


Microphone Impedance

Balanced, low impedance mics are invariably used in live sound work because they can be used with long lead lengths and have good immunity to interference. This is essential when stage-to-mixer multicores can be hundreds of feet long and where electronic lighting systems are being used, as these can generate a significant amount of electromagnetic interference.

Systems that use high impedance microphones are limited to cable lengths of around 10 feet or so before the sounds starts to lose level and become dull. Some small mixer amplifiers still use high impedance mics, though most modern models, even relatively inexpensive ones, have moved to low-impedance, balanced inputs. With microphones, there is no reason to worry about their actual impedance in ohms — they are either specified as high or low.


Series - "Live Sound"

Read the next part in this series:


All parts in this series:

Part 1 (Viewing) | Part 2 | Part 3 | Part 4 | Part 5 | Part 6


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Previous Article in this issue

Music In Our Schools

Next article in this issue

Bass: The Final Frontier


Recording Musician - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Recording Musician - Nov 1992

Donated by: Mike Gorman, Colin Potter

Scanned by: Mike Gorman

Topic:

Live


Series:

Live Sound

Part 1 (Viewing) | Part 2 | Part 3 | Part 4 | Part 5 | Part 6


Feature by Paul White

Previous article in this issue:

> Music In Our Schools

Next article in this issue:

> Bass: The Final Frontier


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