In Part 1 of our new series covering the essentials of PA, PAUL WHITE provides an overview of the main components of a typical system.

In the early days of touring bands, the PA system was likely to comprise a mono mixer amplifier of around 100 watts driving a couple of 4 x 10-inch or 4 x 12-inch speaker columns with no tweeters, no crossovers, and no off-stage mixing. Effects were limited to spring reverb or tape-loop echo, and the microphones employed ranged from dynamic models by the likes of Beyer or Shure to fragile ribbon mics such as the ubiquitous Rezlo. Concepts such as flat frequency response, controlled directivity or stage monitoring were entertained only by the most visionary of audio pioneers, and I still recall going to the first Kempton Park Jazz and Blues Festival in the early '70s and seeing PA towers comprising multiple WEM 4 x 10-inch speaker columns tied to scaffolding towers. Contrast that with today's sophisticated multi-channel concert systems and you'll appreciate that modern PA has about as much in common with its ancestors as Concorde has with the Tiger Moth. Even so, the underlying aim of a PA system is still the same as it ever was, and the purpose of this series is to look at the equipment involved, the principles on which it operates, and the best way to use it in a real world, gigging/touring environment.

ROLE OF THE PA SYSTEM

The first terms that may cause confusion are Sound Reinforcement and Public Address. I've never seen a watertight definition for either term written down in black and white, but I tend to think of Sound Reinforcement as something that helps increase the volume of the existing back line instruments and vocals, whereas a full Public Address or PA system provides the bulk of the front-of-house sound. In gigging circles, however, the term PA seems to be used as a cover-all term for either type of system. PA is also used to describe a system used to carry announcements in a public place such as a railway station or airport, but the technical requirements are quite different and few bands aspire to sound like the station announcements at Paddington!

For larger venues and tours, it's usual to hire in a PA system and a mix engineer, but for pubs, clubs, smaller college venues and the like, it's usually more cost-effective to buy your own. A modern, stereo PA system capable of outputting several hundreds of watts of power can be small enough to fit into the back of a hatchback 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 just voice and guitar, there's no point in getting a system with powerful bass bins that go down to 30Hz — 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, create powerful dance music, or handle a synthesizer setup that is generating deep bass and drum sounds, then you need to have a system that can reproduce low frequencies at high sound levels without distortion.

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 plays more of a sound reinforcement role, augmenting the sound of the on-stage, backline amplification as well as carrying the vocals. For example, in a typical pub gig, the guitarist's amplifier is likely to produce as much volume as is required, so little or no help is needed from the PA system. The PA system still needs to be able to handle a wide audio spectrum, but its main job will be to lift the vocals over the level of the backline and drums. If space or budget is tight, a smaller system with a less extended bass response will handle vocals perfectly adequately, but you'll have to rely on the bass player's own amplification and the natural acoustic sound of the drums to project the bass end of the mix.

One of the benefits of a larger PA system is that you gain more control over what the audience hears. It also follows that if you want your sound engineer to have a reasonable degree of control over a mix in a small venue, it is helpful to use modestly-powered backline amplification and then mic it up. Not only does this give the mix engineer more leeway to turn things up, it also reduces the amount of spill from the backline amplifiers into the vocal mics, resulting in a clearer sound all round. This can be a major problem in real world venues and various strategies for improving on-stage sound will be discussed later in the series. For typical pub work, guitar combo amplifiers rated at around 50 watts each and bass/keyboard amplifiers rated at around 100 watts are usually adequate. Using a 200 watt guitar stack in a pub leaves the sound engineer with virtually no control.

THE SYSTEM

In some ways, you can consider a PA as the equivalent of a giant hi-fi system, but because it often has to work in a less than ideal acoustic environment, it must be specifically designed to cope with the coloured-sounding or reverberant rooms that are so often encountered on the gig circuit. This includes the type of equalisation provided and the directivity of the loudspeakers. If the room is very reverberant, obtaining an intelligible vocal sound can be very difficult and a basic understanding of the relationship between the room acoustics and acoustical properties of the loudspeakers is essential.

In any room, the direct sound from the loudspeakers will become quieter as you move further back; that relationship is governed by the inverse square law and is a basic parameter of physics. By contrast, 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 nominal level of the reverberant sound is likely to be the same, regardless of whereabouts you are in the room. From these two simple facts, it follows that 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.

You might think that the critical distance depends entirely on the room acoustics, but fortunately, that isn't entirely true. The amount of reverberant sound depends on how much sound reaches the walls and ceilings in the first place. If your speakers produce a more tightly focused 'beam' of sound, then it's possible to increase the critical distance by directing more sound onto the audience and less onto the walls. In a given room, the more directional the PA system, the further the critical distance from the loudspeaker will be, but the directional characteristics of the speaker also have to 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 it helps to be aware of this very important factor early on.

THE PA MIXER

A PA mixer need not be complex, but it should be fitted with basic tone controls and should have provision to connect external effects units and signal processors such as equalisers, limiters, and so on. This is true both of separate mixers and those that come as part of a powered mixer package. 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 re-create it live. As you might imagine, powered mixers, effects and signal processors will be the subject of future articles in their own right.

STAGE MONITORING

A portable PA system needs to 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 to minimise the risk of acoustic feedback and to maximise audience coverage, and for all but the smallest systems, some form of on-stage monitoring or foldback is desirable. Note that in PA circles, the terms monitoring and foldback are generally interchangeable.

A foldback system may be considered as an independent sound system, comprising power amplifiers and speakers, running from a foldback (pre-fade send) output on the mixing console. Foldback is needed, because the majority of the sound is projected towards the audience, with a well-designed PA system, which may leave the performers struggling to hear themselves sing or play. An effective foldback system provides the performers with a suitable mix (which may be quite different to the front-of-house mix) to help them keep in time and in tune. Arguably 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.

More sophisticated PA mixers can provide two or more different foldback mixes for the benefits of the different performers. 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.

A further requirement of a monitor system is that its coverage is quite specific; for example, the singer shouldn't be able to hear too much of the guitarist's monitor speakers, and the output from the stage monitors shouldn't get back into the performer's mics, otherwise feedback may become a problem. Conventional monitors usually comprise a two-way loudspeaker system mounted in a wedge-shaped cabinet, which can be aimed upwards at the performer. If the monitor cabinet is placed in the dead zone of a cardioid or hypercardioid microphone, unwanted spill can be minimised.

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. A conventional mixing console can provide as many different foldback mixes as there are pre-fade send controls, but a separate power amplifier and speaker system is needed for each different 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, which are quite separate to the front-of-house mixer, and can provide numerous different versions of a mix over several different monitors. By contrast, a conventional console may only have two or three pre-fade outputs, and all the foldback mixing has to be done from the main mixing position.

Stage monitors often take the form of floor-standing wedges, but there are alternatives, such as small stand-mounted loudspeakers, full-range speaker systems suspended above the performers, side-fills (speakers pointing across the stage), or sophisticated in-ear systems. In-ear systems use specially designed earpieces to provide the performers with a foldback mix, usually via a radio link to avoid cabling, and the benefits include excellent isolation as well as keeping the stage clear. In-ear monitoring is now such an important subject that we'll be devoting a major feature to it in the very near future.

LOUDSPEAKERS

PA loudspeakers vary in complexity from simple, passive two-way systems comprising a 12-inch speaker and a horn tweeter, to huge multi-way, multi-cabinet active systems comprising separately amplified sub-bass, bass, mid-range, and high frequency enclosures. The majority of small, portable systems are based around passive two-way designs, although advances in design technology means that smaller systems can now benefit from active crossovers. Some manufacturers are even building the power amplifiers and active crossovers in to the speaker cabinets to save space and to make setting up easier. The whole subject of PA loudspeakers can be quite involved, which is why the next part of this series is devoted to them entirely.

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 most conventional power amplifiers are only around 60% efficient at best; they also generate a lot of heat when they are working hard, so adequate ventilation is essential. For every kilowatt of power fed to the speakers, something like 500 watts is lost as heat — and 500 watts is about the same heat as produced by half an electric fire bar! A rack system with an integral fan is recommended for systems rated above a kilowatt or so. 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 4 Ohms, although some larger professional models will drive loads of just 2 Ohms. If an amplifier rated at 200 watts into 4 Ohms is used with an 8 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 also be covered in this series.

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 feed the loudspeakers. Separate mixers and amplifiers allow power amplifiers to 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 suitable 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. This is normally accomplished via 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, and there's no easy or cheap way around this.

MICROPHONES

Dynamic cardioid (unidirectional) or hypercardioid 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 punchy 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 pick-up 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.

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.

As this series progresses, I'll be dealing with all the elements in more detail and the next part will cover the basics of loudspeaker systems.