Paul White examines the requirements of a small gigging PA system and offers some useful tips on how to use it most effectively.

As explained last month, current PA systems tend be based around 'all in one box', full-range loudspeaker enclosures; in larger systems, these will be augmented by sub-bass enclosures. Full-range cabinets have the benefit of more predictable performance, plus they are easier to set up. What's more, because they can be designed and built using matched acoustic components, the system performance is likely to be more accurate and there is far less scope for making the wrong connection. For example, a typical small PA cab with an integral, passive crossover requires just a single, two-core speaker cable between it and the amplifier. Active systems are usually fed from the amplifier rack via a multiway cable terminating in a multi-pin connector at the cabinet end. For the ultimate in convenience, however, the new generation of moulded full-range enclosures with integral power amps and active crossovers make a lot of sense.

A QUESTION OF POWER

One of the first things to decide on when choosing a PA is how much power you need. The answer depends both on the size of venue and the type of music involved. Furthermore, the level of sound a system produces isn't only dictated by the amplifier power, but also by the efficiency of the loudspeakers drive units. A 500 Watt PA using very efficient drive units might well put out more actual volume than a 1500 Watt PA built around less efficient drivers. It is also important to appreciate how much sound the backline is contributing to the front-of-house sound — in small venues, this can be considerable.

It is obvious that a rock band with a drum kit is going to need a more powerful system than a folk band. What's more, a PA that is going to be used to amplify the drums and bass guitar is going to need more power at the bass end than a PA being used to amplify only vocals. To put this into perspective, my own pub band gets by with a 300 Watt per side powered mixer system and a couple of full-range cabinets, but the system carries mainly vocals. In most pub venues, getting the drums quiet enough is more problematic than getting them loud enough. On the other hand, if you do want to amplify everything, a couple of kilowatts is probably as small as makes any sense, given that an acoustic drum kit can put out as much power as a 400W amplifier on its own. You'll also need sub-bass cabs to handle the drums and bass guitar effectively.

DIRECTIONALITY

Unless a system is carefully designed, it is easy to end up having a very wide dispersion at the low frequencies and a relatively narrow dispersion at the higher frequencies. Although the high frequencies do make it to the back of the room, they tend to sound rather honky and don't blend well with the lower frequencies from the bass bins — not really the kind of result you want. Furthermore, because of the narrow dispersion pattern of the HF horns, it is likely that members of the audience who are close to the front, but off-axis from the speakers, will receive inadequate coverage. A properly designed full-range speaker should get around this particular problem, but speaker placement is important, especially where multiple cabinets are being used.

SPEAKER POSITIONING

People are better at absorbing sound than reflecting or transmitting it, so if the people at the back are going to be able to hear properly, the speakers need to be above the head height of the front rows of the audience. In a large concert system, separate speakers can be used to cover different sections of the audience, but in smaller venues (which is where most of us play), the same speakers are heard by front and back rows alike. However, simply standing the speaker cabinets on tables or stands that can't be tilted isn't the ideal solution, as Figure 1a illustrates. Here the on-axis sound passes over the heads of all the audience until it hits the rear wall of the venue, whereupon it is reflected back into the room. In practical terms, this means your audience hear less level in the first place — because they are listening to the less powerful off-axis sound — and the reflections from the back wall will interfere with the direct sound making it reverberant and unclear. It also means that more sound gets back to the stage where there'a a greater chance of your mics picking it up and going into feedback.

More effective use of the available sound is made by positioning the cabinets a little higher and angling them downwards so they point at a spot around two-thirds of the way to the back of the audience. Figure 1b shows this arrangement. This way the on-axis sound is directed towards the back of the audience, which helps compensate for the fact that they're further away. At the same time, those people at the front of the venue won't get deafened, because they're hearing the lower level off-axis sound. In other words, the nearer you get to the front, the more off-axis the listeners are, but as they are proportionally nearer to the speakers, the actual sound level is more comparable with what's heard at the back. From this description, you can appreciate why an accurate off-axis response is important — relatively few of the audience are standing directly on the axis of the speakers. Ideally, the off-axis sounds should differ from the on-axis sounds only in level, although in practice, because of the longer wavelengths involved, the low bass end invariably has a wider dispersion than the mid and high areas of the spectrum.

"... getting the drums quiet enough is more problematic than getting them loud enough."

MORE OF EVERYTHING

You might imagine that to double the level of your system, all you need to do is to add another power amp and two more cabinets placed alongside the originals. However, this isn't the best way to do things. Let's say for the sake of argument that these speakers have their -6dB point (this is where the measured SPL is exactly half what it was when measured on-axis) at 15 degrees off-axis. If you visualise the sound coming out of the speakers in beams, as shown in Figure 2a, the beams will overlap, causing an addition of power along an axis drawn between the two enclosures, but there will be level and phase additions and cancellations at various frequencies in the off-axis sound. The result is that you actually narrow the useful angle of dispersion of the cabinets by standing them side by side.

One way around this is to stand the cabinets at an angle to each other as shown in Figure 2b. The angle between the cabinets should be twice the -6dB dispersion angle of the speakers. In our example where the -6dB point occurs at 15 degrees, the required angle would be 30 degrees. This way the dispersion patterns of the speakers overlap at the -6dB point, providing a more even power coverage with a wider angle of coverage. The on-axis sound is no louder, but the angle of coverage is wider, which leads onto the principle of arraying loudspeaker enclosures. By designing enclosures to have a deliberately narrow dispersion angle, the on-axis sound is louder, and multiple cabinets can be arrayed at more gentle angles to provide high intensity, wide angle coverage. Where several rows of cabinets are to be stacked one above the other, the vertical angles are also calculated to give the desired coverage in the vertical plane.

While few pub bands use arrayed PA systems, the underlying theory is useful when it comes to deciding whereabouts to put your speakers for the best results. Providing you can minimise overlap, doubling the number of speaker cabinets should double the overall amount of sound you can project into the room.

REFLECTIVE SURFACES

The problem with nearly all venues is that sound bounces from the walls and ceiling when you really want all of it to fall on the audience. Low ceilings are a particular problem. If you have to work in such a venue, it can be an advantage to try to narrow the vertical dispersion of your system so as to direct as little power as possible upwards. Although enclosures don't come with a dispersion control knob, if you are using two or more identical cabinets per side, you can stack them in such a way as to achieve the desired end as shown in Figure 3a. Here the top cabinet is inverted, which produces a narrowing of the vertical dispersion of the high frequency units, without significantly affecting their performance in the horizontal plane. The physical size of the two cabinets will also tend to narrow the low frequency dispersion to some extent. Back in the '60s and early 70s, PA speakers were often built as columns of four drivers, not only to maintain a narrow profile, but also because the tall, thin nature of a column produces a broad horizontal dispersion and a relatively narrow vertical dispersion.

In venues such as cinemas or theatres, combining high ceilings with tiered seating, it may be desirable to increase the vertical dispersion of the speakers. The easiest way to do this is to stack the cabinets (both the same way up) and angle the top cabinet back slightly. Figure 3b shows how this is done. It helps if you check the vertical dispersion spec of your speakers so that you can select an angle that will minimise the overlap between the two cabinets.

Next month, more practical tips on setting up and operating a small PA system.